Quiz - Recreational Dive Planner - Tissue Compartment and Half-time

[rhwestfall]

Dive Decompression Theory - PADI IDC Koh Tao - Divemaster Course


Dr Raymond E Rogers and the RDP

During the early 1980’s, PADI Divemaster DR Rogers began examining the basis of the US Navy tables. He suspected that they were not ideal for recreational diving. He concluded 3 things:

120 minute halftime: Whilst appropriate for decompression diving, he noted that the 120 minute halftime for calculating washout/surface interval credit, might be overly conservative for exclusively recreational diving.

Test group: The test group used by the Navy, again whilst appropriate for military needs, didn’t represent the recreational diving population.

Conservatism: Doppler ultrasound flow metres found silent bubbles often formed on dives to the Navy limits, and a lower M-value might be more appropriate for non-military diving.

Hence, between the 120 minute compartment and its higher M-values, it was clear that the tables could be overly conservative or insufficiently conservative, depending on dive circumstances.

Following this dive decompression theory research, Rogers went onto develop the PADI Recreational Dive Planner – RDP to meet the needs specifically of recreational diving. The most significant decompression theory change to Rogers adaptation of Haldanes model, was the choice of a 60 minute halftime compartment as the basis of repetitive diving. As a result of this, the RDP gives about twice as much surface interval credit. This means the residual nitrogen time for a repetitive dive is roughly cut in half.

Example:

First dive: 18m for 30 minutes
Surface interval: 2:00

Residual Nitrogen time USN RDP
for a dive to 18m: 24 minutes 11 minutes

The RDP model also has 14 compartments with halftimes ranging from 5 minutes to 480 minutes. Roger implemented the WX and YZ rules to accommodate the fact that any compartment slower than 60 minutes could control a repetitive dive. The table also has lowered M-values.

The US Navy and other tables use different models to the RDP, and so pressure groups and letters are not interchangeable between the RDP and other tables. Letter designations represent different theoretical nitrogen levels.

Testing of the RDP covered a broader demographic range than the USN testing, with females, wider age range and differing physical types included.

 

[scubadada]

I happen to have the PADI DM book in front of me. It doesn't actually fully answer that question (or spell out the answer to the question here directly, although it can be inferred). The Navy tables are listed as being based on a "surface interval credit on the 120 minute compartment." It doesn't state anywhere how many compartments were used for the US Navy tables. Just that Dr. Rogers used 14 compartments in the RDP tables (and based them off the 60 minute compartment).

The answer

 

[saxman242]

The answer

Yup. What I meant in my "it doesn't directly say" is that no where does it outright say that "14/60min" is used, but the answer can be pieced together.

 

[TMHeimer]

Yes, I thought I was right with the 14/60. I remembered that however from the theory section of the DM manual when I took that in 2009, not the OW course/manual.
I was surprised that until now, 14/60 was trailing in votes.

 

[scubadada]

I believe I read somewhere, sometime, that the 40 minute compartment could have been used for SI credit but that they chose to be more conservative and revise the US Navy use of 120 min down to 60 min.

Repetitive diving was not very practical until the PADI RDP, 1988. Those of us old enough to have used the US Navy tables know this.

Edit: The 40 min half life is also discussed in the PADI Encyclopedia of Recreational Diving, cited by @tursiops. Some dives require the 60 min half life

 

[scubadada]

Yes, I thought I was right with the 14/60. I remembered that however from the theory section of the DM manual when I took that in 2009, not the OW course/manual.
I was surprised that until now, 14/60 was trailing in votes.

Ha, see post #2 Quiz - Recreational Dive Planner - Tissue Compartment and Half-time Not many folks voted early either. Still, less than half correct

 

[Brett Hatch]

I happen to have the PADI DM book in front of me. It doesn't actually fully answer that question (or spell out the answer to the question here directly, although it can be inferred). The Navy tables are listed as being based on a "surface interval credit on the 120 minute compartment." It doesn't state anywhere how many compartments were used for the US Navy tables. Just that Dr. Rogers used 14 compartments in the RDP tables (and based them off the 60 minute compartment).

I'm looking at Mark Powell's Deco for Divers, 2nd edition, 2019 printing, chapter 2 "Decompression Principles", page 30.

[...] Haldane's original paper assumed five compartments with half-times of 5, 10, 20, 40, and 75 minutes. US Navy tables published in 1937 and based on research by O.D. Yarbrough used only three compartments as the two fastest compartments were dropped (5 and 10 mins). Later revisions in the 1950s restored the fast 5 and 10 minute compartments as well as adding a slower 120 minute compartment for a total of six compartments. In further revisions Robert Workman expanded the number of compartments to nine by adding slower still compartments at 160, 200, and 240 minutes.

Powell does not offer exact inline citation for every line of text, however he does list references on a per-chapter basis. Chapter 2 includes six such citations. From the context, my best guess is that this chunk of history comes from the last cite of Chaper 2: "Workman, R.D. 1965. Calculation of decompression schedules for nitrogen-oxygen and helium-oxygen dives. Research Report 6-65, U.S. Navy Experimental Diving Unit, Washington D.C." The Rubicon Foundation has this paper on hand, but is down. I did find a copy of it here, which you might find interesting (if you are a huge nerd like me, that is).

 

[scubadada]

Good quiz question @Pedro Burrito

 

[tursiops]

I happen to have the PADI DM book in front of me. It doesn't actually fully answer that question (or spell out the answer to the question here directly, although it can be inferred). The Navy tables are listed as being based on a "surface interval credit on the 120 minute compartment." It doesn't state anywhere how many compartments were used for the US Navy tables. Just that Dr. Rogers used 14 compartments in the RDP tables (and based them off the 60 minute compartment).

You find the RDP info in the PADI Encyclopedia of Recreational Diving....

 

[saxman242]

I believe I read somewhere, sometime, that the 40 minute compartment could have been used for SI credit but that they chose to be more conservative and revise the US Navy use of 120 min down to 60 min.

Repetitive diving was not very practical until the PADI RDP, 1987. Those of us old enough to have used the US Navy tables know this.

Yup. This is briefly touched on in the PADI DM book and in more detail in the PADI Encyclopedia of recreational diving. Essentially computer modeling showed the 40 minute halftime suitable for almost all situations except for a "small percentage of profiles, mainly those that involve repeated, long, shallow dives." As the difference for most dives between the 40 minute and 60 minute halftimes aren't that different, they went with 60 to cover more cases. As mentioned above, the WX &YZ rules cover a few corner cases that require longer halftimes.