DIVE TABLE SPECIAL RULES
Within each set of various dive agency tables there are certain 'safety' rules that divers must know and adhere to. These rules should be emphasized during initial scuba training, enough to be recalled prior to any dive, or be considered prior to any diveplanning. Dive table special rules follow each description of each agency dive table.
U.S. NAVY
HISTORY OF THE U.S. NAVY TABLES.
The first tables for the U.S Navy emerged in 1915. These tables were based on J.S. Haldane's observations that during decompression, pressure could be halved without ill - effect. In the 1930`s U.S. Navy researchers undertook a set of decompression experiments using human volunteers in a recompression chamber. Use of Haldane`s tables was proven effective in preventing the bends, but was not accurate enough to meet the wider range and depths of dives the U.S. Navy Divers were exposed to. Haldane used only a range (5, 10, 20, 40, 75) of 5 to 75 minute half time tissue compartments, and he applied his 1.58/1 supersaturation ratio across all six tissue compartments linearly, when actually each tissue compartment has a different supersaturation ratio. The U.S. Navy then modified Haldane's approach by extending the six (6) half time tissue compartments (5, 10, 20, 40, 80,120) from 5 to 120 minutes, and adjusted higher supersaturation ratios to faster tissue compartments.
Note: The new 1930's tables utilized an ascent rate of 7.5 m / minute, because divers in those days used " hard hats " and faster transport to the surface was not possible. Other noticeable deficiencies were that the tables had no provision for repetitive dives, and mainly configured for single stepped decompression dive profiles.
The U.S. Navy Dive Tables originated based on Haldane's Observations. Later experimentation in the 1930's led to several modifications in which the Navy adopted slower tissue half times and different 'ratios' for different tissues. These ratios are now known as "M" values for the different tissue compartments. Even after extensive experimentation using a larger population of divers and greater environmental variances, the U.S. Navy never achieved a "zero" bends incidence. However, out of 26,035 dives, resulting in 12 accidents equal to 0.046% incidence, or a risk factor of 1 per 2,173 exposures, it seemed quite a reasonable risk factor, and the "Standard Tables" as we call them today were published by the U.S. Navy in 1959.
Note: Complete saturation / de - saturation is presumed to take 12 hours, which is equivalent of six half times of the 120 minute tissue compartment.
DIVE TABLE TERMS
1). BOTTOM TIME. The time from starting your descend until commencing the final ascent to surface or a decompression stop.
2). DECOMPRESSION STOP TIME. Time actually spent at the stop. It does not include the time taken to ascent to it.
3). REPETITIVE GROUP. Is a measure of excess nitrogen remaining in the body after a dive.
4) SURFACE INTERVAL TIME. Time from surfacing to descending to a next repetitive dive.
5). RESIDUAL NITROGEN TIME. Measure of the excess nitrogen found in the body after the surface interval. It`s the time a diver must consider that he has spent at the planned depth of the repetitive dive when commencing a repetitive dive.
6). DESCENT RATE. Descend rate is limited to 30 m / minute.
7). ASCENT RATE. Ascend rate is limited to 10 m / minute.
8). CORRECTED DIVE TIME. Time made up of the actual bottom time plus the residual nitrogen time to find new dive group.
9). NEW DIVE GROUP. Letter designation given by the repetitive dive table.
10). DECOMPRESSION SCHEDULE. Specific decompression procedure for a given combination of depth and bottom time aslisted in the decompression table.
11). SINGLE DIVE. Any dive conducted after 12 hours of a previous dive.
12). REPETITIVE GROUP DESIGNATION. A letter which relates directly to the amount of residual nitrogen in a divers body for a 12 hour period following the dive.
SPECIAL RULES FOR TABLES TO CONSIDER.
1). Ascent rate must not exceed 18 m / minute.
2). For any interim depth use next greater depth an the table itself.
3). When decompressing the divers chest must be located as close as possible to the stop depth.
4). For interim times use the next time on the table.
5). For " strenuous " dives use the decompression prescribed for the next longer time increment. e.g. For a dive to 17 m for 73 minutes look at a dive to 18 m for 90 minutes.
6). Repetitive dives is calculated as any dive more than one (1) in 12 hours.
7). A safety stop of 1 minute at 3m is required at sea level.
8). Remember there are five (5) different tables which are used for different purposes Use the standard air table.
9). Never attempt to interpolate between decompression shedules.
10).If the surface interval time is shorter than 10 minutes add bottom time to previous dive time.
11). U.S. Navy tables are only for sea level diving. If used for altitude diving do the conversions before diving.
12). Remember to use 'variances and factors' when planning these dives. Reduce 10 % from bottom time for each applicable factor, but decompression schedule must be the full time spent at depth.
DIVE PROFILE VARIANCES AND FACTORS.
When calculating your dive profile, there are other additional variances necessary to consider. Any physical effect that will cause a greater uptake of air consumption will increase the partial pressure of nitrogen in the body. Factors will be described in greater detail below.
i). Water temperature < 15 deg C.
ii). Overweight { 20 % above ideal weight }
iii). Female
iv). Strenuous exercise.
v). Age { for each decade after 25 count one factor }
vi). Unfit.
BUHLMANN DIVE TABLES
HISTORY OF THE BUHLMANN DIVE TABLES.
The Buhlmann (1986) tables are the result of more than 20 years research and experimentation by Professor Albert Buehlmann, a world renowned Swiss hyperbaric researcher and decompression pioneer.
The Swiss model is based on years of experimental data collection. The model utilizes 16 theoretical tissue compartments, with half times of 4 to 635 minutes. The tables are based on the modified model and utilize 12 tissue compartments.
There are currently two sets of tables i). Sea level tables ( 0 - 700 m ) ii). Altitude tables (701 - 2500 m ). This table give longer stays at deeper stops than the U.S Navy tables. All dives require a safety stop at 3m for one (1) minute at sea level and 2m for one (1) minute at altitude.
The Swiss system is based on the idea that the shorter dive times will minimize bubble formation on the first dive and therefore enhance off gassing during the surface interval This will allow for more time for repetitive dives than any other table. The repetitive system uses Repetitive groups which are similar than the U.S. Navy RG`s but are not compatible. The maximum ascend rate is limited to 10 m / minute. This will allow for off gassing to start when you ascent.
The Buhlmann Altitude tables is the only set of pure altitude tables available at this stage and it is the only table that has been extensively tested for diving at altitude.
DIVE TABLE TERMS.
1). BOTTOM TIME. The time from starting your descend until commencing the final ascent to surface or a decompression stop.
2). DECOMPRESSION STOP TIME. Time actually spent at the stop. It does not include the time taken to ascent to it.
3). REPETITIVE GROUP. Is a measure of excess nitrogen remaining in the body after a dive.
4) SURFACE INTERVAL TIME. Time from surfacing to descending to a next repetitive dive.
5). RESIDUAL NITROGEN TIME. Measure of the excess nitrogen found in the body after the surface interval. It`s the time a diver must consider that he has spent at the planned depth of the repetitive dive when commencing a repetitive dive.
6). DESCEND RATE. Descend rate is limited to 30 m / minute.
7). ASCEND RATE. Ascend rate is limited to 10 m / minute.
8). CORRECTED DIVE TIME. Time made up of the actual bottom time plus the residual nitrogen time to find new dive group.
9). NEW DIVE GROUP. Letter designation given by the repetitive dive table.
SPECIAL RULES FOR BUHLMANN TABLES.
1). Ascent rate must not exceed 10 m / minute.
2). For any interim depth use next greater depth an the table itself.
3). For any interim repetitive dive depth on the repetitive dive table use the shallower depth.
4). For interim times use the next time on the table.
5). For " strenuous " dives use the decompression prescribed for the next longer time increment. e.g. For a dive to 17 m for 73 minutes look at a dive to 18 m for 90 minutes.
6). Repetitive dives require additional time to be added called the RNT time.
7). A safety stop of 1 minute at 3m is required at sea level and a 1 minute at 2m stop if diving at altitude.
8). Because of the risk of nitrogen narcosis diving to more that 40 m is not permitted without additional security being given from surface. For diving instructors the limit is 50 m.
NAUI DIVE TABLES
HISTORY OF THE NAUI DIVE TABLES.
The NAUI (1990) tables are modified U.S. Navy Tables. Instead of using the U.S Navy NDLs, the NAUI tables utilize Maximum Dive Time MDTs which are in fact the U.S. Navy NDLs reduced by one letter group for dive between 18 metres and 39.9 metres, reduced by two letter groups for 15 metre dives, and three letter groups for 12 metre dives.
The maximum ascent rate is 10 metre / minute and a precautionary safety stop at 5 metres for 3 minutes is recommended after each dive. The time spent at the safety stop does not have to be counted as part of the " Actual Dive Time " { Naui`s new term for Actual Bottom Time } which is used to find the repetitive group after a dive.
Actual dive time is therefore taken as the total time taken underwater and may not or may, include the time at the safety stop. NAUI states that the 3 minute safety stop need not be counted into the time, but doing so will add extra conservatism to the repetitive dive calculations.
The repetitive groups is based on the off - gassing of the 120 minute tissue compartment, as are those of the U.S. Navy. The surface interval tables is essentially the Navy tables but with all numerical errors corrected. With these tables off - gassing is considered to take 24 hours not 12 hours as with the U.S. Navy tables, Group A times have been corrected accordingly. A minimum of 1 hour S.I.T time is recommended between dives. Repetitive dives deeper than 30 metres is discouraged. More than three dives a day as well as " bounce dives " is also not recommended.
DIVE TABLE TERMS.
1). DIVE SCHEDULE.
The schedule of a dive is an abbreviated statement of the depth and duration of a dive.
2). MAXIMUM DIVE TIME { MDT }. The length of time that may be spent at a given depth without being required to stop during ascent to prevent the likelihood of decompression sickness.
3). DECOMPRESSION STOP. The time a diver stops and waits at a specified depth during ascent to allow nitrogen elimination before surfacing.
4). PRECAUTIONARY DECOMPRESSION STOP. Three minutes spent at a depth of 5 metres for 3 minutes as a safety precaution even though the maximum dive time has not been exceeded. This procedure is recommended at the end of each and every dive. Time spent decompressing isconsidered " neutral time " and is not part of the dive time.
5). REQUIRED DECOMPRESSION STOP. The amount of time specified by the dive tables to be spent at a specific depth if the dive limits are exceeded.
6). ACTUAL DIVE TIME. { ADT }. The time form you start your descent until you reach surface again decompression time can be excluded.}
7). LETTER GROUP DESIGNATION. A letter symbol used to designate the amount of excess nitrogen in your system. The nearer the beginning of the alphabet,the less the amount of residual nitrogen in your system.
8). SURFACE INTERVAL TIME. { S.I.T } Time spent on the surface between dives. During thistime excess nitrogen is eliminated from the body and your letter group designation changes, moving closer to the beginning of the alphabet.
9). RESIDUAL NITROGEN. Nitrogen remaining in your system from a dive made in the last 24 hours.
10). REPETITIVE DIVE. Any dive made within the 24 hours of the previous dive.
11). RESIDUAL NITROGEN TIME { RNT }. On repetitive dives, the amount of time you must consider you have already spent at a given depth for a given planned dive. The time is based on the amount of residual nitrogen remaining in your system from a previous dive. Residual nitrogen time is obtained from a table and is based on your letter group designation following your surface interval time.
12). ADJUSTED MAXIMUM DIVE TIME. { AMDT }. For repetitive dives, AMDT is the MAXIMUM dive time for the depth of the dive minus the Residual nitrogen time for the dive.
13). TOTAL NITROGEN TIME { TNT }. This is the sum of your residual nitrogen time and your actual dive time following the repetitive dive. This total is used to obtain the new letter group designation after a repetitive dive. TNT = ADT + RNT.
14). BOTTOM TIME. Time taken from leaving surface until the start of the ascent. { Only to be used for calculations in the air management form }.
SPECIAL DIVE TABLE RULES TO CONSIDER.
1). On any dive do not ascend faster than 10m / minute.This is 0.166 m / second.
Use your instruments to control your ascent.
2). Use the exact or next greater number when referring to the dive tables. If the number is exceeded, use the next greater number. For example: Doing a 12.5m dive will be consideredto be a 15m dive. Also use the greater time limit if the time is exceeded.
3). Use the greatest depth while diving to set up a dive schedule.
4). When doing repetitive dives make sure to do the deepest dive first and the following dives shallower.
5). Consider all dives shallower than 12 m to be 12 m dives.
6). If the surface interval time is less than 10 minutes thefollowing dive will be counted as the same dive.
7). If a dive is particularly cold or strenuous, use the next greater bottom time. If the dive is cold and strenuous use the greater time and depth.
8). Wait at least 12 hours after a dive before you fly.
9). If a decompression stop was omitted, discontinue diving, breath oxygen, rest, and drink plenty of non alcoholic fluids. Keep a eye open for symptoms of decompression sickness. If evident proceed to a nearest recompression chamber, DO NOT RE - COMPRESS IN THE WATER. Also refrain from diving for at least 24 hours.
BSAC DIVE TABLES
HISTORY OF THE BSAC (1988) TABLES.
The BSAC (1988) Tables, published by the British Sub Aqua Club 1988 supersede the old RNPL/BSAC Tables. These tables are based on a decompression model which utilises a single tissue compartment. These tables assumes that off - gassing is slower than on - gassing.
The table designer Dr. Tom Hennessy, believes that bubbles form after every decompression, and that these bubbles affect the gas uptake and release for each subsequent dive. His research has led him to believe that a diver may saturate more rapidly during a repetitive dive than during an initial dive of the same depth and duration, since any bubbles present will increase the uptake of nitrogen during the dive. Therefore the rates of gas uptake and elimination will vary from dive to dive, and it becomes necessary to treat the second and subsequent dives differently from the first when trying to predict safe decompression.
In contrast different decompression models assume that gas uptake and elimination occur at the same rate during any dive, and the models assume that this rate is the same on a repetitive dive as it is on a single dive. This may be true if significant bubbling has not occurred within the blood and tissues, but if bubbles are present, they will slow down off - gassing and may increase the rate of on - gassing.
Hennessy set out to design a set of tables which become progressively more conservative as the number, depth and duration of dives increases. To avoid using a single dive model to predict repetitive dives, he has created a number of different tables to be used for different dives. These tables consist of a set of seven separate tables for use at sea level with additional tables for use at altitude.
The new tables utilizes a depth increment of 3m not 2m, and instead of giving bottom times, gives the time for leaving the surface until reaching 6m during the ascent, or at 9m on dives requiring a 9m stop.
DIVE TABLE TERMS.
1). DEPTH. The deepest depth during the dive.
2). ASCENT CHECK DEPTH. A point reached during the ascent where the Dive Time is checked against the dive plan and appropriate decompression procedures are initiated.
3). DIVE TIME. Time elapsed from leaving the surface to reaching 6m { 5m on level 4 } on the return to he surface. In the case of multiple - stop dives, the time from leaving the surface to arriving at the deepest stop on the return to the surface.
4). ASCENT TIME. Time from leaving the bottom to reach 6m { 5m at level 4 }. Calculated at a rate of 15m / minute, rounded up to the nearest minute and does not include decompression stops.
5). DECOMPRESSION STOP. Time spent at the actual decompression stop depth indicated.
6). SURFACING CODE. The code describing the divers tissue saturation state on surfacing from the dive.
7). SURFACE INTERVAL. Time from leaving the water after a dive of one dive to leaving from surface on a next dive.
8). NO - STOP DIVE. A dive for which the table indicates no need for in water decompression.
9) DECOMPRESSION DIVE. A dive for which the table indicates the need for in water stop or stops.
10). DESCENT RATE. The speed a diver descends, maximum rate allowed 30m / minute.
11). ASCENT RATE. Rate the diver ascents through the water. Maximum rate 15m / minute up to the safety stop and then, one minute should be taken from that depth to surface.
12). ATMOSPHERIC PRESSURE. Pressure chart is in the tables for correct level selection.
13). TRANSFER TABLE. The table enabling a new current tissue code to be obtained following increases or decreases in the atmospheric pressure.
14). NEW LEVEL. The atmospheric pressure level at which a new tissue code is required.
15). CURRENT TISSUE CODE. The code produced by applying a surface interval to your last dive surfacing code. { It indicates the table on which the diver can now dive.}
BSAC DIVE PROFILE SPECIAL RULES TO CONSIDER.
1). The maximum recommended depth for sports diving is 50 metres.
2). If doing two or more dives a day do the deepest dive first.
3). It is recommended that no more than 3 dives may be performed in any 24 hour period, and any dive series involving consecutive days to a depth level of 30m +, should be limited to four (4) days, after which, a 24 hour break should be taken.
4). It is advisable to limit any diving within a 24 hour period to dives which require a total of 20 minutes of water safety stops.
5). Maximum ascent rate to 6m or { 5m at level 4 } or first stop is 15m / minute, and from then, 1 minute to the surface.
6). It is permissible to conduct slower descents and ascents, whilst remaining within the dive profile envelope, but multiple "sawtooth " ascents and descents must be avoided.
7). BE AWARE THAT SMOKING, ALCOHOL CONSUMPTION, TIREDNESS, AGE CAN INCREASE YOUR RISK OF DECOMPRESSION SICKNESS.
THE EXPLANATION OF HOW TO USE THE TABLES IS SET OUT IN THE TABLE BOOKLET ITSELF THE TABLES ARE TOTALLY DIFFERENT TO ANY OTHER TABLES. BE SURE THAT YOU KNOW HOW TO USE THESE TABLES BEFORE APPLYING THEM.
PADI DIVE TABLES
HISTORY OF THE PADI TABLES
PADI Dive Tables once patterned after the U.S. Navy Dive tables, originated early in the 1960's. As new technology developed, PADI Dive Tables were revised to meet recreational diver profiles.
By early 1980, Dr. Raymond E. Rodgers, Institute of Applied Physiology and Medicine, discovered that recreational divers' dive nitrogen levels were almost all within a 40 minute tissue compartment for no decompression diving. Furthermore, a 'New" 60 minute tissue compartment (not one of the standard tissue compartments), covered all possible dive profiles for 'Recreational' diving. Divers following this profile remained well within limits where microbubbles would not form significantly to offset uptake or elimination of nitrogen.
Using updated technology, this was the first agency departure from the U.S. Navy 'Standard Tables' (single decompression dive tables). Dr. Michael Powell, Director of Hyperbaric Research Institute of Applied Physiology and Medicine, conducted research to validate or disprove Dr. Rodger's Hypothesis. Three years of research, involving dives in recompression chambers and open water, use of new technology such as the Doppler Ultrasound Bubble Detector, Hyperbaric Specialists, ie., Diving Science and Technology (DSAT) a corporate affiliate of PADI, expert consultation and review by Dr. Richard Vann of Divers Alert Network (DAN), all validated Dr. Rogers' Hypothesis, resulting in a restructuring of the Dive Table, now called the 'Recreational Dive Planner'.
The PADI Recreational Dive Planner (RDP) used Haldane's Principles, but utilized (6) extended tissue compartments (as with the U.S. Navy Dive Tables), with halftimes of 5, 10, 20, 40, 80, 120 minutes as reference. The 'M' values were also reduced below the values of the U.S. Navy tables, with the controlling tissue compartment centered on the fastest tissue to uptake and eliminate the most nitrogen over a given time. The designers assume that the uptake and elimination of nitrogen is symmetrical, (occur at the same rate within a tissue compartment due to a lack of micro-bubble formation), but also vary between the different tissue compartments (hence, the different halftimes). The PADI RDP was designed around the 'New' 60 minute tissue compartment for all 'recreational dives' based on Dr. Roger's hypothesis, resulting in conservative dive times at depth, shorter surface intervals, but longer more repetitive dives, which appears to more accurately reflect actual recreational diver profiles.
The PADI Recreational Dive Planner has now been in use since 1985. A recent modification is shaded areas three pressure groups prior to the NDL to remind divers to make a safety stop, air permitting.
DIVE TABLE TERMS
1) Actual Bottom Time (ABT): In repetitive diving, the total time actually spent under the water (in minutes) from the time of descent until leaving the bottom for a direct continuous ascent to the surface or safety stop. The safety stop minutes are not counted as part of the dive because due to the reduced ambient pressure, the diver more than likely will be exhausting nitrogen rather than taking in nitrogen.
2) Adjusted No-Decompression Limit: A time limit set for a repetitive dive by subtracting the previous dive's Residual Nitrogen Time from the No-Decompression Limit. Actual Bottom Time should never exceed the adjusted no-decompression limit.
3) Ascent Rate: Ascent no faster than 1 foot per second, or 60 feet per minute(18 m per minute). A slower ascent rate is acceptable, and appropriate, especially when using Dive Computers.
4) Bottom Time: The time from the beginning of descent until the beginning of a direct ascent to the surface or safety stop.
5) Decompression Diving: Diving in such a way that a series of timed depth level stops is mandatory prior to surfacing. Failure to make these stops may cause decompression sickness. In recreational diving (no-decompression diving), a decompression stop is considered an emergency procedure only, and is never an intentional part of the dive plan.
6) Dive Profile: A graphic representation of a dive plan or completed dive, which is used to avoid confusion and omissions when using the dive tables.
7) Multilevel Diving: Planning profiles that credit the diver for slower nitrogen absorption as he moves to a shallower depth. This provides more dive time. Only 'The Wheel' version of the Recreational Dive Planner can be used for multilevel diving.
8) No-Decompression Limit (NDL): The maximum time that can be spent at a depth before decompression stops are required.
9) Pressure Group: A letter used on the Recreational Dive Planner to designate the amount of residual nitrogen in your body.
10) Repetitive Dive: Using the Recreational Dive Planner, any dive made within six hours of a previous dive.
11) Residual Nitrogen: The higher-than-normal amount of nitrogen remaining in your body after a dive.
12) Residual Nitrogen Time (RNT): An amount of nitrogen, expressed in minutes , that is added to the actual time of a dive to account for residual nitrogen from a previous dive.
13) Safety Stop: A stop made at 15 feet for 3 minutes at the end of a dive for additonal safety. The safety stop is recommended after all dives, (air supply and other considerations allowing), and required on those to 100 feet ( 30 meters), or greater, and those coming within 3 pressure groups of the no-decompression limit, (a shaded area on the Recreational Dive Planner).
14) Surface Interval: The amount of time spent on the surface between two dives. It is usually recorded in hours:minutes (e.g. 3:25 - 3 hours, 25 minutes).
15) Total Bottom Time (TBT): The sum of Residual Nitrogen Time and Actual Bottom Time after a repetitive dive, used on Table 1 of the Recreational Dive Planner to determine the pressure group.
General Rules
PADI WHEEL
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General Rules
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