IDC Guide

PADI IDC Revision Dive Theory

The Recreational Dive Planner

You have got a shiny new Dive Computer? Oh, so you can skip this step.

Erm, no, you can’t. You need to know this part inside out. For your own safety and the safety of your students. Not to mention that as a dive professional you should set a good example and carry either the RDP or Wheel in your BCD pocket as a back up and also enforcing good diving practices. Unfortunately you can not take the ERDPML underwater! There is plenty more Dive Revision here about the RDP.


Where different tissues in the body release / absorb nitrogen at different rates.  Different parts of the body absorb and release nitrogen at different rates.  Blood and fat absorbs nitrogen easier and faster than muscle and bone.   Because of these different TISSUES, a decompression model has what we call THEORETICAL TISSUES or COMPARTMENTS.  Compartments are a way to measure / identify how fast or slow our body (and body tissues) absorb and release nitrogen.  FAST COMPARTMENT (blood) = absorb and release nitrogen fast SLOW COMPARTMENT (muscle and bone) = absorb and release nitrogen slowly.

Many dive computers now use the RGBM model which uses theoretical compartments as follows:

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


The rate at which the compartment absorbs / releases half capacity of nitrogen. Each compartment has a halftime for the rate at which it absorbs and releases nitrogen. A halftime is the time, in minutes, that it takes for a certain compartment to reach halfway from its initial tissue pressure to full pressure (saturation) at a new depth.

Compartments “on gas” and “off gas” at the same rate. Hence the controlling compartment for surface intervals of the RDP is the 60 min compartment which takes 6 hours (6 x 60 mins) to reach it’s M value. This compartment also takes 6 hours to empty, there for a a repetitive dive is a dive made within 6 hours of a previous dive, allowing for the 60 min compartment to completely empty.


  1. After 5 minutes the compartment will go to halfway (half full) 
  2. After 5 minutes more the half from the first 20 minutes will half again
  3. This halving can only happen 6 times exponentially. The new compartment will always be considered saturated (full) when it reaches approx. 98.6 percent.  To make it easier the tissue pressure can be expressed or called ‘metres’.
tissue halftimes

Pressure in Halftime Compartments

1          Halftime (50%) in 18m = 9m of pressure (18 x 0.5)
2          Halftime (75%) in 18m = 13.5m of pressure (18 x 0.75)
3          Halftime (87.5%) in 18m = 15.75m of pressure (18 x 0.875)
4          Halftime (93.6%) in 18m = 16.8m of pressure (18 x 0.936)
5          Halftime (96.9%) in 18m = 17.4m of pressure (18 x 0.969)
6          Halftime (100%) in 18m = 18.0m of pressure


A 5-minute halftime compartment will have how much tissue pressure 5 minutes after taken from the surface to 18 metres depth?

After 5 minutes the compartment will go half way to saturation.
So if this is a 18m compartment, then half of 18m is 9m.

Answer = 9m of pressure

To continue: After 10 minutes the pressure will go half way again.
Half the remaining 9m.  Half of 9m = 4.5m.
Add this to the first 9m (9m+4.5m) = 13.5m


The M-Value is the MAXIMUM TISSUE PRESSURE (nitrogen level) that is allowed to be left in the body after a dive.
The faster the compartment, the shorter halftime.
The slower the compartment, the lower the M-value
The higher the M-value, the more nitrogen it is allowed to have upon surfacing.
The M-value represents what does and does not result in DCS.
These M-values are the A-Z on the RDP
A = a low level (low pressure) of nitrogen
Z = a high level (high pressure) of nitrogen.
At 12m our halftimes are shorter, so we can have a high M-value

E.g.  We can dive at 12m for 147 minutes = Z (M-value)
At 30m our halftimes are longer so we have a low M-value
E.g.  We can dive at 30m for only 20 minutes = N (M-value)
The compartment that reaches its M-value first, is called the CONTROLLING COMPARTMENT

Why the US Navy (USN) Table was ‘Standard’ for Recreational Diving

The USN table was developed mainly for military decompression diving, but they became almost the standard in recreational diving until the mid-1980s for several reasons.   Before computers, developing a table was a difficult process that had to be calculated by hand.  Few outside the Navy had the information or the ability to produce tables.  Many early sport divers began as military divers, bringing the USN tables with them.  The USN tables were widely available in the public domain, allowing publishers to reproduce and re-arrange them.  Though they weren’t ideal for recreational divers, they could be relied upon when following accepted conservative diving practices.


The RDP, as we know it today, was developed in 1987 and tested in 1988 by Dr. Raymond E Rogers (a PADI dive master) working with DSAT (Diving Science And Technology).

Difference Between US Navy Tables & RDP

14 Compartments6 Compartments
60 minute surface credit120 minute surface credit
Halftimes have faster gas washoutHalftimes have slower gas washout
6 hours to be clean of nitrogen12 hours to be clean of nitrogen
Designed for multiple dives a day with a shorter surface interval (recreational diver)Designed for limited dives a day with a longer surface interval (military use)
Longer bottom time on repetitive divesShorter bottom time on repetitive dives

How Computers Compare with Each Other & the Rdp With Respect to Surface Interval & M-Values

Computers at times give longer no-decompression limits, because they:

  • Calculate the dive exactly
  • Eliminate unnecessary rounding (that you would do when using a table)

Spencer Limits, EE Washout

EE = ‘Exponential-Exponential’
Approximately the same M-value as the RDP
All compartments release theoretical nitrogen at the surface at their underwater halftime rate, 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 work.  E.g.  3 dives to 40m 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)

Spencer Limits, 60-minute Washout

Based on data for RDP
At the surface, all compartments 60 minutes and faster washout at 60 minute rate, all slower compartments wash out at their underwater halftime rate (like the RDP).
Dives very similar to what the RDP model allows.

Buhlmann Limits, EE Washout

Further reduced M-values (based on the work of Dr. Buhlmann) All compartments washout at their underwater halftime rate With reduced M-values repetitive dives similar to what the RDP supports, though repetitive deep dives with short surface intervals may still permit dives beyond what has been tested to work.  Spencer limits, 60-minute washout and Buhlmann limits, EE washout seem to be the most popular types of computers.

General Rules for Using the RDP

Pressure Groups on the RDP

Letters (pressure groups) CANNOT be swapped (are not interchangeable) between the RDP, USN or any other tables.
You can link pressure groups between different versions of the RDP, such as the wheel and the enriched air 32% and 36% recreational dive planner.

Cold Water Diving

When planning a dive in cold water or under conditions that may be strenuous (difficult), plan the dive as if the depth is 4m deeper than actual.

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.
Max depth of 30m on repetitive dives
Plan your deepest dive first

Limit Maximum Depths to Training & Experience

Discover Scuba Diver / Scuba Diver – 12m
Open Water Diver – 18m
Advanced Open Water / divers greater training and experience – 30m
40m is the maximum training depth for Deep Specialty Course
The 42m on the RDP is for emergency purposes only

Special Rules For Surfacing Pressure Groups

When planning 3 or more dives in a day:
If the ending pressure group after any dive is W or X the minimum surface interval between all next dives is 1 hour.
If the ending pressure group is Y or Z the minimum surface interval between all next dives is 3 hours Limit following dives to 30m / 100 feet or shallower

Safety Stops

Make a safety stop for 3 minutes at 5m 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:
After any dive to 30m or deeper
Anytime you will surface within 3 pressure groups of your NDL
When a dive is made to any limit (black box) of the RDP

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.

Emergency Decompression

An emergency decompression stop for 8 minutes at 5m 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 5m 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.

Deeper Than 40m?

If you accidentally go below 40m immediately ascend (18m per minute) to 5m and make an emergency decompression stop for 8 minutes.

If the no-decompression limit for 40m is NOT exceeded by more  than  5 minutes.  Do not dive again for at least 6 hours.

Missed Decompression Stop

If you accidentally miss a required decompression stop and have already surfaced and exited the water, remain out of the water and stop diving for 24 hours and breath pure (100%) oxygen if available

Diving at Altitude

Using the RDP at altitudes 300m above sea level requires the use of special training and procedures.
Add 4% to the depth for every 300m above sea level.  Conversion table is on page 15 in PADI Adventures In Diving manual.
Rule of thumb:- Subtract 0.035atm for each 300 m of altitude.

A lake is at 1800 m altitude.  A lift bag with 1 Lt of air inside is released from a depth of 10 m.  What will the air volume be on reaching the surface?
——–  =  6 x 0.035  =  0.21atm  – 1 atm  = 0.79atm

Pressure at 10m  = 1atm + 0.79atm = 1.79atm
1.79 x 1 Lt of air
———————- = 2.26 lts

A minimum surface interval of 12 hours is required before flying.
If you make daily, multiple repetitive dives for several days, or make dives that require decompression stops a minimum surface interval of 18 hours is required before flying.

Recreational Dive Planning (RDP) – The Wheel revision

Single Depth Dives

Use the book that came with your wheel for practise and revision
1st dive.            26 metres for 20 minutes

Side 1

Put the white arrow onto 26 on the outer edge.
Put the yellow arrow onto 20.
Read the yellow PG index where it cuts the 26m curved line.  = PG = ‘L’.
Surface time 30 minutes.

Side 2

Locate ‘L’.
Rotate wheel until 30 lines up with ‘L’.
Top of the white curved line = New PG ‘G’.
2nd dive.           24 metres for 15 minutes.
Side 1.
Move the white index until ‘G’ touches the 24m curve.
Move the yellow arrow onto 15.
Read the yellow PG index where it cuts the 24m curved line.  = PG = ‘Q’.

3rd dive.            18 metres for 25 minutes.

Find the minimum surface interval?

Side 1

Line the Yellow index until it touches the NDL cross hatch of the 18m curve

Move the White index until 25 mins lines up with the Yellow arrow, follow the 18 m curve up and check which White letter touches the 18 m curve. In this case it is the letter L

Side 2

Find the Q and then rotate until the top of the white line touches the bottom cross hatch for the L
Minimum Surface Interval time = 21 mins.

Recreational Dive Planning ERDPML Version

1st dive 26m for 20 mins
Turn on and make sure ERDPML is in metric mode.
Select ‘Dive Planning’ mode and press ENTER.
Multilevel ? Select NO, First Dive ? press YES
Enter Depth………Enter 26 m
NDL 27mins
Enter Actual Bottom Time……….20 mins
Pressure Group after dive “L”
Press Enter
Enter Surface Interval Time…………..30 mins and press enter
PG after SI = G……press enter

2nd DIVE 24m for 15 mins

Enter Depth………Enter 24 m……press enter
Adjusted No Decompression Time ANDL 16 mins…….press enter
Enter Actual Bottom Time………..15 mins…….press enter
ERDPML should beep and flash warning of a safety stop, ( RULE 2 )
Press Enter ……Ending Pressure Group R

3rd DIVE 18m for 25 mins

Select Surface Interval Mode….press Enter
First Dive Y/N……Enter No
Pressure Group end of Dive…….press number 7,3 times for R press enter
Enter Depth 2…….Enter 18m and press enter
ANDL = 50 mins…… enter
Enter ABT2……….enter 25 mins……press enter
ERDPML should beep and flash warning of a safety stop,( RULE 2 ) press enter
Minimum Surface Interval = 30 mins

Multi-level Dives (Up to 3 different depth levels)

1st level 28 metres for 10 minutes

As before.

Side 1

Put the white arrow onto 28 on the outer edge.
Put the yellow arrow onto 10.
Read the yellow PG index where it cuts the 28m curved line.  = PG = ‘E’.
(Note maximum multi-level (ML) time is the same as the NDL time = 23 minutes).
By following the 28m curve read at the centre maximum 2nd depth is 20 metres.

2nd depth 18 metres for 15 minutes

18 metres is below 20 so it is OK to continue.
Move the white index until ‘E’ touches the 18m curve.
Move the yellow arrow onto 15.
Read the yellow PG index where it cuts the 18m curved line.  = PG = ‘M’.
(Note maximum multi-level (ML) time is 28 minutes).
By following the 18m curve read at the centre maximum 3rd depth is 12 metres.

3rd depth 10 metres for 20 minutes

10 metres is below 12 so it is OK to continue.
Move the white index until ‘M’ touches the 10m curve.
Move the yellow arrow onto 20.
Read the yellow PG index where it cuts the 10m curved line.  = PG = ‘Q’.
(Note maximum multi-level (ML) time is 131 minutes).

Multi-level Dives ERDPML (Up to 3 different depth levels)

1st level 28 meters for 10 mins

As before, turn on, select mode, Dive Planning, press enter.
Multi level Y/N?……press YES
First Dive Y/N?…….press Yes
Enter level………press 28m…… enter
NDL = 23 mins… enter
Enter ABT………..enter 10 mins and press enter…PG = E after first level

2nd level 18 meters for 15 mins

Press enter
Enter level 2………enter 18m, press enter, ML = 29 mins……press enter.
Enter ABT……enter 15 mins and press enter
PG after level 2 = M……

3rd level 10meters for 20 mins

Press enter….
Enter level 3…..10 m and press enter…..ML = 130 mins….press enter
Enter ABT…….enter 20 mins….press enter
Pressure group after dive = Q

Recreational Dive Planners (wheel, table,ERDP)

1.         They are designed for recreational diving and use conservative limits to minimise bubble formation, even at levels that do not cause DCS.
2.         They were tested using Doppler technology not available when the Navy tables were developed.
3.         They contain more surface interval times for higher precision.
4.         They base surface interval elimination on half time that creates a realistic credit for surface intervals.
5.         The wheel / erdpml makes it possible to calculate multi-level dive times by giving credit for slower nitrogen uptake in shallow water.
6.         The wheel / erdpml allows the diver to more closely calculate time and depth profiles.

RDP Summary

Decompression theory is based on models and doesn’t have any relationship with real people (but it’s the best model we have so we rely on it)
All tables and computers are based on Haldanean decompression model, which are mathematical extrapolations.
First tables produced in 1906.
Different parts of the body absorb and release dissolved nitrogen at different rates.(different compartments / different halftimes)
Haldane’s original model had 5 compartments, from 5 to 75 minutes, modern versions have 14 or more, ranging from 3 to more than 600 minutes.
Each compartment has a halftime for the rate at which it absorbs and releases nitrogen.
Halftime = the time in minutes, for a particular compartment to go halfway from its beginning tissue pressure to saturation at a new depth, in exponential progression.
After 6 halftimes the compartment is considered saturated (actually 98.6%).
The difference between the dissolved nitrogen pressure and the surrounding pressure (whether ascending or descending) is called the pressure gradient.
M-value = the maximum tissue pressure allowed in the compartment when surfacing to prevent exceeding acceptable gradients.  (in our diving we only use the one that applies at the surface).
The M-value is calculated for surfacing at sea level, at an altitude higher than 300m the gradient may be too high unless you use altitude diving procedures.
The US Navy used Haldane’s tables and revised them periodically to meet changing needs.
The 1950 (current) US Navy tables had 2 important differences from the original Haldane model.
6 compartments were used with 120 minutes halftimes.
Surface interval credit added all divers in the previous 24 hours and were treated as one dive.
US Navy table were used until the mid 1980’s because:-
Many early sport divers were ex Military.
The US table were available to the public.
They could be relied on.
The original US Navy tables designed its surface interval credit on the worst case 120 minutes halftime.  This is why, worst case, it take 12 hours to be clean.
In the mid 1980’s Raymond Rogers a Divemaster recognised that the Navy tables might not be ideal for recreational diving.
The 120 minute surface interval credit was excessive for no-decompression dives.
The Navy tables were designed for males, fit and between 20 and 40 and not females etc.
Doppler ultrasound flow meters had not come into being.
During tests in 1987 and 1988 Rogers established:-
The 60 minute surface interval credit concept.
Extensive testing of multilevel diving.
Included a broader demographic test subjects.
The test were based on limiting Doppler detectable bubbles.
Multi-day testing successfully tested the RDP making 4 dives daily for 6 days.
Spencer 60 minute washout and Buhlmann EE washout seem to be the most popular types of computers

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