Air Diving and Decompression 4 SECTION PAGE SECTION PAGE 4.0 GENERAL....................................................4- 1 4.4.3 Making Mandatory 4.1 DECOMPRESSION TABLE Decompression Stops................................4-20 DEVELOPMENT ...........................................4- 1 4.4.4 Omitted Decompression ............................4-23 4.1.1 Table Computation Prediction ....................4- 1 4.4.4. 1 Omitted Decompression 1..............4-23 4.1.2 Computing Decompression Tables ...............4- 1 4.4.4. 2 Omitted Decompression 2..............4-23 4.1.3 Reliability of Tables .................................4- 2 4.4.4. 3 Use of Oxygen 4.1.4 Dive Planning Software ............................4- 2 During Decompression..................4-23 4.2 USING THE U.S. NAVY DIVE TABLES 4.5 DEALING WITH CHANGES TO MAKE SINGLE DIVES ..............................4- 2 IN ALTITUDE...............................................4-23 4.2.1 Single Versus Repetitive Dives....................4- 2 4.5.1 Diving at Altitude....................................4-23 4.2.2 Planning Single Dives...............................4- 3 4.5.1. 1 Altitude Correction Procedure ........4-24 4.3 USING THE U.S. NAVY DIVE TABLES 4.5.1. 2 Correction of Depth of Dive...........4-24 TO MAKE REPETITIVE DIVES........................4- 5 4.5.1. 3 Correction for Decompression 4.3. 1 Recording Repetitive Dive Stop Depths ...............................4-24 Data ...................................................4- 5 4.5.1. 4 Need for Correction .....................4-24 4.3. 2 Accounting for 4.5.1. 5 Depth Measurement at Altitude ......4-24 Residual Nitrogen ..................................4- 6 4.5.1. 6 Correction of Depth Gauges...........4-24 4.3. 3 Finding Repetitive Group 4.5.1. 7 Hypoxia During Altitude Diving .....4-26 Designations Following Single, 4.5.1. 8 Altitude Sickness .........................4-26 No-Decompression Dives .........................4- 7 4.5.1. 9 Breathing Gases ..........................4-26 4.3. 4 Determining a Repetitive Group 4.5.1.10 Equilibration at Altitude................4-26 Designation Following a 4.5.1.11 Repetitive Dives..........................4-27 Surface Interval .....................................4- 8 4.5.1.12 Ascent to Altitude After 4.3. 5 Determining Adjusted No- Diving/Flying After Diving ...........4-27 Decompression Limits for 4.6 BUILDING ADDITIONAL Repetitive Dives ....................................4- 9 SAFETY FACTORS INTO 4.3. 6 Determining Repetitive Group DIVE TABLE USAGE .....................................4-29 Designations Following 4.6.1 Remaining Well Within No- Repetitive Dives ....................................4-12 Decompression and Other 4.3. 7 Why Repetitive Group Limits...................................................4-29 Designations Are Important......................4-14 4.6.2 Making Slow Ascents 4.3. 8 Determining the Minimum and Safety Stops ......................................4-29 Allowable Surface Interval 4.6.3 Taking Advantage of the Dive Between Dives.......................................4-14 Table’s Inherent Margin of Safety 4.3. 9 Exceptions to Normal Repetitive on Multi-Level Dives ...............................4-29 Dive Planning .......................................4-18 4.6.3.1 Reverse Profile Dives......................4-29 4.3.10 Dealing With Surface Intervals 4.6.4 Following Recommendations of Less Than Ten Minutes ........................4-18 Concerning Cold and 4.4 USING THE U.S. NAVY DIVE TABLES TO Arduous Dives........................................4-30 MAKE STAGED DECOMPRESSION DIVES.......4-19 4.6.5 Managing Additional Risk Factors That May 4.4.1 What Is Decompression? ...........................4-19 Contribute to Decompression Sickness..........4-30 4.4.2 Decompression Diving 4.7 NOAA NO-DECOMPRESSION DIVE CHARTS...4-30 Considerations........................................4-19 4.7.1 General.................................................4-30 Air Diving and Decompression 4 4.0 GENERAL physiologist J.S. Haldane developed a hypothetical As explained in previous sections concerning the method for tracking gas in the body and showed how to physics and physiology of diving, body tissues absorb develop decompression profiles or “tables.” At the outset, additional nitrogen from the air breathed during dives it is important to realize that this “model” proposed by and release this excess nitrogen during ascent. After sur- Haldane, and later modified by others, is hypothetical. It facing, body tissue continues to release excess nitrogen is not what really happens in the body, nor was it intended until the level of nitrogen dissolved in the tissue returns to to be; but, it does afford a method of moving from yester- normal. day’s dive experience to tomorrow’s new decompression By keeping the amount of nitrogen being absorbed and tables. This was the first such model; many others have released within acceptable limits, the risk of a serious div- followed, and many are offshoots of the Haldane method. ing malady known as decompression sickness, or DCS, is A well-developed computational method similar to Hal- reduced. Divers have many tools at their disposal to help dane’s was published by the late Swiss cardiologist, Prof. plan and make dives in which the risk of DCS remains A.A. Bühlmann, and it has been widely used by others. within acceptable levels. These tools include dive tables At today’s state of knowledge, the only sound criteri- and dive computers. on for the preparation of useful decompression tables is Even when divers use computers as their primary dive empirical experience. As models improve, prediction planning tool, it is important to have a working knowledge capability will continue to improve, but the judgment as of dive tables. Dive tables can provide an important back- to whether a model is right is how well it actually works, up in case of computer failure or operator error. They can not how sophisticated the math may be. even help divers pre-plan a series of two or more dives— which is something that is generally beyond the capabilities 4.1.2 Computing Decompression Tables of dive computers. Experience has shown that certain profiles, and pre- There is a wide variety of dive tables available, includ- sumably the hypothetical gas loadings produced by such ing versions by the U.S. Navy, other foreign governments, profiles, have or have not produced DCS. With enough and recreational training organizations. For military and experience (data) it is possible to assign limits to various selected scientific and commercial divers in the United ascents from depth. With these tools, table developers States, the standard dive tables are those appearing in the calculate suitably slow ascent rates for a variety of expo- U.S. Navy Diving Manual. A complete set of the U.S. Navy sure profiles; the results of these calculations are decom- Dive Tables can be found in Appendix IV. pression tables. This section of the NOAA Diving Manual is devoted to The limits just mentioned are in terms of the gas the proper use of U.S. Navy Dive Tables for relatively loading that can be tolerated in each compartment at each shallow, scientific and research diving. A working knowl- depth during ascent. Ascent limits are normally consid- edge of U.S. Navy dive table usage will also make it easi- ered in 10 fsw or 3 msw increments, and are known as er to understand and use other dive tables as well. “M-values” (where M stands for “maximum”), the maxi- mum permitted gas loading at that depth in that compart- 4.1 DECOMPRESSION TABLE ment. To calculate a decompression table, the developer DEVELOPMENT needs a set of M-values, usually determined from experi- 4.1.1 Table Computation Prediction ence. The calculated gas loadings in each compartment The most common method used for predicting if a are compared with the M-values, and ascent is adjusted to profile (of pressure and gas as functions of time) will cause keep the loadings below the limits. The diver’s ascent is DCS dates back to around the turn of the century, when halted, with “stops” at specified depths, to wait until the 4-1 hypothetical gas loadings have “decayed” to below the specialists were qualified to produce decompression limits for that depth; the diver then ascends to the next tables, and that all tables needed extensive testing before stop and the process is repeated. operational use. The J.S. Haldane decompression model goes back Several entrepreneurs have prepared and distributed nearly a century, but by using it with continuously updat- computer programs that can be used to generate decom- ed experience, it can be used to produce reliable decom- pression tables. This has been possible because of publica- pression tables. It is not quite correct to consider this a tions by Prof. Bühlmann that outline tested and accepted ‘theory” of how the human body works. Rather, it is a algorithms for computing tables. Most programs available computational tool that allows prediction of tomorrow’s are based, at least fundamentally, on Prof. Bühlmann’s dive from yesterday’s experience. The A.A. Bühlmann algorithms. model uses the same equations for on-gassing, but calcu- The different programs manage the algorithm in dif- lates the ascent limits in a different way; it, too, is firmly ferent ways, especially with regard to introducing extra based on experience. conservatism into the computations. These differences make it difficult to compare programs and to really 4.1.3 Reliability of Tables understand the affects of the conservatism.