Are you struggling with decompression theory? Or would you like to revise some RDP and eRDPml questions in preparation for your Instructor Development Course? This guide has been created to support divemasters or I.D.C. candidates in preparing for their final exams. It's also highly beneficial to better understand dive planning as a professional.
In Part One, we looked at the history of decompression, but now we need to move on to instructor-level theory and do some exams.
The Haldanean Model
The Haldanean model consists of multiple theoretical tissue compartments. Gas always goes from a higher pressure to a lower pressure until equilibrated.
When the diver descends to a given depth, the nitrogen pressure in his breathing air is higher than the nitrogen tissue pressure in his body, so more nitrogen dissolves into the body tissues. These body tissues are represented in the Haldanean model by theoretical tissues (compartments)
With enough time, the nitrogen pressure equalises, and the body cannot take on more nitrogen. This is called saturation. (This process will repeat whenever a diver changes depth upwards or downwards).
When the diver ascends, the nitrogen in the body (tissue pressure) becomes higher than the nitrogen pressure in his breathing air (supersaturation), causing the tissues to release nitrogen to equalise the nitrogen pressure again.
The difference between the dissolved nitrogen tissue pressure and the nitrogen pressure in the breathing air is called the pressure gradient. This happens whether the diver is descending or ascending.
As you ascend, there is less pressure in your breathing air than in the gas dissolved in your mostly liquid body (or more pressure in your tissues than in the air you breathe, depending on how you look at it). The tissues can tolerate some gradient of high tissue pressure (supersaturation) without causing decompression sickness.
If the diver ascends slowly, decompression sickness can be avoided by keeping the pressure gradient within acceptable limits.
If the diver ascends too quickly, the pressure gradient exceeds acceptable limits, and bubbles may form and cause decompression sickness.
This means the diver must stay within the limits dictated by his table or computer and maintain a slow ascent rate as indicated by his tables or computer.
Haldane assumed that all bubbles resulted in D.C.S. as the Doppler Ultrasound had not yet come into existence. However, we now know that silent bubbles do not display symptoms of D.C.S.
There is no direct relationship between the Haldanean model and the body. The relationship is implied based on actual dive data. Like all models, you can only rely on a Haldanean model as far as it has been shown to work in tests and by field experience. Models are imperfect, so there is always some risk, even if you remain within the limits.
THEORETICAL TISSUES or COMPARTMENTS
FAST COMPARTMENTS (BLOOD AND MUSCLE)Â =Release and absorb nitrogen fast
SLOW COMPARTMENTS (FAT AND BONE) = Release and absorb nitrogen slow
Haldane developed a decompression theory for diving by considering that different parts of the body absorb and release nitrogen at different rates. Blood and muscles are faster than fat and bones. He created a mathematical model consisting of multiple theoretical compartments to account for this.
2.5 mins Blood
5 min Brain
10 mins – 40 mins Spinal Cord
40 mins – 120 mins Skin
120 mins – 480 mins Muscle
480 mins + Joints etc.
Blood supply is a major factor.
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To account for the differences, a decompression model uses several THEORETICAL TISSUES or COMPARTMENTSÂ so named because they do not directly correspond to any particular body tissue as the body does not absorb and release nitrogen on a singular basis. It is a means to measure/identify how fast or slow our body absorbs and releases nitrogen.
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Haldane’s original model used 5 tissue compartments. The U.S. Navy revised this to 6 compartments, and the RDP adapted this to 14 compartments to better suit recreational divers (more on this later).
Dive tables and computers track the theoretical amount of nitrogen dissolved in body tissues during and between dives.
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HALFTIMES
THEORETICAL TISSUE = COMPARTMENT
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FAST COMPARTMENTS ARE THOSE WITH SHORT HALFTIMES
SLOW COMPARTMENTS ARE THOSE WITH LONG HALFTIMES
Each compartment (theoretical tissue) has a halftime rate at which it absorbs and releases nitrogen.
A halftime is the time, in minutes, for a particular tissue (compartment) to go HALFWAY from its beginning tissue pressure to saturation (full) at a new depth in exponential progression.
After six halftime intervals, the compartment is considered saturated. For simplicity, tissue pressure is often expressed in msw/fsw gauge.
As we know from Henry's Law, gas always goes from a higher pressure to a lower pressure. So nitrogen dissolves in and out of the tissues until the pressure in the tissues is the same as the surrounding pressure.
Therefore, upon descent, the halftime is the time it takes for a tissue to become half saturated and, upon ascent, for a tissue to become half desaturated.
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Expressed as:
The rate at which tissues absorb and release nitrogen
or
The rate at which gases dissolve into and out of different tissues
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If a diver has not been diving for a while, then all compartments have zero nitrogen loading, but if a diver were to dive to 30 mt/100 ft, then, given enough time, all compartments will absorb nitrogen and will eventually reach saturation at a nitrogen loading of 30 mt/100 ft.
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If we consider a tissue with a 5-minute halftime, it will take 5 minutes to reach 50% (or 50% saturation). It will take a further 5 minutes to reach halfway again (from the current state to halfway to saturation), i.e., 75%
Therefore, after six halftimes, it would take a 5-minute halftime tissue half an hour to reach saturation
6 X 5 min = 30 min (100% saturation)
After six halftimes, we consider the tissue saturated for all practical purposes as it is 98.44% saturated and can never, mathematically, reach 100%. After 24 hours, we would consider the tissue completely saturated.
Each compartment in the Haldanean model will saturate at a different rate. We have seen the 5-minute model above. If we were to consider the 10-minute tissue, it would take 10 minutes to reach 50% saturation, another 10 to reach 75%, and so on until after six halftimes, the tissues reach saturation.
6 X 10 min = 60 min (100% saturation)
 Each tissue, therefore, has a different level of saturation. Based on their halftimes, fast tissues absorb and move toward saturation (and release upon ascent) inert gases faster than slow tissues.
The graph above is based on an immersion of 40 min with 5, 10, 20, 40, 80, 120, 160 and 200 min halftime tissues.
The 5-minute compartment is 50% saturated after 5 minutes; at 10 minutes, it is 75% saturated until, after six halftimes or 30 minutes (6 x 5 minutes), it is effectively saturated (98.44%).
The 10-minute halftime is 50% saturated after 10 minutes; at 20 minutes, it is 75% saturated, but after 40 minutes, it has only been four halftimes, so it is not yet saturated (93.6% saturated). It would take 60 minutes to saturate (6 x 10 minutes).
The 20-minute halftime would only be 75% saturated after 40 minutes (2 x halftimes), and the 40-minute halftime would only be 50% saturated after 40 minutes (1 x halftime).
All slower tissues,80, 120, 160 and 200, have not reached their first halftime and will be less than 50% saturated.
After 40 minutes, the pressure is released (or the diver ascends), and the compartments begin to desaturate in the same manner that they saturated. (E.E., Exponential uptake/Exponential release, more on this later)
The 5-minute halftime will desaturate in the same manner that it saturated, desaturating to 50% in the first 5 minutes, 75% in the next 5 minutes, and completely desaturated after 30 minutes. As you can see, it drops very quickly beneath the 10-minute compartment.
The 10-minute compartment also quickly drops soon beneath the 20-minute compartment.
To make it easy to measure how much saturation a tissue has at each depth, the tissue pressure is expressed or called mt of seawater (msw) or ft of seawater (fsw)
Example: A 10-minute halftime compartment will have how much tissue pressure 10 minutes after being taken from the surface to 18 mt/60 ft of seawater?Â
Answer = 9 msw/30 fsw of pressure
Metric | Imperial |
10 min 50% (1 half time) 50% of 18 mts = 9 msw After the first halftime, the pressure goes halfway, i.e., =9 msw | 10 min 50% (1 half time) 50% of 60 ft = 30 fsw After the first halftime, the pressure goes halfway, i.e., =30 fsw |
Example: A 20-minute halftime compartment will have how much tissue pressure 40 minutes after being taken from the surface to 24 mt/80 ft of seawater?
Answer = 18 msw/60 fsw of pressure
Metric | Imperial |
20 min 50% (1 half time) 50% of 24 mts = 12 msw After the first halftime, the pressure goes halfway, i.e., =12 msw 40 min 75% (2 halftimes) 75% of 24 mts = 18 msw After the second halftime, the pressure goes halfway from 12msw to 24 msw = 18 msw (meters sea water) | 20 min 50% (1 half time) 50% of 80 ft = 40 fsw After the first halftime, the pressure goes halfway, i.e., =40 fsw 40 min 75% (2 halftimes) 75% of 80 ft = 60 fsw After the second halftime, the pressure goes halfway from 40 fsw to 80 fsw = 60 fsw (feet sea water) |
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Example: How long would it take a 60-minute compartment to saturate to a given depth?
Answer: = 360 minutes (60 X 6 halftimes)
M-VALUES
DEEPER DIVES
THE FASTER THE COMPARTMENT (SHORTER HALFTIME)Â THE HIGHERÂ THE M-VALUE
The more nitrogen it can have when surfacing.Â
SHALLOWER DIVES
THE SLOWER THE COMPARTMENT (LONGER HALFTIME)Â THE LOWER THE M-VALUE
The less nitrogen it can have when surfacing.
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FASTERÂ compartments cannot control SHALLOWER dives.
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Within a given model, compartments differ from one another in two significant ways. Firstly, they absorb nitrogen, which is an inert gas, at different rates, and these rates are characterised by halftimes. Secondly, they can tolerate varying amounts of nitrogen, which is known as the M-value or allowable nitrogen loading.
This model works by determining how much each compartment absorbs for a given depth and time; when any compartment reaches its M-value, the dive ends or becomes a decompression dive.
The M-value is determined by test dives showing what does and does not result in D.C.S. or Doppler-detectable bubbles.
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M stands for MAXIMUM and is the maximum value of an inert gas pressure that a theoretical tissue can tolerate without presenting symptoms of D.C.S.
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The M-values are the maximum tissue pressure allowed in the compartment when surfacing to prevent exceeding the acceptable gradient. If a diver exceeds the M-value of any compartment, they have an unacceptable risk of D.C.S.
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A compartment is capable of holding more pressure than its M-value (i.e., does not need to reach six halftimes (saturation) to reach its M-value), but once the M-value has been exceeded, it is dangerous to surface immediately (no-stop diving), and a decompression stop would be required to reduce the compartments nitrogen loading before surfacing (technical diving).
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M-value can be likened to the No Decompression Limits (N.D.L.) on the RDP.
When planning a dive using the RDP, bottom time is defined as the time from the beginning of descent to the beginning of ascent to the surface.
CONTROLLING COMPARTMENTS
The compartment that reaches its M-value first is the controlling compartment.
On deeper dives, fast compartments usually reach M-value first - this is why deeper dives have short no-decompression limits.
The fast compartments (like the 5-minute halftime) will fill the fastest and, as such, have higher M-values.
On shallower dives, the depth may be less than the M-value of some faster compartments. Therefore, a slower compartment controls the dive, and the model allows more no decompression time.
The slow compartments (like the 80-minute halftime) will fill the slowest and, as such, have low M-values. Â
It is because of this that FASTER compartments cannot control SHALLOWER dives.
In this example, for a dive to 30 mt/100 ft, the 10-minute compartment controls the dive because it reaches its M-value first after only 20 minutes
20 min = 2 x 10 min halftimes
One halftime is 50% (100ft/30mt) = 15 msw/50ftsw
Two halftimes are 75% (100ft/30mt) = 23msw/75ftsw M-value
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If we were to dive to 18 mt/60 ft, the 60-minute compartment would control the dive as it would reach its M-value first after one halftime.
1 x 60 min halftime takes 60min to reach 50%
50% of 18msw/60 fsw (the depth) = 9msw/30 fsw. (The M-value is set here for this tissue, hence the controlling tissue).
The 10-minute compartment would never reach its M-value pressure and will never be the controlling compartment to any dive less than (23 mt/75 ft).
Any pressure more than the M-value makes no-stop diving dangerous as you cannot safely ascend directly to the surface (regardless of how many halftimes are left before saturation), and you will need decompression stops (you are outside of the safe pressure gradient and now into technical diving).
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If we were to dive shallower than 12 mt/40 ft, neither the 10 nor 30-minute compartment would ever be the controlling compartment.
And this is why FASTERÂ compartments cannot control SHALLOWER dives.
SURFACE INTERVAL CREDIT AND WXYZ RULE
60-minute surface interval washout vs. Exponential/Exponential
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Exponential/ Exponential (E.E.)
Nitrogen washes out the way that it washes in.
This means that as we ascend, the nitrogen will wash out fast and decrease its rate of release the longer that we are on the surface.
This is the E.E. washout theory (Exponential/Exponential).
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The 60-minute surface interval credit
The RDP surface interval credit table was designed with a 60-minute gas washout tissue compartment.
This means that all 14 compartments are assumed to wash out at a halftime of 60 minutes.
Fast washout (Deep depths/shorter halftimes)
will have an overestimated nitrogen loading in any compartment faster than the 60-minute compartment (i.e., 5-minute, 10-minute compartments). This means it thinks there is MOREÂ nitrogen in the compartment than there actually is.
(overestimation does not pose a problem).
A 60-minute halftime compartment will wash out exactly the same in both.
Slow washout (shallower depths/longer halftimes)
will have an underestimated nitrogen loading in any compartment slower than the 60-minute compartment (i.e., 80-minute, 100-minute compartments. This means it thinks there is LESS nitrogen in the compartment than there actually is.
(underestimation does pose a problem).
The RDP will underestimate the nitrogen levels in slow compartments after a surface interval. The difference is much more pronounced on shallower dives (18 mt/60ft and less) than on deep dives.
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 The W.X. and Y.Z. rule was implemented to accommodate the fact that any compartment slower than 60 minutes could control a repetitive dive.
THE WXYZ RULE
To make allowances for the slow compartments, the W.X.Y.Z. rule enforces a longer surface interval so that you do not exceed the M-value of a slow tissue on a repetitive dive.
You can only end up in a W.X.Y.Z. group after a long dive to 18 mt/60 ft or less.
The W.X.Y.Z. rule applies when you plan three or more dives daily, and the final pressure group after any dive is W, X (1 hour)Â or Y, Z (3 hours).
The special rule for multiple repetitive dives (W.X.Y.Z. rule) was developed to account for extended no-decompression bottom times at shallower depths.
RDP VS. NAVY TABLES
The U.S. Navy tables were the standard in dive decompression theory until the mid-1980s. In the 1950s, the Navy conducted research and made changes to the tables developed by Haldane. One of the changes was the addition of a sixth compartment with a maximum halftime of 120 minutes, as Navy data showed that there were body areas with longer halftimes than Haldane's longest compartment of 75 minutes, which had been previously assumed to be sufficient. Also, credit for surface interval for repetitive diving was taken into consideration. Previously, all dives within a 24-hour period were added together and treated as a single dive.
The U.S. Navy developed dive tables that were designed for decompression diving. They reasoned that the worst case for any possible dive situation (e.g. a repetitive no-decompression dive) was if the slowest compartment controlled the repetitive dive.
 As a result, the U.S. Navy tables and their surface interval credit are based on the 120-minute compartment’s nitrogen elimination. This is why it takes 12 hours (720 minutes or six halftimes) for a dive to no longer be considered a repetitive dive with the U.S. Navy tables.
The U.S. tables were developed mainly for military decompression diving, but they became almost the standard in recreational diving until the mid-1980s for several reasons:
Before computers, making a table was a complex process that had to be computed by hand. Few outside the Navy had the information or the ability to produce tables
Many sport divers began as military divers, bringing the U.S.N. tables with them
The U.S.N. tables were widely available and public domain, allowing publishers to reproduce and rearrange them
Though they weren’t ideal for recreational dives, they could be relied on when following accepted conservative practices
Why the change?
In the mid-1980s, Dr Raymond E. Rogers realised that the U.S.N. tables, although reliable, might not be the best option for recreational diving.
The 120-minute surface interval credit, aimed at repetitive decompression diving, seemed excessively conservative for recreational divers, who make only no decompression dives.
The U.S.N. tables were designed for Navy divers, but this group didn't fully represent the demographics of recreational divers. Recreational diving includes females and people of various ages above and below the Navy.
Doppler ultrasound flow meters had emerged, and they showed that silent bubbles often formed at U.S.N. table limits, implying that lower M-values (which would reduce single-dive no decompression limits) might be more appropriate for non-military diving.
RDP was developed in 1987 and tested in 1988 by Dr Raymond E. Rogers, a PADI divemaster working with D.S.A.T. (Diving Sciences and Technology)
The RDP was developed using a multi-tissue model (as are the U.S. Navy Tables) because body tissues on-gas and off-gas nitrogen at various rates.
The 60-minute surface interval credit concept was established
This was the first extensive testing of the multilevel diving technique.
The test subjects included a broader demographic, more similar to the recreational diver population with females, a wider age range and differing physical types included.
The testing was based on limiting Doppler detectable bubbles, not just bends or no bends.
The RDP was successfully tested over multiple days, with four dives daily for six days. However, it is recommended to dive more conservatively.
Dr Rogers found that the U.S. Navy's 120-minute halftime for the surface interval was too conservative for no-stop diving; a 60-minute halftime was more appropriate.
This means that it offers about twice as much credit for surface interval time (or that the residual nitrogen time for a repetitive dive is roughly cut in half) than the U.S.N. tables.
The RDP model has 14 compartments ranging from 5 to 480-minute halftimes.
Surface interval credit is based on a 60-minute washout. The W.X. and Y.Z. Rules make sure slower compartments remain within accepted limits.
The RDP offers more repetitive dive time, but its maximum allowed nitrogen loading (M-value) is lower.
RDP 60-minute surface interval credit vs NAVY 120-minute surface interval credit
Pressure Groups are NOT interchangeable due to different surface interval tables.
Letter designations represent different theoretical nitrogen levels.
The RDP has more pressure groups than Navy tables. Pressure Group letters designate theoretical nitrogen levels based on the model, and since U.S.N. and other tables use different models, letters are not interchangeable between RDP, U.S.N. tables or any other tables.
Because they are based on the same theoretical model, pressure groups can be transferred between the RDP Table and the E.R.D.P.M.L. You can also link pressure groups between different versions of the RDP, such as the wheel (old), eRDPML and enriched air 32% and 36% recreational dive planner.
RDP Â No stop diving
It was designed for multiple dives a day with a shorter surface interval (Recreational use). Longer bottom time on repetitive dives. It is designed for different sexes, ages, and body shapes. | US NAVY TABLE Â Staged decompression diving
Designed for limited dives a day with long surface intervals (Military use). Short bottom times on repetitive dives Designed for fit and healthy young men primarily in their 20s and 30s |
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Decompression Theory Part 1
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DIVE COMPUTERS
How do computers compare with each other and the RDP with respect to surface interval credits and M-values?
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Dive tables and computers track the theoretical amount of nitrogen dissolved in body tissues during and between dives.
Computers at times give longer no decompression limits because they:
• Calculate the dive exactly
• Eliminate unnecessary rounding (that you would do on a table)
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Spencer,60-minute washout, and Buhlmann, E.E. washout seem to be the most popular types of computers.
(E.E. stands for Exponential Uptake/Exponential Release) All compartments release nitrogen at the surface at their underwater halftime rate
SPENCER LIMITS, EE WASHOUT
Approximate the same M-values as RDP.
All compartments release nitrogen at the surface at their underwater halftime rate (E.E.), as compared to the RDP, which releases theoretical nitrogen at the 60-minute rate for all compartments of 60 minutes or faster.
This washout means these computers can permit dives beyond what has been tested to worthree, e.g., 3 dives to 40 mt/ 130ft in a row for 10 minutes each with only 30 minutes between them.
This washout is not a problem if divers avoid multiple deep dives with short surface intervals (generally not recommended whether using a computer or not).
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SPENCER LIMITS, 60-MINUTE WASHOUT
Based on data for RDP.
At the surface, all compartments 60 minutes and faster wash out at a 60-minute rate; all slower compartments wash out at their underwater halftime rate.
Dives very similar to what the RDP model allows.
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BUHLMANN LIMITS, EE WASHOUT
Further reduced M-values (based on the work of Dr. Buhlmann).
All compartments wash out at their underwater halftime rate.
With reduced M-values, repetitive dives are similar to what RDP data supports, though repetitive deep dives with short surface intervals may still permit dives beyond what has been tested to work.
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R.G.B.M. (Reduced Gradient Bubble Model) and others
Research is providing lots of new information on divers' behaviour and microbubble build-up.
Most dive computer models take this into account.
If a diver exceeds a safe ascent rate on one dive, he will be penalised on repetitive dives, the same with yoyo profiles.
Some take the water temperature into account and adjust accordingly.
Nearly all have altitude settings and settings for conservatism.
Some are integrated with the air supply and consider the diver's breathing rate.
Nearly all models now support Nitrox diving.
Some support gas switch extended range and technical diving.
Some support Trimix and C.C.R. diving.
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RECOMMENDATION FOR DIVING WITH COMPUTERS
Computers have the same theoretical basis as tables, so one is neither better nor safer than the other.
Calls for following the same general recommendations diving with tables, such as deepest dives first.
Follow all manufacturers’ recommendations.
End the dive based on the most conservative computer of a buddy team (you’re supposed to stick together anyway)!
If a computer fails whilst diving, ascend slowly to 5mt/15 ft and make a long safety stop, as long as your air supply permits. You should then remain out of the water for 12 – 24 hours to start clear again with another table or computer.
Make sure it is capable of altitude diving if diving at altitude.
Divers should not attempt to share a dive computer.
Should not be shared by two or more divers on a dive.
Each diver must use the same computer through a series of dives.
Each diver must have their own computer.
Do not lend your computer to another diver if either of you has been diving.
Do not use a computer from another diver if either of you have been diving.
When diving with a group of divers using computers, follow the limits of the diver with the most conservative computer.
Do not try to change the battery between dives or underwater.
If it caters for mixed gas, ensure it is set to the gas you use.
Do not use the computer if it is displaying any error or not functioning correctly.
When you turn your dive computer on, do not turn your brain off; after all, the latter is a better computer.
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FACTORS PREDISPOSING DCS
Body Fat
Exercise Before & After
Age
Dehydration
Injury & Illness
Alcohol Before and AfterÂ
Carbon DioxideÂ
Cold Before and AfterÂ
Hot showers after a dive
Altitude & Flying After Diving
Patent Forman Ovale (P.F.O.) Hole in the heart
History of D.C.S.
Smoking.
Most of these predisposing factors somehow result in or cause a change in circulation.
For more information on why these are factors, check out our blog on Physiology.
GENERAL RULES AND RECOMMENDATIONS FOR THE RDP
The reason for the two versions of the RDP is to have a familiar format (RDP) and one that offers more precision and enables multilevel dives (eRDPML).
No-Stop Diving
A dive during which the diver does not exceed their dive table or computer limits and can, therefore, safely surface (at a safe ascent rate) without stopping.
Decompression Diving
When a diver is required to make one or more stops during their ascent to give their body time to safely release the nitrogen (or other gas, such as helium) that dissolved into their tissues during the dive.
Pressure groups on the RDP
Letters (pressure groups) can NOT be swapped (not interchangeable) between the RDP, U.S.N. or any other tables. You can link pressure groups between different versions of the RDP, such as the wheel, eRDPML and enriched air 32% and 36% recreational dive planner.
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Cold Water or Strenuous Diving
When planning a dive in cold water or under conditions that may be strenuous (difficult), plan the dive as if the depth is 4mt/10 ft deeper than actual.
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Repetitive Dives
Plan repetitive (2nd or 3rd) dives so each next dive is to the same or a shallower depth. Don’t follow a dive with a deeper dive. Plan your deepest dive first—Max depth of 30mt/100 ft on repetitive dives.
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Limit Maximum Depths to Training & Experience
Discover Scuba Diver / Scuba Diver – 12mt/ 40 ft
Open Water Diver – 18mt/ 60 ft
Advanced Open Water / divers greater training and experience – 30mt/100 ft
40mt/130 ft is the maximum training depth for the Deep Specialty Course.
No recreational dive should be planned deeper than 40 mt/130 ft. This is the absolute maximum depth for recreational divers.
The 42mt/140 ft on the RDP is for emergency purposes only.
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Special Rules for, W, X, Y & Z Surfacing Pressure Groups.
To account for extended no-decompression bottom times at shallower depths.
When planning three or more dives in a day:
If the ending pressure group after any dive is W or X, the minimum surface interval between all subsequent dives is 1 hour.
If the ending pressure group is Y or Z, the minimum surface interval between all subsequent dives is 3 hours.
Limit following dives to 30 mt/100 feet or shallower
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Safety Stops
Make a safety stop for 3 minutes at 5mt/15 ft after every dive (recommended). The time at a safety stop need not be added to the bottom time of the dive.
Always make a safety stop: (Required)
After any dive to 30 mt/100 ft or deeper. (Grey boxes)
Anytime you will surface within three pressure groups of your N.D.L. (Grey boxes)
When a dive is made to any limit of the RDP. (Black boxes)
Be a PADI S.A.F.E. diver. Slowly Ascend From Every Dive
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In-Water Recompression
In-water recompression – treating DCI by putting the diver back underwater shouldn’t be attempted. Recompression takes a long time and requires oxygen and often drug therapy.
Normally, the required resources aren’t available at a dive site, and incomplete recompression will usually make the diver even worse.
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Emergency Decompression
An emergency decompression stop for 8 minutes at 5 mt/15 ft must be made if a no-decompression limit is accidentally exceeded by 5 minutes or less. Upon surfacing, the diver must stay out of the water for at least 6 hours before making another dive.
If a no-decompression limit is exceeded by more than 5 minutes, a 5 mt/15 ft emergency decompression stop of no less than 15 minutes is needed (air supply permitting). Upon surfacing, the diver must remain out of the water for at least 24 hours before making another dive.
Missed Decompression Stop
If you accidentally miss a required decompression stop and have already surfaced and exited the water, remain out of the water, stop diving for 24 hours and breathe (100%) oxygen if available. Seek medical assistance if signs or symptoms occur.
Deeper than 40mt/130 ft?Â
If you accidentally go below 40 mt/ 130 ft, immediately ascend (18m per minute) to 5 mt/15 ft and make an emergency decompression stop for 8 minutes. If the no-decompression limit for 40 mt/130 ft is NOT exceeded by more than 5 minutes. Do not dive again for at least 6 hours.
Flying and Ascending to Altitude After Diving
The recommendations about flying after diving at altitude are the same as at sea level.
For a single dive within the no decompression limit, a minimum surface interval of 12 hours is required before flying.
If you make multiple repetitive dives and/or dive for several days, a minimum surface interval of 18 hours is required before flying.
For dives requiring decompression stops, a minimum preflight surface interval greater than 18 hours is suggested.
There are currently no official guidelines for driving to higher altitudes after diving, whether it's after an altitude dive to a higher altitude or from sea level to altitude. The most prudent practice is to be conservative: The longer you wait before ascending to altitude after diving, the lower the risk. Check with a local dive centre, resort, or instructor to see if there is a specific recommendation or protocol that local divers follow.
Diving at Altitude
Above 300mt to 3000mt
Because depth at altitude must be converted into a theoretical equivalent depth at sea level, special procedures must be implemented when using the RDP at altitudes over 300 metres/1000 feet.
Add 4% to the depth for every 300mt/1000 ft above sea level. A conversion table is in PADI Adventures in the Diving manual.
A capillary depth gauge automatically compensates for high altitude because it operates based on Boyle's Law, rather than on a direct measure of the ambient pressure.
Best for when diving at altitude.Â
They are inexpensive and reliable but hard to read accurately much deeper than 10mt/ 30ft. It will show a relationship of atmospheres rather than actual ambient pressure, which is why diving at altitude requires special procedures and training.
Altitude, Boyle, Capillary (A.B.C.):
For more information, check our blog on equipment.
The PADI RDP, other tables and dive computer models assume surfacing at sea level, and so diving at altitudes above 300 mt/1000 ft requires special dive decompression procedures because there’s less atmospheric pressure at the surface, which affects dive table and dive computer calculations, i.e., the ambient atmospheric pressure at altitude is less than at sea level.
When surfacing at sea level, the M-value is calculated accordingly. At altitudes higher than 300 mt/1000 ft, the reduced atmospheric pressure when surfacing could make the tissue pressure gradient (the difference between the pressure of the nitrogen dissolved in the tissues and the surrounding/ambient pressure) too high. This increase in the difference in the pressures of dissolved nitrogen in the body and the ambient pressure raises the risk of D.C.S. To ensure your safety, you'll need to use altitude diving procedures and adjust for the gradient using high-altitude diving protocols. Therefore, it's crucial to know your approximate altitude when diving.
According to dive decompression theory, actual depths must be converted to theoretical depths to find No Decompression Limits (N.D.L.) on the RDP. To use theoretical depth tables, you must know the altitude of the dive, and the special procedures include converting the actual depths to theoretical depths.
There is a decrease in the partial pressure of gases at altitude, but the percentages remain the same.
A slower ascent rate of 9m per minute is used when diving at altitude.
Hypoxia and hypothermia are issues at altitude
Recommended no more than two dives a day
Make a higher altitude dive first, followed by a lower
With the RDP, your first option is to wait six hours after arriving at altitude before making your first dive. Much like after a dive when diving at sea level, for planning purposes, this clears residual nitrogen.
Concerns for flying apply after diving and driving to higher altitude
Safety stop between 4mt – 3mt
When diving in near-freezing water, an environmental seal on your regulators is important to prevent ice buildup in the first stage.
Nitrogen narcosis may occur at shallower depths when diving at altitude!
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Decompression Theory Part 2
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RDP
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Recreational Dive Planner (RDP)
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Would you like to test your knowledge on
Decompression Theory
Part 1
Would you like to test your knowledge on
Decompression Theory
Part 2
Would you like to test your knowledge on
The Recreational Dive Planner (RDP)
Would you like to test your knowledge on
The electronic Recreational Dive Planner multi -level (eRDPml)
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