Non-conservative Dive Computers

[terrydarc]

Doc,
I was reading the thread on buying the best non-conservative dive computer and wondered if you'd like to comment on the idea. Being able to tune a dive computer sounds like a great idea to me, but how about trying to squeeze out the last few minutes of bottom time before DCS sets in (that's kind of the impression I got from the thread)? From what little I know about DCS, the idea of flirting with it gives me the shivers.
Terry

 

[Dr Deco]

Dear terrydarc:

I would also think that squeezing the last second of bottom time is a bad idea. However, the work that we have done at NASA shows that what happens during the decompression has also a great influence on the dive outcome. Remembering that the last stop is on the deck of the boat, the activities up there are also what influence the outcome of the dive.

We are not accustomed to this because we have always thought of decompression as being influenced solely by gas loading. This does not appear to be true. Yes, it is the major determinant of DCS risk, but it is not the sole determinant.

In our studies with altitude decompression, the incidence of DCS could vary ten fold (not 10%) depending on whether the subjects were seated or walking! Now, no one is going to say to a scuba diver that you cannot walk, but you can say that carrying tanks, climbing ladders, etc is a bad idea. :nono:

Most divers do not make long decompressions in the water. This is in the realm of technical diving, and even here virtually all cases of DCS occur after the diver has reached the surface. This is even if the decompression in the water is several hours long. Why? My guess is that the micronuclei in the tech diver’s tissues are compressed during the time at bottom. It is not until the diver climbs up onto the boat and walks around that the DCS arises. My guess is that it is when s/he is on the boat that the nuclei reform. This is speculation, but I bet it is close to the truth.

What does with mean with respect to conservative versus liberal computers? I believe it means that topside influences your dive outcome at least as much as the gas loadings. Thus, conservative is relative to what you do on the boat. Vigorous activity will offset a conservative computer (or table) and give you an “undeserved hit.” Contrarily, refraining from vigorous musculoskeletal activity will offset a liberal computer.

Now you can always participate in moderate activity on the boat, perform slow ascents, and make a safety stop and you will cover all of the bases.

Dr Deco

 

[terrydarc]

Thanks, Doc. Looks like there are other issues than conservative vs liberal settings on dive computers, but I'm still concerned with the attitude of walking the thin line between DCS and a healthy topside SI. Seems to me my PADI BOW didn't cover post dive surface activity very well, which must be at least as significant as gas loading. Physical activity in the water was indicated as a factor - maybe there could be more?

I'm also wondering if there's anything on the horizon that would help demystify DCS threat? I'm thinking of measuring or monintoring the onset of DCS factors before DCS itself becomes a serious problem. The system would involve one or more sensors and you'd continue to wear your dive computer for several hours on the surface. Seems like we already measure disolved O2 - think by shining a light through your fingertip. Maybe you'd need some sort of implant, but that'd beat the guess-and-by-gosh method that seems to prevail now. Anybody working on something like that?
Terry

 

[Dr Deco]

Dear Terry:

There is actually not a “thin line” between DCS and no-DCS. It is a curve where DCS risk slowly rises as bottom time is increased, not one that is an abrupt, “step change.” Your PADI class did not cover this because it is relatively new, found in our NASA research. While there is “nothing new under the sun,” it must be said that some of this “DCS with respect to activity” was known during the Second World War, but they had different goals in mind and different approaches.

Physical activity in the water has indeed been mentioned but this is with respect primarily to gas uptake during the dive. There seems to be little thought paid to the idea that offgassing is still occurring on the surface and that divers still have gas in their bodies. There are not any systems that monitor physical activity at the current time. Divers do have only the “by guess and by gosh method.”

Nevertheless, refraining from stressful activities such as lifting heavy objects, climbing ladders, playing beach volleyball, etc. is a big help.

Dr Deco