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Home , Booster pump, Dive profile, Trimix (breathing gas)

Field Comparison of Open-Circuit Scuba to Closed- PAPER Circuit for Deep Mixed-Gas Diving Operations

Frank A. Parrish ABSTRACT increased training requirements for safe opera- tion (Richardson et al., 1996; Barsky et al., 1998; National Marine Fisheries Service-Honolulu A comparison of open-circuit to Cole, 1998). closed-circuit ("") diving was con- Laboratory Scientific success using closed-circuit ducted while collecting fishery data on black Honolulu, Hawaii rebreathers for research at depths in excess of coral beds in Hawaii. Both methodologies used mixed gas from the same ship-based support 150 ft. has been demonstrated primarily by inde- Richard L. Pyle system. The comparison was based on a series pendent researchers working outside of institu- Ichthyology-Bishop of eight dives, four open-circuit andfour closed- tional diving programs, usually involving dives Museum circuit. These were used to make a direct-com- to depths in excess of 150 ft. (e.g., Pyle, 1998; Honolulu, Hawaii parison of the gear in a square , a 1999; 2000), but also recently within an institu- multilevel profile and two dives of varying pro- tional diving framework (Pence and Pyle, 2002). files. Four general criteria were considered: Most of this success has been in the form of time requirements for topside equipment prep- testimonials to the suitability of rebreathers for aration and maintenance, consumption of expendables, ejficiency, and adoption by institutions. Few rigorous field com- potential dive durations and bailout capabili- parisons of rebreathers to open-circuit gear as ties for each of the two technologies. The open- sampling tools have been made. Given the sub- circuit divers required 4 times as much topside stantial expense and required training associ- equipment preparation as the rebreather ated with closed-circuit rebreathers, their suc- divers, consumed 17 times as much gas, and cess should first be demonstrated under condi- cost 7 times more in expendables. The open-cir- tions and logistics consistent with institutional cuit divers incurred 42% more decompression diving. time for the square profile dives and 70% more This is a report of a field comparison decompression time for the multilevel profUe dives than the closed-circuit dive team. Most of a closed-circuit rebreather system to the of the decompression advantage for the closed- mixed gas or "technical" scuba diving tech- ci?'cuit team is from the benefit of real-time niques currently used by NOAA. This compari- decompression calculations, but some benefit son was a key recommendation from partici- comes from the gas optimization pants of a rebreather workshop held in Hawaii inherent to rebreathers. For a given mass of in April 2000. In attendance were potential users equipment, the rebreathers allow for as much of rebreathers including National Marine Fish- as 7. 7 times more bottom time, or emergency eries Service, National Ocean Survey, University bailout capability (depending on the chosen of Hawaii, and the NOAA Diving Center. Pro- depth of the dive), compared with the open- circuit system. posed applications included improving access to depths beyond conventional scuba and pro- viding divers with silent (bubble-free) operation INTRODUCTION to improve their behavioral observations. losed-circuit technology In August 2000 a team of rebreather Chas expanded significantly throughout the divers participated in a NMFS research cruise civilian diving community over the past 15 years. aboard the NOAA Ship Townsend Cromwell Several manufacturers produce units at retail eTC-OO-lO,leg 2) to assist with preliminary in- prices ranging from about $6,000 to nearly situ surveys of commercial black coral beds off $20,000, and the base of recreational rebreather Maui, Hawaii. The rebreather team functioned divers continues to grow. The most-often cited alongside the NOAA mixed-gas scuba divers advantages of closed-circuit rebreathers com- doing the same operations. Throughout the pared with open-circuit scuba include dramatic cruise, logistic variables associated with equip- reduction of gas consumption (due to increased ment setup, re-setup, and performance of the efficiency), improved decompres- dive teams were detailed. The divers (Richard sion efficiency (due to dynamic and optimal L. Pyle and John L. Earle on rebreathers and breathing gas mixtures), and silent operation Frank A. Parrish, Ray C. Boland, and Michael (due to absence of exhaled bubbles). The most- W. Sawyer on mixed-gas scuba) were skilled often cited disadvantages of closed-circuit with their , so the resulting data rebreathers include increased initial cost of excludes time spent learning how to use or equipment, increased operational costs, exces- prepare the diving gear. The comparison focused sive maintenance and pre-dive preparation, and exclusively on the costs, logistics, and effective-

MTSJournal. Vol.36, No.2· 13 ness of the equipment in meeting data collection Open-Circuit Equipment priorities. Issues of purchase price of the gear The open-circuit dive team, from here and training investment of the two types of div- on termed the "OC team," used conventional ing equipment were not rigorously compared methods, including double and are only briefly reviewed herein. Potential large-capacity, back-mounted cylinders con- advantages of quiet operation afforded by nected with a manifold, filled with -18/50 rebreathers with respect to behavioral observa- (18% , 50016helium, balance ), tions of marine life (e.g., Hanlon et al. 1982) with cylinders of -36 (36% oxygen, balance were not considered. nitrogen) and oxygen carried separately (each This comparison was nested within fitted with its own regulator), for use during programmatic data collection needs as part of decompression. The trimix supply was blended the management of the Precious Coral Fishery by first boosting -16 (16% oxygen, balance Management Plan of the Western Pacific ) into the double cylinders, and then top- Regional Fishery Management Council. The ping off with air. Nitrox-36 was made with the black coral fishery has operated off the Island of same strategy, first boosting oxygen then top- Maui for the last 50 years, and no surveys by ping off with air. General configurations of such NMFS have ever been conducted. Monitoring gear are described in Mount and Gilliam, 1992. efforts have relied on counting and measuring NOAA divers in Hawaii customized their config- the size of coral trees brought to the surface uration selecting high- cylinders (Fig. by harvesters. The diving survey provided fish- 1a) that were smaller and lighter to reduce the ery-independent data on the distribution and degree of diver encumbrance (Parrish, 1999). size structure of coral trees remaining on the As a consequence, the gross of the open grounds. Obtaining these data was timely circuit gear (Table 1) was less than "conven- because harvesters have requested that manage- tional" gear configurations taught in technical ment lower the minimum harvesting size of diving courses (Mount and Gilliam, 1992). coral trees. On all OC dives, Nitrox was breathed In many respects, the black coral beds during initial descent and decompression at are representative of most of Hawaii diving: depths from 110 to 20 ft. Trimix was breathed exposed surface conditions, high currents, and below 110 ft, and oxygen was breathed during steep slopes. The bottom between the Islands the 20- and 10-ft decompression stops. Switching of Maui, Molokai, and Lanai is the remains of a between gas mixtures at predetermined depths prehistoric land bridge that once connected the provides the divers with limited ability to accel- islands. The shallowest depths are plateaus erate their decompression while controlling the found at 130-160 ft and the deepest points are partial of oxygen. the remains of lake beds with a maximum depth Training requirements for this form of of 240 ft. Black coral is known to settle and open-circuit trimix diving include a prerequisite grow on the steep wall faces of these lake beds, certification in advanced nitrox diving tech- particularly in the most -swept environ- niques and a 12-day intensive training program ments. Because the surveys occurred deeper costing $2,500 per student. than conventional scuba, the participating institu- tional diving programs required that divers Closed-Circuit Equipment breathe a helium mixture on the bottom to The closed circuit team, from here on avoid effects of . termed "CC team," used Cis-Lunar Development Laboratories' MK-5P rebreathers (Fig. Ib). As EQUIPMENT AND with any electronically controlled closed-cir- cuit rebreather design, 's exhaled METHODOLOGY breath is recirculated through a chemical car- bon-dioxide (C0 ) absorbent material, and oxy- General 2 gen is maintained at a constant Prior to diving, areas were first sur- throughout the dive (determined by oxygen sen- veyed using a remote camera system until a sors in the breathing loop), with replacement coral bed of significant size was found. After oxygen injected via an electrical solenoid valve. specific dive sites were identified, divers were These units provide upwards of 8-12 h of depth- then sent into this bed to record coral tree size, independent underwater time, depending on

base diameter, and video tape the associated diver workload and type of CO2 absorbent mate- fish community. One of the divers measured tree rial used. For these dives, the CC team used

height using a standard tape measure stretched Sofnolime@ 8-12 grade CO2 absorbent material. from the base to the top of the tree. Base diame- The oxygen "setpoint" used during ter was recorded using dial calipers. The second these dives (including bottom-portions and diver operated a video camera and tended a sur- decompression portions) was 1.4 atm. Multiple face marker. options are available on this unit, but

14 • MTS Journal. Vol. 36, No.2 Figure 1. Divers equipped with open circuit scuba (a) and closed circuit rebreather (b). Photo: (a) Raymond Boland; (b)

for these dives the selected were air Table 1. and approximate cost of open-circuit diving rig components (excluding and Heliox-16. Air was used as the active diluent support equipment such as lights, reels, lift bags etc. that are standard to both DC and CC during initial descent until reaching a depth of diving). 100 ft. At that time, the active diluent was Component description Weight -Cost switched to the heliox (without flushing the Double back-mounted, 120 If steel trimix cylinders (3,500 psi) 1171b $3,425.00 breathing loop), and descent continued. This with harness, BCD, manifold & dual regulators, oxygen resulted in bottom gas composition of constant analyzers.

1.4 atm oxygen partial pressure (P02), constant Single side-mounted, 63 ft3 aluminum nitrox cylinder (3,000 psi) 36 Ib $670.00 with regulator 2.6 atm nitrogen partial pressure (PN2), balanced helium (PHe determined as Single back-mounted, 40 ft3 aluminum oxygen cylinder (3,000 psi) 20 Ib $670.00 with regulator minus 4 atm). Upon ascent to 100 ft, the breath- ing loop was flushed with air, yielding a Total 1731b $4,765,00 dynamic (constant-P02) nitrox mixture during the remainder of the decompression phase. An emergency "bailout" supply of Trimix-lO/70 was Table 2. Weights and approximate cost of closed-circuit diving rig components (excluding carried on all dives, but not used (see Table 2). support equipment such as lights, reels, lift bags etc. that are standard to both DC and CC Standard training protocol for this diving.) rebreather includes a prerequisite certification Component description Weight -Cost in advanced nitrox diving techniques, and an 8- Rebreather unit with onboard 20 ft3 aluminum air cylinder (3,000 76 Ib $17,500.00 day intensive training program costing $1,500 psi), 13.5 If aluminum oxygen cylinder (3,000 psi), BCD, and per student. Certification for deep decompres- other ancillary equipment sion diving with this rebreather requires a mini- Dffboard 40 ft3 aluminum heliox cylinder (3,000 psi), with 20 Ib $300.00 mum of 50 h of dive time following initial train- regulator ing, plus a 4-day training course costing $750 Offboard 15 ft3 secondary oxygen cylinder (2,015 psi), with 7 Ib $300.00 per student. regulator Side-mounted 80 ft3 aluminum trimix cylinder (3,000 psi), with 36 Ib $300.00 Decompression Calculations regulator (bailout) The OC team followed NOAA-approved Total 1391b $18,400.00 hard-printed decompression tables based on the "DCAP" decompression algorithm (Hamil- ton, 1998). Protocols for using these tables were consistent with standard NOAA practice (e.g., differed in two significant ways. First, the assumed maximum depth for total bottom schedules for the OC team were based on con- time). The CC team followed decompression stant oxygen fraction (F02) breathing gas, data as presented by the triple-redundant real- whereas the schedules for the CC team were time decompression computers integrated into based on constant oxygen partial pressure (P02) the rebreathers (following the same DCAP breathing gas. Second, the schedules for the OC model upon which the NOAA tables are based), team were predetermined and assumed total with hard-printed bailout decompression bottom time was spent at maximum depth, schedules as backup. Although both teams fol- whereas the schedules for the CC team were lowed decompression schedules generated by based on real-time calculations of actual time at the same DCAP algorithm, the implementation actual depth and actual gas composition.

MTSJournal. Vol.36, No.2. 15 Comparison Criteria depth-time profiles for all dives were recorded Detailed records of time (minutes) electronically. committed and breathing media used (ft3) were 4. Dive Duration and Bailout Capability: It tracked for all divers throughout the cruise. is general practice within the technical diving Materials, gear and effort used by both the DC community to maintain a minimum breathing team and CC team (e.g., gas banks, , gas reserve that represents one-third of the total staging support divers, , etc.) were gas requirements for the dive (including both not recorded because they represented a con- bottom portions of the dive and decompression stant. Criteria used to compare the efficiency requirements). This practice is often referred to and effectiveness of the CC and DC diving meth- as the "," referring to the fact odologies included: that of the total gas supply carried, two-thirds are planned for use on the dive, and an addi- 1. Equipment Preparation and Mainte- tional one-third is held in reserve for a possible nance Time: The total time required for equip- emergency bailout situation. The planned dura- ment preparation before each dive, and mainte- tion of the dive, therefore, is limited to two- nance between consecutive dives, was thirds of the life-support duration that can be recorded for each diver in each team. Some of carried by the diver. For practical purposes, this the preparation tasks are needed only at the restriction limited the OC team to a maximum beginning of the cruise. Examples of such tasks of 25 min for these dives. Given the constraints typically include assembling the cylinders, har- of carrying gas supplies with them, realistically nesses, compensators, regulators, and only 5 to 10 min could be added to the OC team's other dive equipment. Other preparation must bottom time. For comparative purposes, the CC be done prior to every single dive. This primarily team limited the maximum bottom time of most includes mixing gas and filling cylinders and of its dives to 25 min as well. However, to exam- final pre dive checks (e.g., checking the pres- ine the dynamics of exploiting the longer bottom sures in each of the cylinders, analyzing each times allowed for by the rebreathers, the CC of the gases with multiple oxygen analyzers to team made one dive with a bottom time of 40 confirm breathing mixture, logging data, replac- min. ing rebreather CO2 absorbent material if needed, etc.). Maintenance involves routine rinsing of gear after each dive and repairing or replacing RESULTS AND DISCUSSION any malfunctioning components. ach dive team (OC and CC) conducted four 2. Consumption of Expendables: The pri- Edives. Except for one dive aborted by the mary expendable consumed by both teams was OC team because of unmanageable bottom cur- breathing gas: specifically, oxygen, helium, and rent, all the divers successfully obtained data on air. Cylinder pressures were recorded before the size structure of black coral trees. The two and after each dive to determine the total quanti- dives with square profiles (1 OC and 1 CC) were ties of gas consumed. In addition, rebreather to 170 ft for 25 min. The two multilevel dive profiles (1 OC and 1 CC) began at 200 ft for 5 divers also consumed CO2 absorbent material and batteries, and the quantities of each were min, then ascended to 180 it for 5 min, 160 it recorded and calculated. for 5 min, and finished bottom time at 140 ft for 10 min. The divers endeavored to keep to 3. Decompression Efficiency: The two diving these conceptual profiles; however because the methodologies differed in the management of two teams' dives were not simultaneous, the pre- decompression obligation. Because of these fun- cision in achieving the profile varied some with damental differences, part of the comparison changes in current and other factors specific to was to have the dive teams each do a square individual dives. The remaining dives included profile and a multilevel profile using the same two 25-min dives (1 OC and 1 CC) with profiles dive site, maximum depth, and bottom time. A determined by coral distributions (profiles not third set of dives by both teams was made in planned in advance) and 1 CC dive of increased which the specific profile was not predeter- bottom time to 40 min to illustrate a dive of mined, but was dictated by the distribution of longer bottom time. Pooling all dives, the sur- the coral trees on the . This final set of dives veys represented a cross-section of working was intended to assess the relative flexibility depths ranging from 129 to 226 ft with maximum of the two diving technologies for adapting to dive times ranging from 63 to 130 min. The OC unexpected dive profiles, such as those encoun- team spent 5 h and 22 min underwater, and the tered in many exploratory scientific endeavors. CC team 5 h and 28 min (Table 3). Throughout For comparative purposes, decompression these dives, there were no noteworthy equip- efficiency was defined as the ratio of time spent ment malfunctions, no signs of decompression working on the bottom, to the time required to sickness, nor any other indications of decom- safely decompress back to the surface. Detailed pression stress.

16 • MTSJournal. Vol.36, No.2 Equipment Preparation and Table 3. Summary of dive parameters. Maintenance Time Max. depth Bottom time Deco. time BTIDT ratio The times required for equipment prep- Dive plan DC CC DC CC DC CC DC CC aration and maintenance of each team on each Square profile 176 190 28 32 62 50 0.45 0.64 dive are shown in Table 4. These values repre- Multilevel profile 200 200 27 27 63 37 0.43 0.73 sent average times per diver (i.e., person- Coral distribution 189 192 27 24 61 29 0.44 0.83 minutes). A distinct difference is evident in the Extended duration" 212 226 16 43 38 86 0.42 0.50 time required for gas fills on the first dive, as Max. I Totals I Avg. 212 226 98 126 224 202 0.44 0.62 compared to gas fills on subsequent dives. The 'Extended duration dive applies to CC team; DC team values refer to aborted dive. reason for this difference is that cylinders were Note: Bottom-line values represent maximum depths, total bottom and deco times, and assumed to be empty at the start of the cruise, average Bottom-time/Decompression-time ratios. and therefore needed to be completely filled. On subsequent dives, the cylinders needed only Table 4. Preparation, maintenance, and gas-fill times, in to be topped off from their values at the end person-minutes. of the previous dive. For this reason, average values reflect only the pre-dive cylinder fill Prep.IMain!. Gas fills times and not the initial fill time (a value more Dive DC CC DC CC reflective of typical cruises involving more than 1 64 31 180 50 four dives). 2 58 37 60 8 For the rebreather preparation and 3 52 38 60 7 maintenance times, values for dives 1 and 3 4 45 18 66 6 include time required to repack the CO2 absor- Avg. 55 31 62" 7" bent canister. The absorbent life ranges from 'Excludes initial fills for first dive. 6 to 10 hours of dive time, depending on individ- ual diver and workload. Although the total combined in-water time for all four involved technical scuba, which relies on multi- rebreather dives was only 5.5 h (less than the ple scuba tanks, each fitted with a separate regu- conservative limit of a single canister of absor- lator. It should be noted that no gear was shared bent material), absorbent was changed after the between divers. If gear were shared among dif- first 2.3 h of dive time to represent a highly ferent divers, the turn-around time for rebreath- conservative absorbent replacement schedule. ers would increase by approximately 25 minutes The preparation and maintenance time for per rebreather unit, due to post-dive hygiene of rebreather dive number 2 includes time required disinfecting the breathing loop. to repair a leaky hose on one of the emergency As expected, gas fill times were sub- bailout cylinders and a leaky valve on another stantially shorter for the rebreathers than for cylinder. These maintenance tasks were not spe- the open-circuit cylinders. The initial gas fill time cific to rebreather equipment, and could just as for the rebreathers was about 28% of that likely been needed for the OC team's equipment. required for the initial filling of the open-circuit If this time were excluded from the totals, then cylinders, and the rebreather refill times for the average preparation/maintenance time for subsequent dives averaged about 11%of the time rebreather dive number 2 would have been required to refill the open-circuit cylinders. The 17 min. reason for better comparative efficiency of Despite a common perception that rebreather gas fills on subsequent dives, as rebreathers generally require more maintenance opposed to the first dive, is that the large-capac- time than open-circuit equipment, Table 4 illus- ity rebreather emergency bailout cylinders trates that the rebreathers actually required less needed to be filled before the first dive, but not preparation and maintenance time than did the for the remainder of the dives. An important open-circuit equipment. The rebreathers consideration on gas fill times is that the support required less than half the initial setup and main- equipment was extremely robust, both in terms tenance time for the first dive, and averaged of available gas supplies and in terms of large- about 60% of the preparation and maintenance capacity pumps (oxygenlhelium time for the remaining dives, as compared to and oil-free air ). For operations with the open-circuit gear. If the non-rebreather-spe- less robust gas-filling capability, the rebreather cific gear repair time of the second dive were advantage on gas fill times would increase fur- excluded from the CC total, then the combined ther because a much larger percentage of the preparation and maintenance time for rebreath- rebreather fill times was devoted to basic cylin- ers on all dives averaged less than half that der-filling "overhead" time (removing and required for the open-circuit gear. The notion replacing filling hoses on cylinders, turning of greater preparation and maintenance time for valves, etc.), rather than actual gas filling times. rebreathers may relate to their comparison to With less powerful gas pumps or lower gas sup- conventional scuba rather than the more ply cylinder pressures, the increased actual gas

MTS Journal • Vol. 36, No.2. 17 filling times would penalize the overall open- productive bottom time. The CC team con- circuit gas fill times more heavily than it would ducted 29% more productive bottom time than the rebreather gas fill times. the OC team (Table 3). Per hour of bottom time Combining the preparation/mainte- the OC team actually required 17 times more gas nance times with the gas filling times (the sum and 6.7 times the cost of expendables used by of which represents the total out-of-water, gear- the CC team. related time commitments for these dives), rebreathers required less than a third of the time Decompression Efficiency for the open-circuit gear. However, the A summary of the basic dive profile rebreather advantage increases further when parameters is included in Table 3 and the actual considering the ratio of non-diving equipment dive profiles as recorded by dive computers are time requirements to actual productive bottom shown in Figures 2-5. As was expected, the CC time. Considering actual average per-dive time team had better decompression characteristics expenditures, the CC team required 1.2 min of on all dive profiles. All rebreather dives had a topside equipment time for every minute of pro- bottom time to decompression time ratio of 0.5 ductive bottom time, whereas the OC team or greater, meaning that the required decom- required 4.8 min. pression time was twice or less than twice the bottom time of the dive. Conversely, this same Consumption of Expendables ratio was less than 0.5 for all open-circuit dives, A summary of the consumed expend- meaning the required decompression exceeded able materials is presented in Table 5. The pri- twice the bottom time of the dive (Table 3). The mary expendable consumed by both teams was increased decompression efficiency on gas (specifically Heliox-16 premix, oxygen, and rebreathers was more pronounced for the multi- air). These values represent actual totals of con- level dive profiles (70% improvement; Fig. 3) sumed gas for both members of each dive team than for the square profiles (42% improvement; on all dives, but do not include unused gas held Fig. 2). in reserve for emergency bailout purposes. Cost There are two reasons why the values in Table 5 represent actual cost of gas, rebreather advantage was greater on multilevel and do not include associated costs of compres- profile dives than it was on square profiles. First, sor and other equipment maintenance. The CC the rebreathers continually optimized the

team also consumed CO2 absorbent material breathing mixture according to the depth, and batteries. whereas the breathing gas in the open-circuit As expected, the CC team consumed cylinders is optimal at only one particular depth considerably less gas over the course of these for each different gas mixture. Therefore, as dives than the OC team. The OC team required the rebreather divers ascended to the shallower nearly 10 times as much of the heliox premix depths of the multilevel dive profile, the breath- gas, more than 6 times as much oxygen, and ing gas composition changed to be optimized almost 40 times as much air over the course of for each of the shallower depths. The open- these dives, compared to the CC team. When all circuit divers were forced to continue breathing gases are considered together, the OC team a gas mixture optimized for the maximum depth required more than 13 times as much gas as the of the dive. The second reason involves the CC team. Total costs of expendables for the CC rebreather divers following real-time decom- team were less than one-fifth of the costs for pression computers, whereas the open-circuit the OC team, even when the CO2 absorbent divers followed preprinted decompression material and batteries for the rebreathers were tables. On the multilevel profiles, the rebreather taken into consideration. computers accounted for time spent at shal- Compared to topside equipment time lower depths; the open-circuit divers followed requirements, the rebreather advantage is even standard protocol of calculating decompres- greater when comparing these values to actual sion based on tables designed for the complete bottom time spent at the maximum depth (i.e., no "credit" for time spent at shallower depths). Table 5. Summary of consumed expendables. However, the rebreather did maintain an advan- Quantity consumed Cost· tage even on the square profile because every Expendable DC CC DC CC dive requiring staged decompression is ulti- Heliox premix 525 ft3 55 ft3 $315.00 $33.00 mately a multilevel dive. Oxygen 258 ft3 41 ft3 $15.50 $2.46 As is evident from the profiles shown Air 738 ft3 19 ft3 - - in Figures 3 and 4, even the best efforts to CO2 absorbent - 11 Ib - $22.00 follow a predetermined depth profile on a real Batteries - 11% - $5.50 dive can meet with difficulty. The depth profiles Total 1,521 ft3 115 It! $330.50 $62.96 for the OC team and CC team do not line up 'Costs lor gases represent actual material cost 01 gas only. precisely because of various factors. Thus, the

18 • MTSJournal. Vol.36, No.2 Figure 2. Dive profiles of both teams for the square profile comparison Figure 3. Dive profiles of both teams for the multilevel profile comparison dive. dive.

Square Dive Profile Comparison Multilevel Dive Profile Comparison , 211 ,'-,•....'-••...... •.. ,..". ...-, .0 , ,-,~.. 10, I-• J 10 : I _ 100 I ,, .t:. 1211 ,

Q11.0 110 - Closed Circuit 110 - - Open ClrdJlt 200

220 10 20 30 4l) 50 60 70 80 90 100 0 10 2lI 10 .0 10 10 ~ 10 10 ~ Dive Time (minutes) DIve Time (minutes)

Figure 4. Dive profiles of both teams for the coral distribution profile dive. Figure 5. Dive profile of CC team for the extended bottom time dive.

Coral Distribution Profile Closed-Circult Dive· Extended Bottom TIme

20 ,.'-'- .,.....•..---" 20 40 " ..; .0 ,,, 10 10 "" - 10 80 •••" '1 100 ~ 100 1 ti 120 Q.l.o I:: c!l 110 110 110 110 - Closed Circuit 200 Z20 200 - - Open Circuit 220 lI40 o 10 20 30 .0 10 10 ~ 10 80 100 110 120 ,. o '0 2lI XI .• 50 eo 70 100 ••• ••• Dive TIme (mlnutnl DIve Time (minutes)

task of distinguishing the relative importance of is more substantial than the advantage afforded the decompression advantage factors (optimal by the dynamic breathing mixture of a closed- gas mixture vs. real-time decompression com- circuit rebreather. Comparing the OC schedules puters) is best addressed by mathematical with the CC schedules for both the square profile interpretations of idealized dive profiles. A series and the multilevel profile results in CC decom- of decompression dive profiles are shown in pression advantages of 23% and 300A>,respec- Table 6. All profiles were generated using the tively. The CC advantage on the square profile same DCAP algorithm followed during the comes from the optimized breathing gas during dives discussed herein. Parameters for all dives ascents and intermediate decompression stops include a 100 fUmin descent rate; 30 fUmin (90-30 ft), and the greater CC advantage on the ascent rate; a nitrox gas mixture (EAN) for initial multilevel profile is a result of optimized breath- descent to a depth of 110 it and during ascent ing gas during the shallower four depth levels and decompression from 110 ft to 20 it; a trimix of bottom time in addition to the intermediate gas mixture of 18%oxygen, 500A>helium, balance decompression stops. However, when compar- nitrogen (Tri-18/50) at a depth of 225 ft, and pure ing the multilevel profiles to the respective oxygen at 20 ft and shallower during the final square profiles (analogous to diving a multilevel stages of decompression. Rebreather setpoint is profile and comparing the decompression 1.4 atm. Profiles were calculated for both open- advantage of a real-time decompression com- circuit mode (constant F02) and closed-circuit puter to hard-printed decompression schedules), mode (constant P02) and with both a square pro- there is a 700A>decompression advantage for the file (25 min at 225 ft) and a multilevel profile (5 OC profile and an 82% decompression advan- minutes each at five 25-ft ascending depth incre- tage for the CC profile. This means that the ments, starting at 225 ft). decompression advantage afforded by real-time It should be clear from Table 6 that the decompression computing on multilevel dive decompression advantage afforded by real-time profiles is approximately 2.5 times greater than decompression calculations of a the decompression advantage afforded by gas

MTSJournal. Vol.36, No.2. 19 Table 6. Comparison of decompression profiles. CO2 absorbent material, which in this case was Square profile Multilevel profile Gas mixture Sofnolime® 8-12 grade material. The various gas Depth (It) DC CC DC CC DC CC supplies as carried by the CC team were all more than sufficient to extend the bottom time well 10 23 21 18 15 Oxygen Oxygen beyond what would be allowed by the CO absor- 20 12 11 11 10 Oxygen Oxygen 2 30 18 11 7 4 EAN-36 EAN-73 bent material. In this case, the limiting diver 40 6 4 5 3 EAN-36 EAN-63 routinely achieves 6-h durations out of a single 50 5 4 2 EAN-36 EAN-56 canister of CO2 absorbent material with this 60 3 3 EAN-36 EAN-50 absorbent type. Allowing for at least one-third 70 2 1 EAN-36 EAN-45 of the total life-support system to be held in 80 1 2 EAN-36 EAN-41 reserve, a maximum total dive time of 4 h could 90 2 1 EAN-36 EAN-38 be achieved. Because the total dive time of the 100 EAN-36 EAN-35 CC team on the square profile was 82 min long, 110 EAN-36 EAN-32 77% of the actual life-support was held in 125 5 5 Tri-18/50 Tri-29/43 reserve by the CC team. If the duration of the 150 5 5 Tri-18/50 Tri-25/46 dive was extended to exploit the full 4-h of dive 175 5 5 Tri-18/50 Tri-22148 200 5 5 Tri-18/50 Tri-20/49 time afforded by the rebreathers, the bottom 225 25 25 5 5 Tri-18/50 Tri-18/50 time could have been extended to more than 90 min (which would have resulted in a decom- Total deco.· 80 66 47 36 1 pression time of less than 2.5 h). BTIDT ratio 0.31 0.38 0.53 0.69 J If the CC team wanted to extend dive ·Total decompression time includes ascent time between stops. durations they could switch to a more effective

CO2 absorbent material, such as Anhydrous Lith- optimization from closed-circuit rebreathers ium Hydroxide (LiOR), doubling dive time to 12 for this particular profile. Dive profiles differ in h. Unlike the open-circuit scenario, this modifi- their degrees of multilevel bottom-time, which cation to extend the rebreather (using LiOR varies the relative advantage ratios of real-time instead of Sofnolime®) would not have increased decompression calculation to optimal breath- the overall weight of carried equipment ing gas mixture. However, the overall beneficial because the gas supplies already carried by the trend remains the same because both factors CC team were ample to allow for dives of this are affected by depth in similar and propor- duration. In fact, the weight would have been tional ways. reduced by several pounds because LiOR is a lighter material than Sofnolime®. By compari- Dive Duration and Bailout Capability son, if the OC team wished to extend its bottom A common practice among open-cir- time to 3 h on this dive, six times as much trimix cuit mixed-gas divers is to reserve at least one- (1,440 ft;3) and more than 12 times as much third of the breathing gas supply for emergency nitrox (770 ft;3) would be required and would purposes. This limits the amount of bottom time have increased the overall gear weight of that to two-thirds of the diver's total gas supply. Typi- team to more than 1,000 lbs just for trimix and nitrox supplies. Indeed, if gear weight is fac- cally bottom time is ended when the first person tored into the relative effectiveness of the two on a dive team reaches the prescribed gas limit. systems (given that the OC team's gear weighed The "limiting" OC diver doing a square profile consumed 130 ft;3 of the 240 ft;3 trimix supply 29% more than the CC team's gear), the rebreather performance increases by 7.7 times carried for the 25-min bottom time. This means that of the OC gear. that 46% of the trimix was held in reserve on that dive. This dive team could have extended its bottom time to a maximum of 30 min before CONCLUSIONS exhausting 160 ft;3 (two-thirds) of the carried trimix supply. The additional decompression Projected Efficiency obligation of a 30-min bottom time would have The bottom-line consideration of this required a slight increase in the nitrox gas supply comparison is the ratio of total support time (and with a larger 70 ft;3 cylinder (instead of the 63 cost) to total productive time when determining ft;3 used during the 25-min dive). Thus, the OC which system (open-circuit mixed gas, or team could have extended its effective bottom closed-circuit rebreathers) is optimal for a given time by 20%. To extend the bottom time beyond field operation. If the logistical support compar- 30 min, the OC team would need substantially ison is combined with the decompression effi- more gas (particularly with respect to the carried ciency comparison (i.e., time spent decom- trimix supply), increasing the weight of gear pressing is considered as support time, rather carried by the divers. than productive time), then the OC team The limiting factor for dive duration devoted an average total of 7.1 minutes of sup- when using rebreathers was imposed by the port time for each minute of productive time

20 • MTS Journal • Vol. 36, No.2 at an average cost of $3.37 per minute of produc- ever, many of the divers in these institutions tive time for expendables alone. The CC team routinely carry air and nitrox computers and at devoted an average of only 2.8 minutes of sup- times are likely to be relying on them. There port time for each minute of productive time are a number of reasons to adopt computers. at an average cost of only $0.50 per minute of First, computers are not necessarily a more lib- productive time. With a cost savings of $172 eral form of decompression but rather a balanc- per hour of productive underwater time, the ing of the risks between square profiles and additional equipment purchase cost for multilevel profiles. Second, the computer tracks rebreathers can be recovered within 80 hours of unintended deviations (max depth/time) in the productive operational time, based on cost of dive plan and provides the diver real-time expendables alone. When additional cost sav- options to alter the planned decompression and ings associated with man-hours dedicated to mitigate the impacts of the deviations. Third, the support time, as well as excess costs associated computer is analogous to a flight data recorder with support gear maintenance and logistical providing a detailed record of the dive, which overhead are considered, the cost-recovery time is important if the diver becomes symptomatic for closed-circuit gear is significantly reduced. and requires hyperbaric services. Finally, unnec- essarily prolonged decompression has its own Logistical Overhead risks, including exposure to cold, risk of drifting Adopting CC gear will significantly off, and harassment by sharks. Matching total reduce some of the logistical infrastructure decompression times to actual decompression needed to support dive operations. The bulk and requirements reduces these risks. expense of dozens of gas storage flasks and specialized gas pumps can be replaced by CC RECOMMENDATIONS gear, a minimal number of high-pressure stor- age cylinders, a small booster pump, and a con- ased on the results of this effort, we make ventional scuba compressor. (An oil-free air Bthe following three principle recommenda- compressor would not be required because no tions: mixing of air and oxygen is required for CC 1. Any additional evaluations undertaken operations.) Our comparison did not consider should continue to support programmatic the significant effort and cost (-$30,000) of operations. The black coral data collected dur- obtaining and assembling the pumps and banks ing this gear comparison provided valuable required for OC diving. If instead, CC is insight to the local fishery management council. adopted, the money and time typically spent on Evaluations conducted in the context of actual this conventional infrastructure can be used to programmatic operations allows "real-world" defer the initial start-up costs of obtaining CC comparisons, which sometimes reveal unex- gear and the necessary training. pected trends that would not have been uncov- ered in sinlpler proof-of-concept projects. Dive Computer Advantage A substantial part of the savings in sup- 2. The multilevel dive profile paradigm port time for the rebreathers comes from should be accepted and dive computers reduced decompression time afforded by real- should be adopted for primary decompres- tinle decompression calculations-especially sion guidance. Dive computers should be when multilevel diving operations are involved. adopted for both conventional and mixed-gas Several dive computer manufacturers have OC diving. Recent development of computers already introduced (or will soon be introducing) that accommodate switching among different breathing mixtures could provide the majority of dive computers (-$1,200 US) capable of calcu- lating real-time decompression information for the decompression benefit that the CC gear pro- vided. An obvious next step is to field evaluate mixed-gas dive profiles (including trinlix, for these new mixed gas computers at sites with both OC and CC operations). Although applica- good hyperbaric facilities. tions of such computers for open-circuit trimix dives would not close the effiCiency gap with 3. Scientists should begin working with closed-circuit dives, they would substantially rebreathers in conventional depths on reduce the total required decompression times selected projects. Even though decompression for dives with multilevel bottom profiles, times can be shortened simply by adopting a thereby improving the efficiency of mixed-gas decompression computer, CC gear still provides diving operations in general. a significant benefit by eliminating cumbersome Decompression computers are only support infrastructure and improving dive effi- now being adopted by some institutional pro- ciency (in terms of reducing the support time grams, including the NOAA dive program. The to productive dive time ratio). Selection of a CC lack of resources to evaluate the many available system should include the capability to expand units has prevented their formal adoption. How- to deeper applications as experience is acquired.

MTS Journal • Vol. 36, NO.2. 21 Costs of the unit and time involved with training of the Sixth International Scientiji.c Symposium of will be a consideration in this selection and may CMAS, Proceedings of the Diving Science Sympo- vary between applications. Specific projects sium. (J. Blanchard, J. Mair and 1. Morrison, eds.) involving use of rebreathers in conventional pp. 43-52. London: National Environmental Research Council. depths should include comparative evaluations Mount T. and Gilliam, B. 1992. Mixed Gas Diving. The of the potential advantage of quiet operation, in ultimate challenge for technical divers. Watersport the context of marine life interactions and Publishing, Inc. 391 pp. behavioral observations-a parameter not con- Parrish, F.A. 1999. Use of technical diving to survey sidered in this report. forage habitat of the endangered Hawaiian monk seal. Hamilton. R.W., Pence D.E., and Kesling D.E. Editors. Proceedings of the AAUS Technical Div- ACKNOWLEDGEMENTS ing Forum: Assessment andfeasibility of Technical e appreciate the assistance of the NOAA Diving Operations for Scientiji.c Exploration. WDive Center and the NOAA American Academy of Underwater Sciences. 83 pp. Board in facilitation of this project. We also Pence, D. and Pyle, R.L. 2002. University of Hawaii dive team completes Fiji deep reef fish surveys thank D. Dinsmore, D. Pence, D. Crosson and using mixed-gas rebreathers. The Slate. April, R. Boland for reviews and criticisms of drafts 2002:1-3. of the manuscript. Photos were provided by R. Pyle, R.L. 1998. Chapter 7. Use of advanced mixed-gas Pyle and R. Boland. Reference to trade names diving technology to explore the "Twi- in this article does not imply endorsement by light Zone." In: Ocean Pulse: A Critical Diagnosis, the National Marine Fisheries Service or the eds. J.T. Tanacredi, and J. Loret (Eds.) Ocean Pulse: NOAA Diving Program. A Critical diagnosis. Plenum Press, New York XII +201 pp. Pyle, R.L. 1999. Mixed gas closed circuit rebreather REFERENCES use for identification of new reef fish species from Barsky, S., Thurlow, M. and Ward, M. 1998. The Simple 200-400 fsw. In: Proceedings of the AAUS Technical Guide to Rebreather Diving. Flagstaff: Best Pub- Diving Forum: Assessment a.nd feasibility of lishing Company, 228 pp. Technical Diving Operations for Scientific Explo- Cole, B. 1998. Rebreather Diving. Liverpool: Sub-Aqua ration. American Academy of Underwater Sci- Association, 112 pp. ences. 83 pp. Hamilton B. 1998. NOAA Trirnix I Decompression Pyle, R.L. 2000. Assessing Undiscovered Fish Biodiver- Tables. NOAA Diving Center 7600 Sand Point Way, sity on Deep Coral Reefs Using Advanced Self- NE Seattle, Washington. 69 pp. Contained Diving Technology. Marine Technology Hanlon, R.T., Hixon, R.F., Hendrix, Jr., J.P., Forsythe, Society Journal. 34(4):82-91. J.W., Sutton, T.E., Cross, M.R., Dawson, R. and Richardson, D., Menduno, M. and Shreeves, K (eds.). Booth, L. 1982. The application of closed circuit 1996. Proceedings of rebreather forum 2.0. Diving scuba for biological observations. In: Proceedings Science and Technology, Inc. 274 pp.

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