CKVVER 4-2

INTRODUCTION TO DIVE TABLES

INTRODUCTION spent only thtee minutes at 100 feet, the no- decompression time for that dive was 25 Diving usedto be so simple. In thoseearly minutes. ycM3 there were only a few brandsand models of masksand regulators. There werc no BCDs. In the mid-70s a technique of stepping- And basic black was Ihe color. Today, the-tables, similar to one that had previously must literally sort through scores of masks, fins, been used in oil field diving, emerged in the snorkels, regulators, BCDs and diving suits. recreationaldiving community. Using this ncw Oncethc diver determinesthe make and model, technique a scuba diver could dive to a fashionable color coordination may also be maximum depthof 100feet at the beginningof a considered in final equipment selection. dive and progressivelymove to shallowerdepths without exceeding a no-decompression limit. The selectiondilemma is not just litnited to The no-decompression time for a 100 foot .Considering that the avcragc maximum depth dive could now be cxtendcd to individual dives only 10 days pcr year, it is 60 or more minutes, Although this technique possibleto usea diffetentdecompression table or never received an official endorsement by the device for each day. At present, there arc Navy or training agencies, it was used for approximately seven distinctly diffcrcnt thousandsof scubadives eachyear, especiallyin decompressiontables and numerous elccttonic the Caribbean. The recreational scuba diver is dive computers available to thc diver. All of the by nature a multilevel diver. availablctables and many of the dive cotnputcrs arebased on diff erent decompressionmodels. Meanwhile, there appeared to be increasingcontroversy as to the safety of the U. The U. S. Navy Standard Air S. Navy tables for a population. In Decompression, No-Decompression, and 1976, Dr. Spencerpublished the results of his Repetitive Dive table:spublished in the 1958 work on ultrasonically detected bubbles in the edition of thc U. S. Navy Diving Manual served blood following U.S. Navy no-decompression as thc standard for recreational and scientific dive exposures and proposed new, more scubadiving for nearly duce decades[6, 7, 24]. conservativelimits [22]. In 1981,Karl Huggins Even though they have been highly criticized in of the University of Michigan developed a thc past decadeand errors have been discovered completesct of no-decompressionrepetitive dive [23], they have served the diving community tables based on Spencer's values [9]. Other wcIL authorities also endotsed a mote conservative approach to no-decompression diving and Several excellent format rearrangements severalncw no4ccomptcssionlimits appcarcdin simplified the use of the Navy tables for recreationaldiving litcratutc. Sport divers and repetitive . Thc tables were instructors now had to decide what no- matketcd under names such as PADI Tables, dccomptession limit to usc. NAUI Tables,Nu-Way Tables, Tablesand so on, however, they were all just Although electronic dive computers had tetlrangcmentsof the U.S. Navy tables.In those been available for many yeats, it was not until carly yeats it was consideredunacceptable to thc early 1980s that two electronic deviate from the exact ptoccdutesprescribed in decompression microproccssors successfully thc Navy manualfor using the tables. Tberewas entered thc recreational and no allowance for table intetpolation. Many matkct at an affordable price. Each of these scuba divers felt limited by the fact that a dive computers was based on a diff'erent schedulehad to be basedon the maximumdepth decompression modeL In a review of one of attainedduring the dive, even if the diver only thesedevices, the EDGE, Dr. Bassett Skin Diver stayedat that depth for a few minutes. If a diver Magazine, July 1983! wrote "an innovation to University of Michigan Diving Manual

sport divers equal to the original introduction of for dive time selection to one minute intervals scuba." Since then other electronic devices and has five foot depth incrementsto a depthof based on still other decompression models have 100 feet compared to 10 foot increments on been released. most other tables!. The Wheel may also be used to make multilevel dive calculations. Not every diver will be able to afford an electronic . Consequently,today In 1985 the Canadian Defense and Civil the majority of the diving community must still Institute of Environmental Medicine's DCIEM! rely on a table for determiningproper and safe releasednew air diving tables. These tables dive schedules. In the United States, most resultedfrom a continuous 15 year evolution of instructors begin to be teaching a conservative the Kidd-Stubbs decompression model. The approachto use of the U. S. Navy tables in the tablesare simple,easy to use,induded correction 1980s. Some groups have adoptedthe Huggins factors for diving at altitude, and appearedto be tables. A few use the British or Royal Naval more conservativethan the U. S. Navy tables. tables. From technical reports, it was evident that the tables had been calculated on a relatively In 1983 Florida dentist Ray Rogers conservativedecompression model and subjected proposeda new dive table model based,in part, to considerable human subject testing before on the earlier work of Spencer, Diving Science release. Thesetables are used in Canadatoday, andTechnology Corporation, a corporateaffiliate however,they have not been adoptedby any of of the Professional Association of Diving the United States training agencies. Instructors PADI!, funded a project to finalize and test tables based on this new model [1, 2, 16, Today, there are basically two air dive 17, 19,20]. The RogersTables were testedby the tables used by the United States recreational and Institute of Applied Physiology and Medicine in scientific diving communities the US Navy Seattle under the direction of Michael Foweii, and the PADI Recreational Dive Planner. PhD [15]. The test population was selectedto Currently, the National Association of include typical recreationaldivers basedon sex, Underwater Instructors NAUI! has designated age, and physical condition and all testing that their instructors will teach a conservative use includedDoppler monitoring. of the US Navy dive tables. NAUI haspublished a rearrangementof' the Navy tablesand redefined In the mid-1980s the PADI Recreational dive time and repetitive dive. The Professional Dive Planner, based on Rogers' work, was Association of Diving Instructors PADI! releasedto the:general public. These tables are requiresthat all studentsbe trained in the useof designedsolely for no-decompressiondiving the Recreational Dive Planner. [19]. This table provides the diver with a more conservationinitial dive no decompressionlimit The YMCA and Scuba Schools comparedto the US Navy tables!. However,the International SSI! apparently remain with the diver will soon discover that these tables allows US Navy tables. SSI apparently endorsesthe must shorter surface intervals and longer Jeppesenarrangement of the US Navy dive repetitive dive times than the US Navy tables. tableswith conservativenoMecompression limits This results from the fact that Rogersuses a 60 based on doppler research. The Jeppesen/US minute half-time compartment for computing Navy dive table was used in University of surface interval credit and residual nitrogen as Michigan training programsuntil June 1990. At comparedto the 120 minute compartmentused that time the University instructors elected to by the US Navy. provided instruction in the use of the NAUI/US Navy dive table and the PADI Recreational Dive The Recreational Dive Planner is available Planner in order to compliance with the in two formats a Table and 'Ihe Wheel. 'Ihe requirementsof theserecreational Table format is identical to PADI's prior US agencies. Navy reamaged table format however it has 26 repetitive! groups rather than 14. The 'Ihe dive table controversy continuesinto diver must still select the exact or next higher the 1990s! Unfortunately,the controversymay time and depth figurc. The Wheel format allows be more political than physiological. The US

4-20 Decompression

Navy table have 30 years of in field use with W'hen the diver begins to ascend, the hundredsaf thousandsof dives. People have process is reversed as the partial prcssure of cxpcricnced decompressionsickness using the nitrogen in the tissue exceeds that in the US Navy tables, however, the incidence is not circulatory and respiratorysystems. The prcssure nearly as seriousas someindividuals would lead gradient between the tissues and the blood and you to think. When usedproperly the incidence lungs must be carefully controlled in order to of decompressionsickness on US Navy tablesis prevent too rapid of a of nitrogen. If said to be less than 0.01 percent [28]. It is too ptcssutc changes take place too rapidly and a soon to establishan incidenceof decompression sufficiently steep gradient develops, nitrogen sickness for the Recreational Dive Planner. bubbles can form in thc blood and tissues. These Although the initial test results were apparently bubbles ultimately can lead to the development excellent,the true test lies in long term usc by the of . general diving populatian. To minimize thc possibility of All divers must keep in mind that no decampression sickness, special tables and decompressionmodel, no dive table, or no dive procedures have been cstablishcd. These tables, computer can assure complete freedom from based on the amount of nitrogen absorbedby incidenceof deconrprcssionsickness. Thcrc are various thearctical companments at various simply too many vanables. The best that wc preSsuresfOr givCntime pe:iadS,,are deSignedtO can hope for is to keep the incidencc to an maintain a pressure gradient that, in theory, absolute minimum through proper training, preventsharmful bubble formation. They allow understanding basic diving physiology, and for natural elimination of nitrogen through commonsense . normal circulation af dissolved gases in the blood stream and normal gas diffusion in the At the present time we are all a bit lungs. confused by the number of alternatives in dive tables and computcts available to the beginning Today, no4ecompression is the buzz word diver. Today it is possible for a group of 10 af recreationaldiving. Divers are encouragedto divers ta enter the water and for eachta be using never exceed the no-decompression limits of a dive table or computer that is based on a dive tables. One popular dive table is based differen mathematicalmodel. In this training solely on no-decompressiondive limits. In program you will be inuoduccd to the basic reality, there is no such thing as a no- physiologicalprinciples of nitrogenabsorption decompressiondive. Basically, you decompress and elimination, instructed in the usc af bath US fnm any exposureto prcssure when you ascend Navy and Recreational Dive Planner tables, cvcn in a s~rimmingpaol. Thc British usc the provided with an overview of diver computers, term no-stop dive. 'iMs is probably a matc and provided with insight into thc proper appropriate term. application of dive tables and computers in routinediving activities. There are thrcc primary methods of decompression or pressure reduction following exposure to depth. Linear decompression is DECOMPRESSION: WHY AND HOW! probably the oldest foan of decompression.In linear decompressionthe rate of pressurechange When air is breathedunder pressurethe is constant.Linear decompressionis used today inert component, nitrogen, diffuses into thc for dives which do not exceed the no- various tissuesof the body. Nitrogen uptake by decompressiondepth-time limits. In this casethe the body continuesat various ratesas long as the diver is permitted to ascenddirectly from depth partial ptessutcof the nitrogenin the inspiredgas to surface pressure at a constant rate not lo is higher than the partial pressureof the nittogcn exceed 60 fpm. already absorbed in the tissues. Thc amount of nitrogen absotbedis primarily dependentupon Madified linear decompression is the af the inspired nitrogen sametimcs used for ascent from a saturation dive. depth! and the duration of exposureto pressure Saturationdives involve extremelylong exposure time!. to ,generally exceeding24 hours, The

4-21 University of Michigan Diving Manual

body tissues become fuHy saturated with inert curve in a stepwise fashion. Stage gas at a prcssure equivalent to the depth of decompressionascent profiles are relatively easy exposure. One approach to decompressionof to maintain in both the water and chamber as saturateddivers is a very slow constantrate of comparedto linear and curvilinear profiles. pressure reduction. In reality, such a decompressionis generally modified to mvolve Recreational and scientific scuba divers up to three different rates through three depth are encouraged to remain within no- Imgcs. decompression no-stop! limits of dive tables , and computers. Dive erasure which prohibit Curvilinear decompressionis one in which directascent to the surfaceare of muchhigher the rate of pressure change is constant1y nsk to the diver. decreasing. Essentially, a curvilinear decompressionattempts to follow the model's nitrogen elimination curve. Theoretically, it UNDERSTANDING AND USING provides a fixed, optimum gradient bctwecn DIVE TABLES tissueinert gas tensionand alveolarinert gas partial prcssine.Curvilinear decompressionis thc Dive tables are reaHy quite simple. Thc optimal technique that can be executed when diver simply reads the no-decompressiontime using somedive computer. limit or the decompressionstop requirements for a given depth. If the diver is making only one In simple terms, the dive computer is dive within a specifie time period .i.e. 12-hours designed to simulate nitrogen absorption and for the US Navy tables, 24-hoursfor the NAUI elimination during exposure based on a tables and 6-hours for thc PADI Dive Planner! inathematicalmodel and provides the diver with the procedure is straight forward and decompressioninformation basedan this model. uncomplicated. Thc diver must not exceed Using the curvilineardecampression technique prescribedrates-of-ascent and is encouragedto the diver may ascenddirectly to the ceiling, or makean ascentcontrol stop for all dives. safe ascentdepth, indicatedby the computer. This establishesan optima1gradient for nitrogen Pmceduresfor using three dive tableswill elimination.The diver continuesto decompress bc includedin this discussian thc US Navy ascend!by keepingthe actualdepth equa1 to the dive tables, the NAUI dive tables, and the PADI ever-decreasing ceiling depth. Although this RecreationalDive Planner. 'Ihe proceduresfor techniqueproduces the optimal decompression useof thc US Navy Dive table ate a conservative prafil, it does maintain the diver at the limits of modificationof standardUS Navyprocedures as model throughout decompression. given in the US Navy Diving Manual! basedon Consequently,the diver experiencesthe highest ncw findings and trends in recreational and level of decompressionstress allowed by the scientifi diving. The NAUI dive tables is a modcL conservativemodification of the US Nay dive tables. Since the proceduresfor the use of the Stagedecompression is the mast widely RecrearionalDive Planner have beenpublished used method of decompression from a dive and specifically designatedby the Professional wherelinear ascentdirectly to thc surface a no- Association of Diving Instructors, the decompression or no-stop dive! cannot be procedures designated with that table should be accomplisheddue to the amount of exposure. used and are aLsoaddressed in this publication, Stagedecompression involves ascending,at a fixed rate,from the maximumdepth of the dive Specifi proceduresand values endorsed to a decompressionstop at sameshallower depth. by the University of Michigan instructors for After a givenperiod of timeat the stopdepth, thc using US Navy dive tables tue presentedin bold diver ascendsto the next decompressionstop. type. This processis repeateduntil the diver reaches the surface. In air diving, the decamptcssioa stops are usually at 10 fsw msw! depth No-Decompression Time Limits: intervals. Stage decompressionrepresents an Unfortunately, all of the dive tables commonly attemptto follow the body's nitrogen elimination used by recreational and scientific divers have different no-decompression no-stop! limits. &or

4-22 Decompression

some this becomes quite confusing. The the NAUI dive table consider any dive depth following valuesare the no-decompressionlimits shallower than 40 feet as a 40 foot dive. for ! Jeppesenmodified limits for conservative usc:of the US Navy dive tables MN!, ! NAUI Dive Table Depth is the modified limits for US Navy Tables NAU!, ! Maximum Depth Attained the PADI RecreationalDive Planner RDP!, ! During the Dive the Canadian DCBIM Sport Diver Table DCEIM!, and ! the original 1958 US Navy dive tables USN! depth in feet, time in For cold and/or arduous dives yau are minutes!: encouraged to use a mare conservative dive schedule. 'I%isis easily accomplishedby adding 10 feet to the measured dive depth for purposes DEPT MN NAU RDP DEC!M USN of determiningtable: entiy depth c.g., if 56 feet is thc measureddepth, use the 70 foot dive table 30 205 360 depth!. 35 160 205 3 40 130 130 140 175 200 50 70 80 80 75 0 Cold and/or Arduous Dive: 60 50 55 55 50 Add 10 Feet 70 40 45 40 35 80 30 35 30 25 NAUI has electedta use thc next greaterbottom 90 25 25 25 time to address cold or sttcnuous dives. For example, for example if you wish to remain 100 20 22 20 15 25 within the no-decompicssiondive time limit for a 110 15 15 16 12 20 depth of 60 feet and you are caid, limit the dive 120 '10 10 12 13 15 to 50 minutes or less, In arder to determine the 130 5 8 10 8 lcttcr group at the end of the dive read the letter for thc next gicater bottom time i.e., use 55 These atc the most common dive tables and minutes!. limits that you will encounterin United States icctcationaland scientificdiving at this time. Time: The U.S. Navy and PADI Recreational Thc limits designatedin under column "NAU" Dive Planner! define bottom time as the total are to be usedwith the NAUI arrangementof the elapsedtime in minutes! from when the diver US Navy dive tablesand the limits undercolumn leaves thc surface in descent to the time that the ''RDP" are to be used with the Recreation Dive diver beginsascent. NAUI definesdive bottom! Planner. Most authoritics recommend that divers time as thc total time spentunderwater exclusive never exceed prescribe limits and whenever af the precautionarydecompression stop time. possibleavoid diving to the limits of the tables. Many scientific and recreational scuba divers now use a more canservative definition of bottom or dive time. They defin bottom time as total AvoidDiving to the dive time or &ne spent underwateron any given Limits of the Tables! dive. This is more pr3ctical in that most divers now usc dive timers or computers that are automatically activated and deactivated by Depth: When readingdive tablesdepth is thc pressure water depth! at the beginning and cnd maximumdepth attained during the dive and is of thc dive. Since it is nat practical to publish a expressed in feet or meters of sea water. Most dive table with minute-by-minuteschedules, the dive manualsand tables published in the United diver mustuse the exactor nextgreater time to Statesusc feet of sca water FSW!. Since most dctenninc the appropriatedive schedule e.g., a tables are presented in 5 or 10 foot depth 42 minute dive to 60 feet would be read as a 50 intervals, the diver must use the exact or next minute time on thc US Navy dive table!. greater depth to determinethe appropriatedive schcdulc c.g., if 56 feel is thc mcasuicd depth, In the UM training programs and use the 60 foot dive table depth!. When using operationaldiving we use the morc conservative definition of bottom time basically surfaceto

4-23 University of Michigan Diving Manual

surface for no-decompression dives. However, Although the VGE-decompresson sickness time spent at an ascent control stop or relationship is still contested, research is precautionary decompression stop!, to be revealing a probable correlation between high discussed later, is considered neutral time and levels of VGE and decompression sickness, neednot be included as bottom time for purposes Revised and teduced no-decompressionlimits of teading tables. initially resultedfrom this discovery.

Later teseatch suggested that stopping for Bottom Time: a few minutes during ascent was as likely to Total Time Underwater reduce or preclude detectable VGE formation Exclusive of Time Spent at the than reducing the no-decompression limits I 14]. PrecautionaryDecompression Stop The next subject of controversy was. how long and at what depth? Participants in a workshop on diver ascent sponsored by thc American Rate of Ascent. The rate at which the diver Academy of Underwater Sciences October returns to the surface is specified for specific 1989! concluded that divers should stop in a tables. Both the US Navy and PADI depth zone of 10 to 30 fcct for 3 to 5 minutes RecreationalDive Planner specify 60 feet per during ascentfrom all dives. Ihis procedurewas tninute. NAUI specifies the rate of ascent as "not taught at thc University of Michigan for scvcral to exceed60 feet pcr minute." Rateof ascenthas years and is still used by many University divers. bccn a controversialtopic over the last few yeats. However,the procedurescurrently recommended Some authorities suggestthat the dive should in thc instructional program have been adjusted ascendat a slower rate such as 30 to 40 feet per to teflcct those p~ribed by NAUI and PADI, minute. Others suggest that ascent rate is not a major factor from a standpoint of bubble From a practical standpoint, allowing the fotmation. US Navy tables used prior to 1958 diver to stop in a depthzone is far more realistic specified a 25 foot per minute ascent rate for than establishing a fixed depth. In typical deep-seaor hard-hatdivers. As compressedair Caribbeandiving, a diver can relax and explote scubadiving becamemorc popular with special the surroundingsand not worry as much about warfare groups, a faster ascent rate was prccisc depth or depth maintenance. tequested. The 60 feet per minute ascentrate Furthermore, if them are tough high wave! was a comproiniscbetween hard hat divers and surface conditions, the diver can more scuba divers. comfortably stop at 25 to 30 feet than 10 to 15 feet. The time ffcxibility can telate to thc nature Presently, most authorities have agtecd of the dive, the number of dives, or environment that thc rate of ascentshould not exceed60 feet conditions. 1' example, some divers will stop per minute and that the diver may ascendat a for 3 minutesduring ascentfrom the first dive of slower tate if so desired. Physiologically, it thc day and5 minutesfor subsequentdives. appearsthat an ascentcontxol stop can be mote imp+tant than slowed ratesof ascent. AscentControl Stop: 3 Minutes at 15 Feet! Rate of Ascent: Neutral Tune} Not to Exceed 6G Feet/Minute

For all pt3ctical purposes,this stop time AscentControl Stop. Researchershave shown may bc considered as neutral time. In other that tiny bubbles venous gas cmboli or VGE! words, it need not be included as bottom time for form in the divers blood stream during ascent purposesof readingdive tables. However,some from many, if not most, dives. Although a diver conservative divers do included it in their may often develop VGE during ascent, surface-to-surface time or bottom time. In our symptoms of decompressionsickness may or opinion, if the stop time exceeds5 minutes, the may not pursue. These tiny bubbles indicate a additional time should be considered as bottom degree of decompressionstress on the diver. llnle.

4-24 Decompression

At present, divers using the PADI By strict US Navy procedure,the ascent RecreationalDive Plannerare required to rnakc:a time and the surface time need not be included as safety stop for 3 minutes at 15 feet if the diver dive or bottom time. Strictly speaking,the diver comeswithin 3 prcssuregroups definedlater! of couldadd the bottomtimes of the two segments a nodecomprcssiontime limit, andfor any dive of the diver to deteimine the actual total bottom to a depthof 100feet or greater. Diversusmg time for the dive. However,from a practical the NAUI dive tablesare encouraged to makea 3 standpoint,most divers simply include both the minutesprecautionary decotnpression slop al a ascent and surface time in their bottom times and depth of 15 feet during ascentfor any no- treat theentire eventas a single,continuous dive. decornprcssiandive. Thc times 2 hours and 6 hours! are determined by the theoretical half-time RepetitiveDives compartment on which the Surface Interval Credit Tables calculations are based. For Most scuba divers make morc than one example, the US Navy designates the 120- divein anygiven diving day. Somewill makeas minutehalf-time compartment as the controlling many as 5 to 7 dives. However, most authorities compartmenL Consequently,in theory, a 12 now agree that three dives pcr day is a more hour time pc:riodis requiredto rid the body of reasonable limit. accumulatednitrogen. In reality, this may or may not be so. It appearsthat NAUI arbitrarily elected to use 24-hours in a morc conservative Number of Dives: Limit to 3 Per Day adaptationof thc US Navy dive tables. On the other hand, the mathematical mode! RepetitiveDives: A repetitivedive refers to any used to develop thc RecreationalDive Planner dive conductedwith a specified time of a assumesthat all no~ompression dives can be previousdive. The U.S. Navy repetitivedive controlled by a comparunentwith a faster than tables define a repetitive dive as any dive 60 minutehalf-time. Consequently,in theory,a conductedwithin l2-hours of a previousdive. 6 hourtime periodis requiredto rid the bodyof On the other hand, thc Recreational Dive accumulatednitrogen. In reality, this may or Planner PADI! defines a repetitivedive as any may not bc true. You will find that thc dive conductedwithin 6-hours of a previous RecreationalDive Planner will give shorter dive. To the contr3ry,NAUI considersany dive surfaceintervals and longer repetitive dive times conductedwithin 24-hoursof a previousto be a for the same depth-time situationsthan the US repetitive dive. The Canadianand British dive Navytables. Please do not lct this confuseyou. tables require repetitive dive calculation Keep in mind that eachof thesetables was based proceduresup to 18 and 16 hours,respectively, on a different mathematical model. dependingon the diver'snitrogen retention level value at the completionof the previousdive. In order to computerepetitive dive times, Post-diveinert gas retentionand elimination will we must considerthe amauntof nitrogenstill retainedby the body fmn previousdives and the be discussed in another section of this manual. surface interval betweendives. Using this In order to bc designatedas a repetitive information, thc diver will compute: the dive, the US Navy specifiesthat a surface maximumallowable no-decompression time for intervalof at least10 minutes must elapse. Let' s a repetitive dive. assumethat a diver, working at a depthof 50 A day of repetitive diving should be feet, accidentallyreleases a buoyantobject an it plannedso that the deepestdive is performed Boatsto the surface. The diver ascends,retrieves first and succcssivcdives are progressively the object within 5 minutes,and wishesto return shallower.Thc placesless decompression stress to thework site at 50 feet. Returningto 50 feetis on thc diver and allows for longer no- not consideredto be a rcpetitivc dive; it is a decompressiondives throughout the day. continuation of the dive! Why? Thc dive spentless than 10 minutesan thc surface.

4-25 University of Michigan Diving Manual

Make the DeepestDive First buffer associated with more conservative use of and Successive Dives tables. Squeezingevery last minute out of a ProgressivelyShallower! table is, in my opinion, eventually going to get some divers in trouble.

RepetitiveGroup Designation: Basedon depth Some instructors and researchers have also and time of a dive, a letter is usedto designatea expressed concern with regard to short surface theoretical level of nitrogen in the body intervals and the practice of bouncediving i.e., immediately following the dive. This is a repetitive deep, short duration dives with limited repetitivegroup or pressuregroup. For example, surface intervals!. Since this is an area of if a diver surfaces frotn a 40 minute dive to 60 concern that must still be explored by feet, thc US Navy repetitive group is "G". The tcseatchets, it is our recommendation that the higherthe letter,the higherthe level of nitrogen minimum surface interval betwccn dives be retainedin the body. established at 1 hour until more information is available. Letters are used for most dive tables. Theseletters are «ot t'nterchangeable.Do «ot air tables and table values! Your companions Minimum Surface Interval: pressure group "G" on the Recreattonal Dive I Hour Planner is a totally different value that your repetitiveor letter group "G" on the US Navy dive tables. The PAD! Recreational Dive Planner also has special rules for multiple dives. If you are planning three or more divesper day, beginning Surface Interval: 'Ibis is the time between with your first di ve,if the endingpressure group repetitive dives. Official/y, the surface interval is W or X, the muunuun surface interval between begins as soon as the diver surfaces and ends subsequent dives is I hour. If the ending when the diver begins the descent of the pressure group is Y or Z, the minimum surface following or repetitivedive. From a practical interval betweenall subsequentdi vesis 3 hours. standpoint most divers will simply compute repetitive dives based on an approximate and conservative surface interval, Let's assume that PADI Recreational Dive Planner: a group of divers return to their boat between ObserveSpecial Rules 10:30 and 11 AM the last diver is on board a for Multiple Dives! few minutesbefore 11!. Practicallyspeaking, the surface interval can begin at ll AM. The gtoup leaderinforms the divers that they will be leaving the dock for the afternoondive at 2 PM SurfaceInterval Credit and New Repetitive and be on site and ready to dive no later than Group: During ascent and on the surface 2:15 PM. In this situation, more conservative foi!owinga dive,you atc og-gassing.In other divers will simply use a three hour surface words, your body continues to release excess interval to compute the repetitive dive even nitrogento the atmospherewith eachbreath until though the surfaceinterval may exceed3 hours you return to prcssure dive again! or reach by 15 to 20 minutes. In most casesthis would be equilibrium with the atmosphere.Consequently, consideredas conscrvativc diving! your repetitive group letter decreasesas the length of surfaceinterval increases.If your were Modern trends in appearto a "Giver" when you surfaced at 11 AM, you bc toward morc precisenessin timing and will be a "C-diver" for your 2 PM dive computation. For example, the Recreational NAUWS Navy tables!. DivePlanner has 26 prcssureor repetitivegroups compared to of the US Navy tables. Consequently,the diver can usc very precise Residual Nitrogen: Practica!!yspeaking, this is surface interval, ahnost to the minute in some the amountof nitrogen that remainsin you body following a specificsurface interval. %hisexcess cases, to optimize repetitive dive time. Optimizing time also mnoves someof the safety nitrogen must be accountedfor when computing

4-26 Decompression a repetitive dive. Dive tables specijy this any dive in excessof 60 feet shall be considered residual or remaining nitrogen in terms of as a deepdive. This simply meansthat the diver minutes of time that must be added to the shall take an extra degteeof care in planning the repetitive dive in order to adjust for the nitrogen dive and computingdive schedules. remainingfrom a prior dive s!. The value, in minutes,will vary for the samerepetitive group dependingon the depth of the repetitive dive. DeepDive: For example, if our "C-diver" using the Depth in Excessof 60 Feet NAUJ/US Navy tablesplans to dive to a depthof 60 feet on the afternoon dive, the residual nitrogen time is 17 minutes. ln other words, the The absolute limit for recreational divers diver begins the dive with an assumption that 17 has been placed at a depth of 130 feet and an minutes of bottom time have already elapsed, increasing number of instructors and physiologically speaking. If the afternoondiver organizations now endorse a 100 foot depth was limited lo 40 feet, thc residual nitrogen time limit. For our purposes, recreational scuba would be 25 minutes. diving shall bc limited to depths of 100 feet or less. Even if the diver exceeds 100 feet on the initial dive of the day, many authorities now Repetitive Dive No-DecompressionLimit: encourage divers to not exceed 100 feet on The most frequent computation in scuba diving is repetitive dives. determination of rcpetitivc dive no- decomprcssion time. This is quite simple. The no4ccomprcssion time for a specifi depth is RecreationalDive Depth Limit: designatedon the dive table. Keep in mind that 100 Feet this limit is for the initial dive only. To determine the no-decompression limit for a repetitive dive, you must subtract the residual Multilevel Dives: The scuba diver is a nitrogen time from the original no- multilevel diver. A scuba diver seldom goes to decompressiontime for the planned depth of the one specifi depth and remains there for the repeaive dive. duration of thc dive. Various mechanism stepping-the-tables i.c., interpolating! have been Let's assume that our "CCiver" above! used by commercial and recreational divers over actually wishes to return to 60 fcct on thc thc years. Many authorities discourage this afictnoon dive. The revised no-decompression procedure because, in some cases, dive times limit for 60 feet is 55 minutes. Subtract 17 indicated safe by table-stepping are shown to minutesof residualnitrogen from 55 minutesand exceed the safe times hmits of thc table model, you have the maximum noMecompressiondive time for the repetitive dive 38 minutes. Currently, the Wheel version of the Scvcral practice repetitive dive problems are RecreationalDive Planner is used to compute includedin the Appendicesof this Section. planned recreational multilevel dives. In theory, the diver computes the various stages levels or steps!of the dive and recordsthis information on Drawing the . Oneof the bestways a s1ate prior to entering thc water. The diver to prevent mistakes and avoid confusion in must now accuratelymorutor time and depth at repetitive dive computation is to graphically eachlevel to assuresafety. In reality, divers do a representthe seriesof dives as a drawing a poor job of momtoring depth and time even at dive profil. Enter all information on the dive just one level. The task of monitoring time and profile. Ideally, both membersof thcbuddy team maintainingspecific depth limits for two or three should draw dive profiles and compare thcln lcvcls can become complicated and is subject to when finalizing the dive plan. Dive profiles ate increased error. included in the appendicesof this section. Considering thc complexity and potential for error, it is our opinion that multilevel divesbe Depth Limits: Scuba diving depth limits have made only when using an electronic dive been discussed previously. F'or our purposes,

4-27 Universityof MichiganDiving Manual

computerthat continuouslytnonitors the diver's is unavoidable, a conservative approach is theoreticalnitrogen status. suggested. Fm example, some divers use a dive table depth that is 10 to 20 feet deeper than the actual dive depth to dctcnnine decompression. Multilevel Dive Profiles: Furthermote,if at all possible,avoid making Only with a Dive Computer repetitivedives following a decompressiondive.

ExceedingNo-Decompression Limits: Both DivesRequiring Decompression: recreational and scientific scuba divers are Not Recommended! cncouragcd to remain within the no- decompressionlimits no-stoplimits! of dive tables. The PADI Recieational Dive Planner is Multi-Day Diving: Most scientific and strictlya no4ecomptessionand exceeding a na- vacationingtecreational divers tend to make anc decompiessionlimit is cansidetedan emergency to two weeklong diving trips. Sincemost divers situation. Fbr example,if you exceedthe no- are payinga highprice for their divingholiday, dccamptessionlimit by no morethan 5 minutes, they wish to make as many dives as possible an 8-minutestop is mandatoryat a depthof 15 duringthis time period. Physiologistand diving feet andupon surfacing you mustremain out of autharities are now expressing increasing the waterfor at least6 hoursprior to making conccm about divers who make several dives another dive. If you exceed the no- daily for severalconsecutive days. Thereis dccomptessianlimit by mote than 5 minutes,a concetn that slower level tissues will became 15 foot decompressionstop of no lessthan 15- progressivelysaturated and ultimatelyinvalidate minute air permitting!is recommendedand the the decompressionmodels. Consequently,some diver must remain aut of the water for 24 hours authort'ties recomnaend that divers alkrw a 18 to priorto makinganother dive. 24 hourperiod of non-divingactivity +er every 3 consecutivedays of repetitivediving. In NAUI's adaptationof the US Navy Dive tables decompressiontime tequirements ate included for selecteddive times that cxcccd ReadingTables: 'Ilie proceduresfor tcading thc no-decompression time limits. The dccompicssiontables are fairly straightforward decompressiondepth is designatedas 15 feet. and are includedincluded in the appendicesof the Section.In addition,they will be addtessedin Thc U.S. Navy Standard Air detail in lectutc sessions. DecompressionTable gives stage decompressian stopsat 10 foot intervalsfor dive times that cxcccdthe U.S. Navy'sna4ecompressian limits. HOW SAFE ARK DIVE TABLES? For example,this tablerequires a stap of 14 minutes at 10 feet for a dive to 120 feet for 30 Severalpublications and many recreational minutes. These tables have been used for more scubadiving instructors cite thata highincidence than30 years.lbe US Navydoes acknowledge of dccomptessionsickness can be expected even an increasedincidence af decompression when the diver is using the U.S. Navy dive sicknessassociated with decompressiondives tables. A figure of 5% incidence is most and discouragesdecompression dives when frequentlystated and figutes as high as 10% are usingscuba not uncommon. Such figures are without foundation.Some claimed that the US Navy The recreational and scientific diving accepteda 5% incidenceof bendswhen initially community has also discourageddives that developing and testing thc tables. This is requitedecompression. Furthamote, ahhough absolutelyuntrue. In fact, if decomptession various "experts" have dealt with no- sicknessdid developduring testing, the schedule dccompiessionlimits, rates of ascent,and safety was adjustedand rctestedtill the incidence was stops, little information is availablc on air 0.00% [6, 7, 13]. decompressiondiving. At present,we canonly encouragescuba divers to avoiddives acquirin Until 1970 there was simply a hack of decompression.In the eventthat decompaession reliable statistical data on bends incidence in air

4-28 Decompression

diving. The unfortunate combination of Why have so many new tables been developed? unfounded incidence percentagesand lack of Why werc the Navy tablesso severelychastised? statistical data led to distrust, fear and in some First of all, many divers simply refuseto accept cases disregard of the U.S. Navy tables, blame for their own mistakes. Divers are Atbitraiy safety factors were promoted. Some careless. Many simply fail to monitor depth and individuals used this uncertainty to promote the time correctly. Some are under the imptession usc of dive computersand other tables, that tables and rules do not apply to them. Others simply cannot read tables or compute In 1970the U.S. Navy adopteda reporting repetitive dive schedules. &or the experienced system for both the number of decompression divers and instructors that I have offended with sickness cases and the number of dives. For the thc above statements. I do not apologize! You period of July 1, 1970 to June 30, 1971, the U.S. know it is true as well as I do. Navy documcntcd 25 cases of decompression sicknessout of 30,039dives. Air dive exposures Furthermore, modern trends in diver accounted for 26,035 dives and only 12 education, in my opinion, preclude adequate decompression accidents were noted in air training in the use of dive tables. In today's short diving, or an overall 0.046% incidence. The courses,trainccs simply do not havetime to learn incidence for open-circuit scuba diving was and pracace use of dive tables. What many slightly less. Expressedin terms of a risk factor, traincesleam may be soon forgotten. I feel that this is one caseper 2,857 exposuresfor all dives many Caribbeandive guides would support this [3] statement. But of even greater concern, in many casesnew divers are nol being provided with In the 24-monthperiod from January1972 sufficient information to understand and through December 1973, U.S. Navy divers appreciatethe gravity of the subject. tecotdcd 127,103dives or 97342 person hours underpressure. Only 35 casesof decomptession Second,our great America legal system sicknesswere reported during this study period is unable,in many cases,to acceptthe fact that giving an incidencc of about 0.03%. It is ultimately individuals must take responsibility significant to note that only 12% of all dives for their own actions. Even with thc excellent wcte in the depth range of 100 to 200 fsw; safety record of the US Navy tables and the however,dives in this depth rangeaccounted for acknowledged certainty that it is impossible ta 57% of the cases of decompressionsickness. develop a mathematical model of the human Only about 7% of the annual dives involved body that will enable development of an decompression,but gave an incidenceof 0.41% absolutely bend-free table, injured divers have decompressionsickness [4]. still suedcvcryone in sight. Consequently,some, if not many. changeshave been driven by "cover These early figure refiect an excellent your hind cnd" factors rather than physiological record for the U.S. Navy divers and thc U.S. factors. Navy tables and are included for historical peeve. As data reporting and analysisof 'Hurd, in 1976 Spencerpublished the fact statistical data improved, the incidcncc of that little bubbles are rushing around in thc decompressionsickness appears to be evenlower circulatory systemfollowing many asymptomatic for Navy divers. Mote recentreports indicate that dive exposures[22]. The Doppler technology decompressionsickness does occur, even on was not available 20 years prior when the Navy appatcntly safe schedules. However, it should bc tables were developed. This fostered an noted that statistics for over 240,000 dives immediate,response of concern in the diving conducted by Navy divers indicated an average community. Scvcral individuals published decompressionrisk of lessthan 0.01 percent[12. tcduccd nMccompression limits and even new 28]. dive tables based on these findings [9]. Later tesearch tevealed that making a stop an the way If the US Navy tables are apparently so to the surface,even if the table was pushedto thc safe, why is there such a concerntoday? Why maximum no-decompression limit, indicated have thc no-decompression limits been modified? significantreduction in bubble formation [14].

4-29 University of Michigan Diving Manual

Fourth, recreatinonal dives often push There are a number of factors to consider. tables and dive computers to their absolute First, is it virtually impossible to develop a limits. On the other hand Navy divers by nature practical, totally bends-free table to fit every are morc conservation. ~y either back off individual and situation. The times and from the limit or "jump schedules" i.e., use limitations would be prohibitive. Nitrogen deeper depth and longer times that actually absorption and elimination in the human body encounteredduring the dive to read tables!. If are dependent upon a number of variables. enough people go to the edge enough times, Tables have been developed to best protect the someone is going to fall off. Divers who push normal, healthy adult diver. Physiological thc table limits run a higher risk of deviations associated with poor physical decompressionsickness. conditioa, aging, and obesity are sufficient to precipitate bends under thc same diving Fifth, recreational divers may increase conditioas that would bc safe for a normal, their susceptibility to decompressionsickness healthyyoung adult. due to physical compromises. Basically, they invalidate thc table models. Social habits alone Second,the level of physical exertion and compromisemany divers. We live in an alcohol the thermal status of the diver aller nitmgen dominated society. Far too often divers are absorption-elimination. Persons who have physiological compromised by thc effects of worked hard and/or chilled significantly on a alcohol and subsequentdehydration when they dive are morc susceptible to decompression enter the water, This can significantly alter the sickness. nitrogen absorption-eliminationmechanism s!of the body. Third, daily diver condition is important. A higher incidence of decompressioncan be Finally,one cannot escape the inter-agency expectedin individuals who are suffering from politics and potential for economic gain minor illness colds, diarrhea,etc.!, lack of sleep, associatedwith today's dive tables. Them is alcohol intoxication and hangovers!,alcohol or both community statusand economicadvantage drug consumptionprior to diving, and the like. in producingand successfullymarketing the "so-called" best dive table. We must also Fourth, some table schedules show a contendwith the diving community's-oae-ups greater tendency to produce tmuble than others. personship" game and ego factors among it' s For example,the noQecomprcssionlimit of 100 leaders. Ncw waterproof dive tables are minutes for 50 fsw is probably questioaable. madcetedat $8 to $10each. Specialversions of Other possibly questionableschedules, beyond the dive tables sell for $13 to $35. In theory, the range of recreational divers, include 140 when a new dive table is issued hundreds of fsw/30 min; 140 fsw/40 min; 150 fsw/30 min; thousandsof diversare encouragedto purchase aad 170fsw/30 min [10]. new ones. Fifth, many factors exist in the civilian The fact remains that civilian divers have recreationaldiving commuaity that are lesslikely cxpericnccd decompression sickness while to confront thc Navy diver. For cxamplc, apparentfy following the tables exactly. Thc recreationaldivers are not bound by established exact number aad incidence percentageis not rules and rigid supervision. Deep air diving that known since there is currently no effective data would not bc perinitted under Navy regulations, collecting system that identifies both the actual is commonplace ia civilian diving. Furthermore, nurabcrof decompressionsickness cases and the Navy divers are required to have a medical actual number of dives for civilian divers. I office and hyperbaric chamber at the dive site suspect that the number of cases is relatively for dives beyonda depthof 190 feet. small, but probably higher than thatof the U.S. Navy. Currently, it appearsthat as maay as400 Many divers take a haphazard approach to 500 U.S. recreational divers cxperiencc towardmonitoring their and dive decompressionsickness annually. 'Ihis includes timer. Consequently,it is not possibleto select those using all dive tables and computers,aot the appmpriatedive scheduleif you do not know just the U.S. Navy tables[29, 30]. the precise depth and tirnc. Furthermore, depth gauges used in may have an

4-30 Decompression

accuracydeviation of up to + 5% of full scale formation would result when the diver returns to depending upon the gauge model and the surfaceprcssure sea level!. If the diver then manufacturer.'Ms meansthat a newgauge 50 immediately ascends to a higher altitude in an fsw model! could have a variation of 37.5 fsw to aircraft or drives into the mountains, the 62.5 fsw at an actual depth of 50 fsw and 117.5 reduction in can result in fsw to 142.5 fsw at 130 fsw. In addition, nitrogentensions within the body that exceedthe mechanical damage from use and abuse can critical level for bubble formation. cause even greater variation. Until the recent Consequently,the diver who was safe at sca dcvclopment of bcttcr depth gauges and lcvcl, can develop serious decompression mechanical- or electronic-device that sickness upon ascent to altitude. automatically recorded or marked maximum depth, many divers did not really know their allowing an exposure to pressure the exactmaximum depth on any given dive. diver must remain at sea lcvcl for somespecific period of time before ascendingto altitude in Many recreationaldivers take exceptional order to allow the body lo equilibrate. Exactly liberties with personalmodification of the U.S. how long apples to bc a subjectof considerable Navy tables. Several modification schemesfor contiovcrsy. multi-depth level scuba diving have been advanced in recent years. Although these The U.S. Navy Diving Manual 973 schemes may appear logical and apparently edition, p. 644! statedthat the diver "definitely work, they are generally not properly conceived must not fly for at least 12 hours [25]." NOAA or tested. Diving Manual published in 1979! stated, -Before flying in an aircraft in which the Conservativelyused for no-decompression atmospherewill bc less than 8,000 fcct usually dives in depths of 100 fsw or less and with the case in most flights!, a diver who has accurate depth-time determination, the U.S. completed any number of dives on air, and Navy tablesshould provide thc civilian diving decompressedfollowing the U.S. Navy Standard population with a very low, if not negligible Air Decompression Tables, should wait at sea incidence of decompression sickness. level air for the computed surface Conservativerecommendations have been given interval that allows him to be classified as a in the precedingdiscussion. However,all divers GroupD diverin the U.S.Navy Repetitive Dive must understandand acceptthe fact that no dive Table [11]." table or computercan guaranteebends-free diving. A more recent edition of the US Navy Diving Manual 985 edition, p. 7-22! indicates, Thc same questionsare now being asked "Hying in aircraft with cabin pressuresabove regarding the safety of the PADI recreational 2300 feet altitude may be done after a 2-hour Dive Planner. Preliminary testing in the surface interval for no-decompressionair dives laboratory and open water with modern and 12 hours for decompressiondives. If aircraft ultrasonic Doppler equipmentthat was not cabin pressureis below 2300 feet altitude, then available when the US Navy tables werc flying may be done immediatelyafter an air dive developed suggest that thc model and table [26]." Other dive manualsand rematch reports maybe valid. However, thc true test lies in statetime rangingfrom 2 to 24 hours[8]. extensive usc by the general diving public. Insufficicntdata exists at this time to draw any Thc PADI Rccrcational Dive Planner final conclusions. designatesthat the diver must wait a minimumof 4 hours following a single noMcotnpression dive with less than 1 hour of bottom time. Ror a FIX'ING AFTER DIVING single no-deempressiondive with a bottomtime of more than onc hour or after any repetitive When a diver surfacesfollowing a dive,an dive, the diver must wait a minimumof 12 hours. elevateddissolved nitrogen tension exists in the Eollowing any dive that required emergency body's tissues and fluids. The dive tables are decompression,the diver must wait a minimum calculated to keep the nitrogen tension bc:low the of 24 hours before flying. The instructions theoretical critical level at which bubble

4-31 University of Michigan Diving Manual further state that, wheneverpossible, a 24 hour measured from the surface is used and stop wait is generally recommended in most cases depths are increased by one foot [26]. [19].

The general lack of agreement among NAUI Recommends that the various authorities tends to leave the average US Navy Dive Tables be Limited diver with a degree of unccnainty. When in to Altitudes of Less than 1000 Feet doubt, take the more conservativeapproach. If Above Sea Level! for some reason the cabin pressurewere to be OOOO% lost during a flight, the diver flying within a short period of time following any dive would be The PADI Recreational Dive Planner extmncly susceptibleto decompressionsickness, ls not Designed for Use at Altitudes Greater than 1000 Feet Above Sea Level! The following recommendations were made by a group of diving physiology and medicine authorities at the Undersea and For diving at higher altitudes, two Hyperbaric Medical Society's "Flying after modifications must be made to correct for Diving Wotkshop" 4 February 1989! for no- differences in when dccomptessiondiving f 21]: standard sea-level tables are used. The diver must compute, or refer to a table to obtain, the a. If you have less that 2 hours of theoretical depth of the dive and the theoretical accumulated dive time in the last 48 depth of dccomprcssion stops for a given hours,wait 12 hoursbefore flying. altitude. Both the theoretical diving depth and decompression stop depths will vary with b. For unhmited, multiday diving or if altitude. In addition, the diver's tissue nitrogen you have greater than 2 hours bottom tensionschange when traveling from sealevel to time in the last 48 hours, wait 24 altitude. An adaptation period or special dive hoursbefore flying. table considerations are required. Also, conventionaldepth gaugesmay give erroneous If you made a decompressiondive, wait at least readings when used at altitude and conection 24 to 48 hours 8 hours if possible!before factorsatc tequited. flying. There ate variousptoccdutes and tables for computing no-decompression limits and Minimum Surface Interval decompression schedules for high altitude BeforeFlying diving. However, at the presenttime for diving No-DecompressionDive!: at altitudes in excess of 2300 fcct 00 meters! 12 Hours we recommend using the Buhlmann "Decompression Tables for Dives with Air at Various Altitudes" 8uhlmann, A., Becauseof the complexnature of decompression Decompmsion-DecompressionSickness Ncw sicknessand becauseunverifiable assumptions Yotk: Springer-Vcrlag,1984! [5]. are involved in decompressionschedules, there can neverbe a flying-following-divingrule thatis guaranteedto prevent bendscompletely! CONCLUSIONS At present the risk of decompression DIVES AT HIGH ALTITUDE sickness can be minimized, but not totally eliminated,through the proper use of dive tables U.S. Navy air dive tables were computed and computers. Unfortunately, divers must for diving with tcfctcnce to sea level. select from among many dive tables and current US Navy tables addressed in the 1985 computers. This selection is difficult and editionof theUS Navy Diving Manual,p. 7-22! debated even among the most knowledgeable may be usedfor diving in fresh waterat altitudes experts in thc flcld of diving. Dive tables and up to 2300 feet provided that the actualmeasured computers are aggressively advertised and depth, not depth gauge depth is used, to marketed for prestige and profit, rather than dcterminc dive schedules and that if simply the safety of the diver. I am certain that decotnpressionis tcquircd, a decompressionline no one is advocating a table or device that is

4-32 Decompression knowingly unsafe. However, the consumer is dcs Grange, M., Standard Ai r simply overwhelmed at this point in time. Decompression Tables, NEDU Research Fortunately, responsible individuals under the Report No. 5-57, Project NS 185-005,Test leadership of the , Duke 3 956!. University Medical School, and the Undersea and Hyperbaric Medical Society are attempting des Grange, M. Repetitive Diving to sort out this entire issue. Decompression Tables, NEDU Research Report No. 6-57, Project 105405, Subtask Although this is only a superficial 5, Test 3 957!. ovcrvicw, you now know morc aboutdive taMcs thanmost instructors. Most entry level divers are Edel, P, Carroll, J., Honaker, R., and sheltered from the controversiesthat rage Beckman, E, Interval at Sea Level Pressure throughoutour diving community. In many Required to Prevent Decompression classesdive tables are marketed,not taught! Sickness in Humans Who Fly in Instructors and organizationstend to overlook Commercial Aircraft after Diving, the fact that it is your body and that you have a Aerospace Medicine 400!: 1105-1110 right to know something, actually a lot, about 969!. what you can expectto happenwhen you go underpressure. Huggins, K., New No-Decompression TablesBased on No-DecompressionLimits Hopefully, thc issues of decompression Determinedby Doppler Ultrasonic Bubble will soon be sorted out. Some of the information Detection,Report MICHU-SG-81-205 Ann presented above may soon be obsolete. Arbor. Michigan ScaGrant Program,1981!. However,this is the picture as I see it now. AH divers are cautioned to remain abreast of the 10. Lanphicr,E., Medical Aspectsof Diving latest developments through continuing Underwater Physiology pp. 51-99 in education courses and seminars, attending Empleton,B. ed!, The New Scienceof Skin lectures,and following diving literature. and Scuba Diving Chicago: Follett PublishingCompany, 1974!. REFERENCES 11. Miller, J. ed!, NOAA Diving Manual Washington, DC: U.S. Government I. Anonymous, The Recreational Dive Printing Office, 1979!. Planner: Two Versions Available, The UnderseaJournal, First Quarter988!. 12. NavalSafety Center, Navy Diving Statistics and Accident Analyses for the Period 1 2. Anonymous ed!, Recreational Dive January1977 through 30 Junc 1981981!. Planning: The Next Generation Santa Ana, CA: Diving Scienceand Technology 13. Neuman, T., United States Navy Dive Corporation, 1987!. Tablesand No-Stop Diving, pp. 169-172in Lang, M., and Hamilton, R. eds! 3. Bassett,B., Theoryof Air Decompression Proceedings of Dive Computer Workshop for ScubaInstructors, pp. 50-57 in Fead,L. Los Angeles, CA: University of Southern ed!, Proceedingof the Eighth International California Sea Grant College Program, Conference on Underwater Education 1989!. Colton, CA: National Association of Underwater Instructors, 1976!. 14. Pilmanis, A, Comments at the American Academy of Underwater Science Dive 4. Berghagc,T., Rohtbaugh,P., Bachrach,A., ComputerWorkshop 6 September1988!. and Armstrong, F., Navy Diving Summary Statistics,pp. 285-303in Proceedingsof rhc 15. Powcll, M., Scientist Test Ncw Dive Tables 1976 Working Diver Symposium for Recreational Diving, The Undersea Washington, DC: Marine Technology Journal, Fourth Quarter 987!. Society,1976!. 16. Richardson, D., The Recreational Dive 5. Buhlmann, A., Decompression Planner: History and Development, The Decompression Sickness New York: UnderseaJournal, First Quarter988!. Springer-Verlang,1984!.

4-33 University of Michigan Diviag Manual

17. Richardson, D., Questions and Answers on 26. U.S. Navy, U.S. Navy Diving Manual, 'Ihe Recreational Dive Planner, DSAT, and Volume 1: Air Diving, NAVSEA 0994- the Table Research, The Undersea Journal, LP-001-9010 Washington, DC: U,S, Mird Quarter 988!. Government Printing Office, 1985!.

Richardson, D., Multiple Dives over 28. Vann, R., Dovenbarger,J., Bond, J., Bond, Multiple Days: An Area of Growing Interest B., Rust, J., Wachholz, C., Moon, R., and Concern, The Undersea Journal, Third Camporesi, E., and Bennett, P., DAN' s Quarter 988!. Results and Perspectiveof Dive Computer Use, pp. 133-143 in Lang, M., and 19. Richardson, D., ed!, PADI Open Water Hamilton, R. eds! Proceedings of Dive Diving Manual Santa Ana, CA: Computer Workshop Los Angeles, CA: Professional; Association of Diving University of SouthernCalifornia SeaGrant Instructors 988!. CollegeProgram, 1989!.

20. Rogers, R., Renovating Haldane, The 29. Wachholz, C., How Safe is Rectcational UnderseaJournal, Third Quarter988!. Diving?: An Estimate of the Incidence of Nonfatal ScubaDiving Injuries, Aler Diver Sheffield, P., UHMS Workshop 2!: 1-2 985!. RecommendsGuidelines for Flying after Diving, Pressure18! 989!. 30. Wachholz, C. What is the Incidence of Nonfatal Diving Injuries? Alert Diver 4!: Spencer, M., Decompression Limits for 1 988!. Compressed Air Determined by Untrasonically Detected Blood Bubbles, J. Applied Physiology40!:229-235 976!. ACKNOWLEDGMENTS

23. Thalman, E., and Butler, F., A Procedure I wish to expressmy deepestappreciation to Karl for Doing Multiple Level Dives on Air Huggins for his inspiration, many helpful Using RepetitiveGroups, Report No. 13-83 suggestions, and critical review of this PanamaCity, FL: Navy Experimenta manuscripL Diving Unit, 1983!.

24. U.S. Navy, U.S. Navy Diving Manual Washington, DC: U.S. Government Manuscriptcompleted: 30 December1989 Printing Office, 1958!. Manuscriptrevised: 17 April 1990 Manuscript revised: 18 June 1990 U.S. Navy, U.S. Navy Diving Manual Washington, DC: U.S. Government Printing Office, 1973!. Decoepression

INTRODUCTION TO THE RECREATIONAL DIVE PLANNER TABLE VERSION!

INTRODUCTION thtoughout this 2 hour period, you can receive credit for this time. Table 2 Surface Interval Terminology and basic proceduresfor Credit Table is used to determine the amount using thc Recreational Dive Planner werc of credit. LocatePressure Group "N" from Dive discussedthroughout this Chapter4-2. The ¹1 on the diagonal linc of letters on the left side following is a step-by-stepproyession through of Table2. Enterthe "N" row horizontallyto the thc various tables of thc Recreational Dive box containing thc numbers I:31/2:18. Your Planner. surfaceinterval is betweenone hour, thirtywne minutesand two hours, 18 minutes. Now, follow that column to the bottom of the table to TABLE I determineyour ncw Prcssuie Group "B". Recreational Dive Planner Table 1 is used to determinemaximum no-decompression and TABLE 3 repetitivedive group. If you are planningonly one dive in a six hour period,this is thc only Table 3 Repetitive Dive Time Table- tablethat you will needlo use. Thebest way to providcsyou with residualnitrogen time and explaina divetable is througha example. maximum no decompressiontime allowed for your repetitive dive time. Find the letter "B" Lct's assumethat you intend to dive to a alongthe top of Table3. Now, lct usassume that maximumdepth of 55 feet. Depthis given in you plan to dive to a depthof 47 feet on Dive ¹2. 10-footincrements aficr the 35-footcolumn! at &liow thc: -B" column down to the 50-foot the top of table one. Since55 feet is between50 depthrow of figure depthsate on thc left side and 60 feet, you must use the higher depth of Table 3!. You will find two numbers. The cotumnforplanning your dive. uppernumber is thc residualnitrogen time in minutes! remaining from Dive ¹1. The bottom The maximumno-decompression time for number is the maximum allowable actual no- that dive depthis displayedin the box at the dccompression dive time for Dive ¹2 in this bottom of thc:column. In this casethe maximum case67 minutes. This figurc 67 minutes is no-decompressiondive time:for 55 feet 0 foot the maximumnoMecomprcssion dive time for 50 column! is 55 minutes. feetless the residual nitrogen time or 80 13 = Uponcompletion of yourdive Dive ¹1! to 67!. 55 feet your timer reads35 minutesbottom time. Let us assumethat your plannedbottom If you enter the 60 foot column to 35 minutes. time for Dive ¹2 is only 60 minutes. In order to you can determineyour PressureGroup at thc determineyour PrcssureGroup at the end of end of that dive by readingthe letterat thc left Dive ¹2 you mustadd the actual bottomtime and sideof Table1. In this caseyou arein Pressure the residual nitrogen time. In this case 60 GroupN. Thisinformation is only requiredif minutes plus 13 minutes RNT! equals 73 you intendto dive againwithin 6 houis. minutes. This is thc Total Bottom Time or Equivalent Single Dive Time ESDT!. TABLE 2 Returningto Table1 youfind thatyou arenow in PiessurcGroup "W" since 73 is not on the table Two hours after completingyour initial you go to thc next higher number!. You only dive of thc day your plan to make anotherdive. needto determinethis figurcif you planto make Sinceyour body has beenoff-gassing nitrogen another dive within 6 hours.

4-35 Universityof MichiganDiving Manual

PLANNING A THIRD DIVE Now find your new PressureGroup at the bottom of this column "I". Let us assumethat you are planning to make a third dive and you wish to return to the Dive ¹3 is planned for a maximum depth water as soon as possible. Keep in mind that of 40 feet. Entering the ''I" column at the top of there are "Special Rules for Multiple Dives" Table 3, you proceed to the 40-foot row of statedbelow Table 3. If you are planning 3 or numbersand find that yow tnaximum allowable more dives in a day, beginning with your firs actual bottom time ABT! is 100 minutes dive if your pressuregroup after any dive is "W" bottom figure!. or "X', the minimum surface interval between all subsequentdives is 1 hour. If your ending Most authorities recommend that you pressuregroup after any dive is "Y" or "Z", the make no more than 3 dives per day. Sine you minimumsurface interval between all subsequent have made a seriesof repetitive dives you must dives is 3 hours. In this caseyou were in wait at least 12 hours before flying in a PressureGroup "W' and according to the rule commercialair craft. Ideally, observea 24 hour your minimum surfaceinterval must be one hour. surfaceinterval beforeflying.

To plan Dive ¹3, enterTable 2 on row "W" and move horizontally to the box 0:57/I:02.

DIVE PROFILE Decompression

APPENDIX B

INTRODUCTION TO THE NAUI DIVE TABLES

The National Association of Underwater the bottomnumber is the requireddecompression Instructors has modified and rearranged the US time. Eor example, if your actual dive time Navy dive tables and aow market than under the ADT! was 62 minutes you would have to make NAUI trademark. Unfortunately, this a mandatorydecompression stop at 15 feet for 7 anangementand the various abbreviationsused allautcs. differ from those use with the Recreational Dive Planner PADI!, the JeppesenAtraagemcnt of thc Upon completionof your dive Dive ¹1! to US Navy Tables SSI!, aad the traditional US 55 feet your timer tcads 35 minutes actual dive Navy Tables. time exclusive of thc precautionary decompressiontime; in reality automatic timers would continue to fuactioa throughout the 3 TABLE 1: END OF DIVE LETIXR GROUP minute stop at 15 feet!. If you cater the 60 foot row to 35 minutes in this case 35 minutes is NAUI Table 1 is used to dctcrminc betwccn 30 and 40; you must used the higher maximum no-decompression dive time and valve!, you can determine your Letter Group repetitive group. In addition, the number of designationat thc cnd of thc dive by following minutes of decompression required in the event the columa in which 40 appears down and that you exceedthe no4ecomprcssiondive time reading the letter at the bottom of the table. Ia is also included. The best way to explain a dive this case your Letter Group is "G". This table is through an cxamplc. information is requited only if you intend to dive again within 24 hours. Lct's assumethat you intend to dive to a maximumdepth of 55 feet. Depth is given in 10 Please take note that NAUI defines foot and 3 meter increments at the left side of thc a repetitive dive as any dive within table. Since 55 feet is between 50 and 60 feet, 24 hours of a previous dive. Thc you must use the higher depthfor planning your Recreational Dive Planner is based dive. on a period of 6 hours and the US Navy tables usc 12 hours. The maximum no-decompression time maximum dive time or MDT! for that depth is displayedas a circled number. In this casethe TABLE 2: SURFACE INTERVAL TIME maximum no-decompression dive time for 55 SIT! TABLE feet 0 foot row! is 55 minutes. Two hours after completing your initial The value used for dive arne or dive of the:day you plan to make anotherdive. bottom time is definedby NAUI as Since your body has been off-gassing nitrogen the ACTUAL DIVE TIME the throughout this 2 hour period, you caa receive total time spent underwater during credit for this time. Table 2 Surface Interval a dive except for precautionary Time Table is used to determine the amount decompression stop tune. of credit. Locate Letter Group "G" from Dive ¹1 at thc top of Table 2 or bottom of Table 1!. The values to the right of this circled Enter column "G" downward to the box number 60 over 5 and 80 over 7 represent containing the numbers2:58/2:00. Your surface the amount of marufatory decompression that interval is 2 houts. Now move horizontally to would bc requited at a depth of 15 feet in the the left to determine your new Letter Group- event that you exceededthe no-decompression eeQOS time limit. The top number is the dive time and University of Michigan Diving Manual

TABLE 3: REPETITIVE DIVE TIME your Adjusted Maximum Dive Time AMDT! is TABLE 57 minutes bottom figure!.

Table 3 Repetitive Dive Time Table Most authorities recommend that you providesyou with residualnitrogen time and the make no more than 3 dives per day. Since you maximum noMecomptessiontime allowed for havemade a seriesof no-required-stoprepetitive your repetitive dive. Find the letter "D" in the divesNAUI recommendsthat you wait at least24 column at thc tight side of Table 3. Now, let us hours beforejoying in a commercialai r craft. assumethat you plan to dive to a depthof 47 feet on Dive ¹2. Follow the "D" row to the left to the 50 foot column depthsare indicatedat the top of COMPUTING MINIMUM SURFACE Table 3!. You will find two numbers in a box. INTERVAL The uppernumber light face type! is the residual nittogen time in minutes! remaining from Dive Many diver wish to complete two ¹1. The bottom number bold face type! is the succcssivcdives with know depth and dive time Adjust Maximum Dive Time AMDT! or requircmcnts and remain within thc no- maximum allowable no-decompressiontime for decompression limits for both dives. Lct's Dive «2 in this case51 minutes. This figure assume that wish lo make Dive ¹1 to 60 feet for 51 minutes is the maximum no- 50 minutes and Dive ¹2 to 50 feet for 60 decomptcssion dive time for 50 feet less the minutes. You must determine the minimum residualnitrogen time or 80 29 = 51!. surface interval bctwcen the two dives.

Let us assume that your planned actual Upon completion of Dive ¹1 you are a dive time for Dive ¹2 is only 40 minutes. In Letter Group "H" diver Table 1!. ln order to orderto detetmineyour LetterGroup at the end determineyour LetterGtoup at thc beginningof of Dive ¹2 you must add the actual dive time: Dive ¹2 you must enter the 50 foot column of ADT! and the residual nitrogen time RNT! to Table 3 determinethe total nitrogen time TNT! of Dive to thc box which indicate an adjustedmaximum ¹2. In this case40 minutes ADT! plus 29 dive time bottom number, bold type! of 60 minute:s RNT! equals 69 minutes. This is the minutesor greater. Readingto the right you find TotalNitrogen Time TNT!. Returningto Table that you are in Letter Group "B". Now continue 1 youfind thatyou arenow in LetterGroup "I". to the right into Table 2 until you intersect the You only need this figurc if you plan to make "H" column. The minimum value in this box is another dive within 24 hours. 490 or 4 hours 50 minutes.

PLANNINC A THIRD DIVE SPECIAL RULES AND RECOMMENDATIONS Lct us assumethat you are planning a third dive and wish to return to the water as soon as Rate of Ascent. The maximum rate of ascent is possible. Althoughthe tablesprovide for surface 60 feet per minute. intervals as short as 10 minutes, the policy of NAUf is that divers observea minimum of one Dives Shallower Than 40 Feet. Dives shallower hour surfaceinterval betweendives. than 40 feet are to bc: considered as 40 foot dives for table usc. You were a Lcttcr Group "I" diver upon completionof Dive ¹2. To plan Dive ¹ 3 cater Dive Depth. Dive depth is the maximum depth Table 2 in column 'T' and move down to the box attainedduring thc dive. Use the exact or next 1:29/1M. Now you can find your new Letter greatervalue. For example,thc 50 foot schedule Gtoup by readingthe letter to thc left of Table 2 would be used for a dive to 42 feet. "G". Dive Time. Dive time is the total time spent Dive ¹3 is planned for a maximum depth underwater exclusive of the precautionary of 40 feet. Entering the "G" row into Table 3, decompressionstop time. Use the exact or next you proceed to the 40-foot column and find that greatervalue. For example,for a single dive of

4-38 Decompression

31 minutes at 60 feet you would usc the 40 PRACTICE DIVE COMPUTATIONS minute column to determineyour Letter Gtoup. USING THE NAUI DIVE TABLES Precautionary decompression stop time is considered to be neutral time. Maximum Actual Dive Time no- decompression! for a dive to 66 feet. PrecautionaryDecompression. A precautionary decompressionstop of 3 minutes at 15 feet is You havebeen diving to a maximumdepth recommendedduring ascentfrom eachdive. of 72 feet and discover that your ADT is 45 minutes. What, if any, special Mandatory Decompression Stop Dives, Dive procedurewould you take in ascendingto requiring a mandatory stop ate discouraged. the surface'7 Decomptcssiontimes tuc includedon Table 1 for use in the event that the recommended Maximum Dive ¹1: 72 feet/30 minutes Dive Time no-decompression time! is Dive ¹2: 58 feet accidentallyexceeded. Determine the Adjusted Maximum Dive Omiled Decompression.In the eventthat you Time AMDT! for Dive ¹2 following a cxcccd thc Maximum Dive Time and omit the Surface Interval Time SIT! of 1 hour 30 decompressionstop you should refrain from minutes. physical activity test!, drink plenty of fluids, breath 100% , and watch for signs or Dive ¹1: 55 feet/40 minutes symptoms of decompression sickness. If Dive ¹2: 35 feet symptoms are evident immediately acquire medicalattention and hyperbaric treatment. If no Determine the Adjusted Maximum Dive symptomsare evident,do not dive for at least24 Time AMDT! for Dive ¹2 following a houts. Surface Interval Time SIT! of 1 hour 30

Flying After Diving. Wait at least 24 hours following decompression diving. Wait at Dive ¹1: 95 feet/20 minutes least 48 hoLus following a dive that required Dtve ¹2: 50 feet/40 minutes dccomprcssion or following an omitted dccomptcssion. Determine the minimum surface interval time SIT! require in order to complete Cold or StrenuousDives: Usethc nextgreater Dive ¹2 without making a mandatory bottom time for cold or strenuous dives. Bar decompressionstop. example,if you wished to remain within the no- decomprcssiondive time limit for a depth of 60 Dive ¹1: 80 feet/35 minutes fcct and you are cold, limit the dive to 50 Surface Interval: 1 hour minutes or less. In order to detetmine the Letter Dive ¹2: 50 feet for Adjusted Maximum Group at thc cnd of the dive, usc thc next gtcater Dive Time no-decompression! bottom time for a 50 minute dive use 55 Surface Interval: 1 hour minutesor Letter Group -I". Dive ¹3: 40 feet

Diving at Altitude: NAUI recommendsthat the Determine the Adjusted Maximum Dive use these tables bc limited to altitudes of less Time no-decompression! allowable for than 1000feet abovesca level. Specialtables are Dive ¹3. requitedfor diving at higher elevations. ANSWERS: l. 45 min; 2. mandatorystop at 15feetfor10nun; 3.25min; 4.81 min; 5.46 min; 6. 43 min

4-39 University of Michigan Diving Manual

APPENDIX C

COMPUTING MINIMUM SURFACE INTERVAL

The following instructions will address the NAUI!. Using the RDP follow the 50 foot tow general procedure for determining minimum horizontally from left to right until you find the surfaceinterval using both the RecteationalDive first adjustedno-decompression limit that is 60 Planner RDP! and US Navy Dive Tables: NAUI minutes or greater gower number in box! and Artangement NAUI!. You will discover that the follow that column up vertically to find the values will vary considerably depending on the pressure group and enter it on your dive profile. Dive Table that you use. Answer: RDP Group "D" PROBLEM Using the NAU1 Table follow thc 50 foot column Dive I: 60 feet/50 minutes down until you find the first adjusted no- Dive 2: 50 feet/60 minutes decomptcssiondive time that is 60 minutes or gicater gower number in box! and follow that Compute the minimum surface interval that role to the right margin to find the tepetitive enables you to complete Dive 2 within thc "no- group and enter it on your dive profile. decompression limit" for 50 feet. Answer: NAUI Group "B"

Step1: Draw a dive profile and enter all known Step 4: On the Surface Interval Credit Table infoanation depth and time of both dives!. RDP Table 2! find pressuregroup "U" on the diagonal and pressuregmup "D" at the bottom Step2: Using the No-DecomptcssionLimits and and find the surfaceinterval where they intersect. Group DesignationTable RDP Table 1! find the At this intctsection you find I:26- 194. pressure group repetitive group! at the completion of Dive 1 and enter letter on your Answer: RDP "1 hour 26 minutes" dive profile. Using thc NAUI Table follow tepetitive group Answer: RDP = "U" NAUl = "H" row "B" Table 3! to the right to Table 2 Surface Interval Time Table! and tcpctitivc group "H" Step 3: Pity~ to the Repetitive Dive Time bottom of Table I! down to thc box where they Table RDP Table 3! or Repetitive Dive intersect. At this intersection you find 7:59 to Timetable NAUI Table 3! and find 50 feet at the 4:50. left side of the table RPD! or top of thc table Answer: NAUI = "4 hours 50 nunutes" De~mpression

CHAPTER 43

USES AND ABUSES OF DIVE COMPUTERS

Karl E. Huggins

INTRODUCTION Pressure Transducer, which converts the ambient pressuresurrounding the diver to a Since the intmductionof dive computers signal which is fed into the input of the A/D DCs! divershave been developingimaginative Converter. methodsof use,not all of themvery smart.'Ms article will describe what a DC is, some of the A/D Analog to Digital! Converter, which misconceptionsthe general diving population changes the pressure transducer signal to a have regardingthem, and how they are being digital "word" which can be "read" by the used. Examples of how dive are being used are nilclopfo cessor. presentedalong with recommendationsadopted by the American Academy of Underwater Microprocessor, the "brain" which controls the Sciences AAUS! for DC usein scientificdiving signal Bow and performs the mathematicaland programs. logical operations.

Of major concern is the accuracyof the ROM Read Only Memory!, a non-volatile information passed on from instructors to memorywhich containsthe program stepswhich students,salespersons to customers,and divers to "tell" the micropmcessorwhat to do. The ROM other divers with regardsto DCs. Too often in also containsthe constantsused in the program the diving community we seeheatsay, biased, or which determines the diver's decompression inaccurateinformation presented on piecesof status. equipmentas insteadof factual information. DCs look as if theywill be aroundfor a while.They RAM Random AccessMemory!, containsthe shouldnot be condemnedoutright, nor should storage registers in which variable data and they be worshiped as a panacea for tesultsare stored during computations. decompressionsickness DCS!. They ate only tools, and as such are no better or wot3e than the Display, which presents the diver' s personusing them. decompressionstatus. Clock, whichsynchmnizes the operationalsteps %HAT IS A DIVE COMPUTER? of the microprocessorand is used as the time inpuL A DC is just that,a computer.It doesnot, as some people think, monitor the amount of PowerSupply, which runs the device. nittogenin a diversbody. All it doesis compute decompressionstatus. This is done by sensing DeviceHousing, which protects the components depthand time during the dive andthen, by using from the environment. a tableor model,the decompression status is read or computed. This decompression status Many people believe DCs just read informationis disp1ayedto the diver, who can establisheddive tables.'niis is not true. Only one use it as an additional source of information in of the DCs available is table based the execution of a dive. SME-USN!. All the rest use a decompression model algorithm!to computethe decompression The basicdesign of a DC is presentedas a status. block diagram in Figure 1. The general componentsare the: The algorithms used in DCs are mathematicaland logical formulaswith variables

441 University of-lvtlchiganDiving Manual of depthand time, which makesthem much more However,many of the advantagesproduce flexible than tables. A pure mathematicalmodel the disadvantagesof DCs. If the device is pushed affords an infinite number of depth/time to its limit, the model is pushedto its limit. There .Dive tablesare finite listing of someof ate no safety factors programmedinto the units the solutions produced from a mathematical except for the models which are more modeL conservativethan the U.S. Navy model. In using tablesthe maximum depth entire dive rule adds The implementation of a decompression a safetyfactor if the diver is at shallowerdepths model in a DC is not "putc." As with tables thc during most of the dive. Another safety factor DC deals with depth and time increments,on a inherent in table use is the rounding up to a depth much smaller scale, based on thc update interval or time value greaterthan the actual depth and of the computer how often it recalculatesthe time of the dive. By using 60 fsw when the divers decompressionstatus! and the resolution tnaximum depth is 51 fsw additional safety is of thc prcssuretransducer circuitry the smallest added. changein depth it can detect!. The U.S. Navy no-decompressiontable has only 135 depth/time Other disadvantagesptcsent themselves. A combinationsfor depthsbetween 0 and 140 feet diver nccds to read the dcvicc, understand the of seawater fsw!. A modelbased computer that information that is being presented,and act upon updates its status every 3 seconds and has a that information. There is also thc possibility that depth resolution of 0.5 fsw can distinguish 400 the DC will becomea crutch.Some divers might possible"square-wave" depth/time combinations use it as an excuse to not teach, learn, or use in a one minute period over a 10 fsw depth tables just like BC's are being used to range. circumvent the teaching of proper weighting!. The major disadvantage,shard by tables and Tables also base decompressionstatus on DCs alike, is the fact that all the DC or table the assumptionthat the entire dive was spent at knows about is depth and time. the maximum depth. Most tecreational divers spend only a small fraction of their dive time at the deepestdepth achievedduring the dive. This DECOMPRESSION MODELS VS REALITY meansthat during most of dive the diver is taking on less nitrogen than assumedby the tables. Dccomprcssion models do not actually Model based DCs that update thc divers status represent what is happening in thc body. All the every few seconds will compensate for thc models do is attempt to produce depth/time changesin depth. This allows thc diver to bc combinations that are safe for most divers most presentedwith a decompressionstatus based on of the time. Nearly all decompressionmodels to the actual dive that was performed. The date use these two variables, depth and time. advantagesof computingdecompression status in Theseused to computelhe decompressionstatus this manner includes: displayed to the diver. Many other factors can changethe divers susceptibility to DCS. These A. Profile Integration no maximum include ascent rate, physical exertion, water depth entire dive assumption!. temperattue,physical condition, hydration level, blood alcohol, age, gender, breathing mixture, B. Shallow portions of dive safety ctc. stops! are taken into account. If two divers pedorm thc samedepth/time C. Actual Depth used in Calculation dive profil, one being low exertion by a young, 1 fsw not 60 fsw!. healthy diver in a warm Caribbean enviionment and the other, performed in cold water by an D. All compartmentsof the model ate older, out of shape, hung over diver, who was taken into account when working heavily then the same decompression calculating multi-level dive profiles statuswill be computed by a DC if the same DC most table basedtechniques utilize model is used!. All the DC "knows" is depth and thc compartmentrepresenting their time. A DC also has no memory of how it has repetitivegroups!. icached its present decompression status. It does not keep track of the dive profiles that have been Decompression previously perfotmcd to modify decamptessian Using outsideoperating range: About half of status calculations on subsequentdives. All it the DCs on the market are designedto be usedat "knows" is the gas loading the model has sea level ar thc fust few thousand feet of altitude. calculated up ta that point. The DCs based an the Swiss mactel generally adjust tio altitude. However, some divers use the A MATHEMATlCAL EQUATION DOES sca level DCs "as is" at altitudes outside the NOT A BODY MAKE! Divers must be aware model's operatingrealm. Anotherproblem in this that they need to add safety factors based on their category is diving to depths that exceed the own physiological state, thc diving cnvirtmnent, maximum depth range of the DC. Why have a and their previousdive profiles,just asthey have DC if it is being used on dives where it can't beentaught to do whenusing tables. calculate properly or is placed in ERROR mode?

Abusingsafety features: Some of the DCs have GENERAL MISUSES OF safety featutes that allow a diver to get out of Drm COMPUTERS situations outside the model or clcctrical limitations of thc DC. Casein point, the EDGE Thereseems to be samegeneral techniques DC bas a maximum depth resolution of that have been developed by "c/ever" divers to approximately 165 fsw. At that point the depth squeezeevery second they can out of a DC. Ice tegister is storing the largest number it can, If the reasoning behind these abuses ranges from dive:r were to dcsccnd further the DC would nat stupidity to blissful ignorance. Some af these be able tecognize the fact that thc diver was at a misuses follow: deeper depth. However, a safety feature was added that assumes that the diver is at Regularly pushing unit to limits: There are appraxitnately 200 fsw any time the maximum many divers wha run their DCs down to zero depth has been cxcecded. Bus assumption will decompression time, ascend to a shallower only be "safe" which is a questionableterm for depth, and then run the time back down ta zen. any dive to that depth! for a minute or so. After This pushesthe decompressionmodel in the unit that time the compartmentpressure registers will to its reachthe maximumvalue they can storeand they limit. will not bc able to distinguish any increasein Ignoringascent rate warnings:Most of thc compartment pmsure, making the calculations and decompression information cnoneaus. 'Ilare DCs use ascent rates that are slower than the 60 ate divers wha pervertthis safety featureto tnakc fsw/min. U.S. Navy standard. Using a faster dives to 200 fsw with the EDGE and some who ascent rate than suggested may place thctn use it to depths deeperthan 200 fsw. Why? In autside thc limits of the model. Following the most cases another diver has told them they slower ascent rates has the added benefit of could do it. Most of the time the divers don' t requiring thc diver to have good really understand how the DC works, even control. though somethink they do and theywill believe Not reading or ignoring the information: what theywant to believe. Somedivers will just ignote thc information Hangingthe Dive Computer: One of the most provided by the computerif they da not like tbe ludicrous techniquesobscrvcd. Some divers who infotmatian that is displayed. violate the DC's ascent rate or have suxfaced while tbc DC still indicate:s rc:quired %@ruingoff snit to dear residual nitrogen: Somedivets who do not like the repetitivedive decompression will tic a rope to the DC and hang infortnationbeing shawm by tbeDC will actually it over thc side of boat to clear the warningsand turn it aff to clear the residualnittagm ftam tbe ptevcnt thc DC fram gaing into ERROR mode. What can one say? The computer understands computer'srcgistc33 and give themmate timecut thc repetitivedive. Clearin the residualnitrogen depth and time. It has no idea if it is attached memory fttm the DC does not clear it from the diver or a rop. diver's body! Use with gas mixturesother than air: At this time a!1 the DC models are based on the use of University of Michigan Diving Manual air as the breathinggas. Use of enriched limits was55 minuteson a singledive average air} mixtures with less nitrogen content than air of 23.8 minutes!. will theoretically add conservatism to the decompression calculations. However, the The profi!e data indicated that 48.5% of camputers do not know anything about the the dive time was spent at depths which were increased potential of while 75-100% of the maximum depth of the dive. using these mixtures. Other gas mixtures can 26.2% of the dive time was spentin the 50-75% actually increase the risk of DCS if of maximum depth range. l6.3% of the time was decompressionstatus is determinedwith an air in the 25-50% rangeand anly 9.0% was spentin based DC. the shallawestquarter of the dive. 'Hus indicates that, for this youp of divers, the DCs were not Blind trust in numbers:Many divers think that being used to make a short excursionto a deep becausethe DC is telling themsomething that it depth followed by the remainder of the dive in is "TRUE." Thc numbersproduced by the DC shallowerwater. Also, thc profile did not follow are only a guide to a divers decompression the dccp-to-shallowrule as can bc scenin Figure status,based on a decompressionmodel using 2. depth and time variables. We diver must be aware of the other factors that may infiucnce Thc results of thc Doppler monitoring susceptibilityto DCS and add their own safety indicatedone defiiute and three possiblecases of factors. As pointed out at the AAUS Dive Grade I bubbles. No symptoms of DCS were Computerworkshop last year,' Key are like a observed in the divers. small television,and peoplebelieve what they sec on television." Mike Emmcrman reported dive profiles used on the Andrea Doris !. What hc obscrvcd Turn thinking over to a machine:Some divers was frightening. Divers would do a 210 fsw dive, do not want to worry, or think about their wait 4 to 6 hours, and then do thc same dive aver decompicssionstatus so they lct a little box made again, Some did two dives a day and othersdid out of silican, metal, and plastic take over their three! This was done three days in a raw. Over thinking requirements.As statedbefore, thc DC 50% of thc divers used DCs. Some used DCs that informationis only a guide, not the gospel! had maximum depth rangesthat werc shallower Divers need to be able to think for themselves than the depths of the dives. Of thc 16 divers on andunderstand the risks they aretaking. the trip six of them presenteddefinite signs and symptomsof DCS! These six divers did express some concern for their condition, but none of HOW DIVE COMPUTERS them sought treatment! ARE BEING USED Dr. Tom Neuman at the University of In 1987 a group ten sport divers were California San Diego related a case of DCS monitoredduring a 14 day dive trip. All the where a diver and his buddy had been diving divers,except ane, used a DC. Following76 of with DCs !. The dive history obtainedfrom the the dives the divers were monitored using diver was first a dive to 254 fsw followed by a Doppler ultrasanic bubble detector ta check for dive to 160 fsw 3-l/2 hours later. Dr. Ncuman "silent bubbles." On 65 dives the actual dive found this hard to believe until it was discovered profile was recorded maximum depth every 3 that the computer thc diver wore also recorded minutes!. the dive profile. The profil was recalled and the dives werc confirmed, except for showing a Whencompared to the U.S. Navy tables maximumdepth on the first dive of 230 fsw. This 52 of the divesindicated omitted decompression. wasduc to the factthat thc maximumdepth limit Thc maximum omitted decompressionfor a for thc computer was 230 fsw. However, when singledive was 71 minutes.Thc averagewas thc infoimation was retrieved from the 23.0 minutes.For an entire day the maximum buddy'sDC one that has a depthlimit of 300 omitteddecampressian was 145 minutes and the fsw! a maximumdepth of 254 fsw was obtained. average was 46.2 minutes 'Ac maximum time extendedpast the U.S. Navy No-Decompression Another caseinvolves a 26 year old male diver on vacation in the Caribbeanusing a DC Decompression

On the day 1heproblem developedthe first dive dive plan. The goal of such an assessmentis to was a multi-level dive to 140 fsw for a total dive maximizethe bcncfit while mizdrnizingthe risk. time of 56 minutes. Four hours later a second dive was performed to 160 fsw for 47 minutes, The operation and limitations of the DCs The diver noticed an onset of fatigue 2 hours being used need to be understood. 'Ihe more the following the second dive, however he decided diver understandsabout the equipment being to perform a third dive following a 3 hour ural, thc morc educated and safe the decisions surfaceinterval. This third dive was a night dive will become. to 47 fsw for 67 minutes. That night he had a restlesssleep, cold sweats,and minor pain in thc Dive computers should not be pushed to elbow. In the morning the fatigue and pain their limits. Divers should add safety factors just mmained so he proceededlo perform another they are added with table usc. Remember, all a dive. Thc dive was to 65 fsw for 40 minutes and DC knows about is depth and time. DCs am not during the dive felt pain relief. It was at this point anti-DCS talismans. They will not ward off that hc concludedthat he probably was bent. Thc bubble formation or suck thc nitrogen from thc next morning the pain and fatigue remained body. Most of all, a diver needs to employ along with a headache.That day he ffew home to commonsense in all phasesof diving. thc statesand sought tmatmcnt, four days after the dive seriesthat producedthc problem. After being treatedwith a Table 6 tmatmenttable thc CONCLUSIONS diver had no apparentresidual problems. At all time thc diver was within thc "no- It should be mmembered tha1 thc advent of decompression" realm of his DC, but was mliablc DCs should not give the "train em fast pushingthose limits. and easy" people in diving an excuse not to teach tables and underlying decompressionconcepts, The final case involved a 53 year old nor should they provide lazy divers an excuse not experience fcmalc diver in excellent physical to learn and practice table use. I have talked to condition.Shc performedthmc dives to depthsof instnucors who would have no qualms about 70 80 fsw using a DC. At no time was there having their studentsjust strapon a little box that less than5 minutesof no- decompressiontime tells them their decompressionstatus instead of mmaining on the DC. However, on the second teachingthe usc of dive tables.Them am places dive the sleeve to her ripped exposing whem Basic studentsam showing up to the first hcr ann to very cold water. R>llowing the dive pool sessionswearing DCs. What incentive do she had pain in her arm, but attributed it to the they have for learning the COnccptsand use of exposure to cold water. The suit was fixe and dive tables? Thc introduction of DCs means that she performedthe third dive. Later that evening, along with table instruction them is now the need thc pain in her arm became intolerable and she to teach students how to use and understand soughttreatment. Besides the cold water another computers. extenuatingfactor was that she had not had any hydration during the day when she finally No dive table or computer 100%effective! passed urine it was dark brown!, There was no Divers nccd to understand how and whem the way for the DC to know that thc dry suit had numbers are coming from, be it with tables or rippedor that the diver was dehydrated. cornputcrs.They aced to malize that all these dcviccs understand am depth and time applied to a mathematical model. Common sense and THE DIVER'S RKSPONSIBILlTY understandingneed to be part of the equation. Dive computersare good tools and as such can Divers must realize that they aced to take be used to enhancethc: diving experience,but msponsibility for their actions and safety.They they arc only tools, not dcmigods to be must acknowledgethc fact that every rime they worshiped and followed religiously. With hard dive there is risk involved. One of these: risks is work, training, and educationwc may be able to the possibility of dcvcloping DCS. A diver needs eliminate thc: event where a diver states, "I don' t to make a risk/benefit assessmentas part of the understand,my computertold me I could..." University of MichigartDiving Manual

AAUS RECOMMENDATIONS G. Ascent r3tcs shall not exceed 40 fsw/min. in the last 60 fsw. In October 1988 the AAUS held a workshopon the use of DCs in scientific diving H. Whenever practical, divers using a DC !. The recommendationsthat were agreedupon shouldmake a stopbetween 10 30 fsw for that apply to recreationaldivers are listed below: 5 min. especially for dives below 60 fsw.

A. Each diver relying on a DC to plan dives I. Repetitive and multi-level diving and indicate or determine decompression procedures should start the dive, or the status must have their own unit. series of dives, at the maximum planned depth, followed by subsequent dives of B. On any given dive,both divers in the buddy shallower exposures. pair must follow the most conservativeDC. J. Multiple deepdives shouldbe avoided. C. If the DC fails at any time during the dive, the dive must be terminatedand appropriate surfacing procedures should be initiated REFERENCES immediately. l. Emmerman, Michael E., personal D. A diver should not dive for 18 hours before communications. activating a DC to use it to control his/her diving 2. Neuman.Tom, personalcommunications.

E. Once a DC is in use, it must not be switched 3. Lang, Michael A. and Hamilton, R.W., Proceedingsof the American Academy of off until it indicates complete outgassing Underwater Sciences Dive Computer has occurred or 18 hours have elapsed, Workshop, University of Southern whichever comes first. California SeaGrant Program,1989. F. When using a DC,non-emergent ascents are to be at the rate s! specified for the make C! CopyrightKarl E. Huggins1989- Reprinted andmodel of DC beingused. wrth permission from author.