Deco Model tested by Dr. Ray Rogers and Dr. Michael Powell

[Scuba]

Dr Deco,

I have a computer, Sherwood Scuba Logic, that uses "the no decompression multilevel repetitive dive schedules successfully tested by you and Dr. Ray Rogers". These tests did not include repetitive dives deeper than 90 feet or Deco dives.

Do you know if the No Deco Limits (NDL) used by this computer or others that use your study reflect you findings up to the 90 ft. level? Or are your findings only considered when "multilevel repetitive" dives take place?

The reason for this question is that the manual offers a comparison of this computer's NDL's to the U.S. Navy NDL's, in which the computer's NDL's are consistently 20 - 31 % more conservative than the Navy's NDL up to the 90 ft level. (Actually up to 110 ft level) After which, from 120 to 160 ft, this conservative margin diminishes and even becomes more liberal than the Navy's NDL.

Will you please shed some light on this NDL change at deeper depths, aside from the obvious use of a different study or model? Ok, better, worse, insufficient tests, other.

Also, please shed some light on how the guidelines for the various deco models are determined? For example: the NDL may be 30 mins. at 80 ft. Why not 29 or 31 or X mins?

On your particular study referred to above, where subjects tested until they exhibited symptoms of DCS? Any comments on the size of the buffer zone protecting from DCS beyond the NDL limit guidelines?

 

[Dr Deco]

Dear Scuba:

Do the NDLs from your study reflect your findings up to the 90 ft. level ?

Actually there were test dives to 120 feet in the chamber during the multiday test series.

The NDLs are derived from the US Navy test program, and these values were employed to calculate the US Navy Dive Tables. When Dr Merrill Spencer performed his dive experiments, he noted that a given percentage of divers developed Doppler-detectable gas bubbles in Navy Do-d dives. Dr Spencer proposed that, for additional recreational diver safety, the USN NDLs should be reduced so that a smaller percent of the divers would develop gas bubbles. Dr Rogers used then used a reduction in the calculation of his table. [Curiously enough, while Spencer proposed the “Precordial Grading scale” for Doppler bubbles, he did not use it as a criterion for safety. I was actually the first to link Doppler score to DCS risk.]

The Navy NDLs can be plotted to yield a curve, but the USN limits do not produce a smooth curve, as one might suppose they would. These “wiggles” reflect the Navy test program more than anything. Ray Rogers simply smoothed the curve and thus developed a more regular set of points. To render this into a mathematical form, the “Root-t equation” (or the British diving scientist Dr Val Hempleman) was used. This relates the NDL to depths according to

Constant = (Depth)(square root of bottom time).

A more general form is

Depth = (Constant)(time)[exponent (-x)].

An equation similar to this last was used. One will note then that the pedigree of the Rogers NDLs lead back to the US Navy by way of revisions (by a given percent) derived from the Spencer Doppler test series.

Are your findings only considered when "multilevel repetitive" dives take place?

Since all recreational scuba currently avoids decompression and always allows for a direct ascent, the NDLs apply to both square wave and multilevel dives. Multilevel dives simply allow one to prolong the bottom time if one is willing or capable of making an ascent towards the surface (“repeating up”).

Please shed some light on this NDL change at deeper depths, aside from the obvious use of a different study or model?

The change in the deep NDLs with respect to the US Navy simple reflects a desire to produce an orderly set of NDLs without “wiggles,” which were thought to reflect non-physiological reasons. The reasoning is consistent in the Rogers development.

How are the guidelines for the various deco models determined? For example: the NDL may be 30 minutes at 80 ft. Why not 29 or 31 or X minutes?

The NDLs are generally traceable to the USN research and the modifications by Spencer’s work. This is true for the RDP and their derivatives, e.g., Oceanic. The NDLs are numbers that are rounded out. That alone should tell you that there is not a “knife edge” in decompression. More than calculated gas loads are involved.

Many of the manufacturers will tell you the pedigree. These are really straightforward. What are more difficult to determine is such things as medications because of cold water. There is not any data given to show what is really done or why. Is the bottom time lengthened because the circulation to the extremities shuts down in cold water. Or is the bottom time shortened because cold causes divers to move about more to warm themselves?

On your particular study referred to above, where subjects tested until they exhibited symptoms of DCS?

The diver test subjects were not exposed to longer and longer dives (bigger doses of nitrogen) until they acquired DCS. This is not done for recreational table development.

Any comments on the size of the buffer zone protecting from DCS beyond the NDL limit guidelines?

It is not easy to measure the “buffer zone” for a large group. The probability of DCS depends on the dose of nitrogen and the size of the tissue micronuclei present in the body. This is a continuous function; there is not a step from “no problem,” to “definite problem. You can develop tables that will yield DCS in 100% of the individuals. (This would not be simply 100% “bends,” because some subjects would also become paralyzed and some would die.) In the other direct, the dose can be reduced to such a low point that micronuclei do not exist that are capable of growing.

It is very difficult to determine safety margins when one deals with very low doses of inert gas. This is because the test database becomes progressively smaller. Clearly, if the DCS incidence is one in ten, this is easy to determine experimentally. If it is one in 100, the test is more difficult but feasible. When we get a good determination of a one in 1,000 incidence, laboratory tests are not really financially possible. For this discernment, it is necessary to resort to field data. However, many dives are needed, and they must be accurately logged.

The problem lies in a cut-off point, the minimum where DCS is possible. Certainly this is on the order of approximately 18 – 20 feet for a saturation exposure. For shorter dives (that is, those in the scuba range) many, many experiments would be needed.

Dr Deco :doctor:

 

[Scuba]

I was actually the first to link Doppler score to DCS risk

Way to go Doc. Thanks for all the good info.