Dear Readers:
As far as the discussion of which computer is best, Dr. Deco cannot really provide much assistance since it appears that much of this discussion is centered on hardware. At NASA [at least in the area in which I work], we do not deal with diving and thus have not had any occasion to deal with aspects of dive computers. Our research is devoted to altitude problems. Thus what I am saying is really only directed at the underlying, theoretical aspects of this science (or is it really an art?).
As readers of this BB will know, I have treated
decompression as a two-part problem:
- gas tensions, and
- nucleation.
As indicated in other postings under
Ask Dr. Deco, dive computers only deal with the first aspect, since this controls the major determinant of decompression sickness, that is, the dose of dissolved inert gas (generally nitrogen). However,
gas bubble formation is always a possibility, although in the usual case, recreational diving employs such small supersaturations that gas phase (= bubble) grow is negligible.
Many developers of dive computers will tell you publicly that their devices contain algorithms that are different from the standard Haldanian types. While this is well and good, my only question is, where can I see the data upon which this is based? The only extensive test series that was made of recreational dive schedules was that performed by Diving Science and Technology (DSAT) for the
Recreational Dive Planner and
The Wheel of PADI. This was then published in a booklet (that believe is still available)[Hamilton, RW, RE Rodgers, MR Powell, and RD Vann.
Development And Validation Of No-stop Decompression Procedures For Recreational Diving. Diving Science and Technology. (pp. 78 + appendix). February 28, (1994).
This is not to say that others may have not used those tests for their devices (Dr. John Lewis did for
Oceanic DCs) or that a different database was not assembled and analyzed (as Dr. Bruce Weinke did for his Reduced Gradient Bubble Model). The algorithms are not always given, although all can be derived by reverse analysis if one so desired.
What is important to remember is that all of these models are using paper bubbles - - bubbles that may not exist with the designated properties. Some devices claim that they will adjust the dive program for gas bubbles that form and
interfere with gas elimination. I personally have not seen any evidence for or against this concept, and I have never seen any data. Thus, when this is advertised as a plus, I must simply say that they are adding a total hypothesis that this is actually occurring. That a degree of conservatism has been added is, in my opinion, a good idea.
BUT, is there any laboratory substantiation for this? In most cases, I do not believe there is any strong foundation for the superiority of the algorithm. The individual features of the dive computer (ability to down load dive profiles, altitude diving, etc.)is a different matter.
:nono:
With respect to nucleation, the diver must always remember that he or she can control what is not in the dive computer,viz, tissue nucleation. The following should be observed after the dive:
[*] take the safety stop and move your arms and legs in the water;
[*] do not climb from the water with full gear;
[*] do not lift tanks and heavy objects when on the surface;
[*] move your limbs and do not lie down or sleep;
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[sp]Michael Powell, PhD Dr. Deco
[Edited by Dr Deco on 11-14-2000 at 02:19 PM]
