Decompression controversies

[admikar]

I said numbers are random. What I want to know is it safer to be more aggressive at depth or closer to surface?

 

[fsardone]

I said numbers are random. What I want to know is it safer to be more aggressive at depth or closer to surface?

Probably the true answer to that question is 'it is not known'.
Dr. Pyle said it was feeling better adding a deep stop from deeper dives.

What is is known for 'almost sure' is that adding deeper stops does not allow to shorten shallower ones.

Hence I do the deep ones (lowering the GF low) and the shallower ones conservatively. Especially if diving helium and deep (50 or more meters).

It takes time and a lot of reading to learn what it is known on deco, it is not possible to truthfully answer your question. Sorry.

 

[KenGordon]

I said numbers are random. What I want to know is it safer to be more aggressive at depth or closer to surface?

If you are doing no stop dives, as you seem to imply, then only the low GF value is involved. The actual GF used as the limit is interpolated between the low value at the first stop and the high value at the surface. If there is never a stop then you cannot have exceeded the low value. Thus for calculation of NDL 30/85 is more conservative than 40/70, but for doing stops it may be the other way round.

 

[leadduck]

I have the opposite opinion. Some of the decompression trials conducted by the US Navy in the last couple decades are among the few studies where the exact same dive profile has been conducted hundreds (in some cases) of times under rigidly controlled laboratory conditions (depth, time, gas, work, temperature always practically the same). Because there are limited number of experimental subjects, some individual divers will dive the exact same profile on a few (or many) occasions. If ten divers undertake a dive profile one day, diver "A" might get DCS. On the next occasion the same ten divers undertake the exact same dive profile, diver "A" can be fine and diver "B" will get DCS, and so on. This sort of data is incontrovertible evidence that there is intra-individual variability in susceptibility to DCS (i.e. day-to-day variability - or "randomness" - in the same individual). The data that does not exist (as far as I know), is the same ten divers have not conducted the identical dive profile, say, 100 times, to identify if there is a diver "F" who gets DCS more often than everyone else, and a diver "J" who never gets DCS. Some inter-individual variability surely exists, we just have no idea how large it is, or if it is more important than the intra-individual variability.

Everyone thinks they are diver "J" until they get DCS.

David

I didn't mean to say there's no intra-individual variability. I agree there is. I meant that the DCS risk measured in these studies contains both intra-individual and inter-individual variability. Say, if there's a profile tested that has 10% DCS risk for a PFO patient, and 0% DCS risk for others, and 30% of test subjects have a PFO, then we'll observe 3% DCS risk.

I thought it was commonly accepted that some physiological risk factors significantly increase DCS risk, and that there's a significant portion of divers with these risk factors. I'm surprised that this is not an issue in these studies. Couldn't commercial divers save a lot of deco time on average by individually adjusted schedules?

 

[Kazeone]

Teaching decompression theory this way makes some sense.



Phys

What is most interesting to me is that all the suggested further reading is dated from 1978-1996 being the most recent. How long will it take to answer the questions of decompression accurately. I don’t mind being a human guinea pig when I dive “deep”. Every individual being unique means that you must experiment on yourself and find the profile that works best until some scientific conclusions can be drawn. Just my insignificant opinion.