NOAA Nitrox 32 No-Decompression Table Questions

[100days-a-year]

The important thing to do is think about your physique, conditions etc when deciding how conservative you want to be.

What a dive computer can't do is know exactly how your body is reacting to the decompression stress. The best they can do is give a mathematical line in the sand where statistical analysis has confirmed there are an "acceptable level" of DCS hits. It doesn't know how YOU are doing, but how statistics suggest an average person will probably do.

What you as a diver have to do is look impartially at yourself and your potential dive and decide do I have any risk factors such as overweight, unfit, poor circulation, lack of hydration, cold weather, strong currents (which suggest a lot of strong kicking might be required) etc. If you do see risks there, increase your conservatism. If you get out unusually tired, it could well be subclinical DCS.

An oft quoted phrase here - new divers comment that they know very little about DCS, experienced divers comment about bubble formation, gas permeability, compartment types etc and scientists and professionals who study diving physiology comment that they know very little about DCS.

No offense intended to anyone but...most of the divers I know,have met or see on SB are subject to 1 or more of those risk factors and most are subject to several and more.Even with that given,my experience with recreational dives is that DCS is almost entirely unpredictable from day to day and individual to individual.

DISCLAIMER: DO NOT REGARD THE FOLLOWING AS A SUGGESTED DIVE PLAN

In my industry(commercial spearfishing)there is a huge body of anecdotal evidence that indicates that single dive BT exposure at depth is more critical than total daily dive BT exposure.We now make more short dives and get more daily bottom time with decreased incidence of DCS ie:6 dives of 20 minutes BT vs. 3 of 35 minutes at a given depth per day.The guys who did longer dives,even within NDL using nitrox saw far greater risk which reduced on the trips when they finally caught on.Only a couple dozen guys but over a lot of years averaging 400 dives a year per person.The least experienced of which has several thousand dives and most with well over 10K dives.

 

[boulderjohn]

It doesn't know how YOU are doing, but how statistics suggest an average person will probably do.

What you as a diver have to do is look impartially at yourself and your potential dive and decide do I have any risk factors such as overweight, unfit, poor circulation, lack of hydration, cold weather, strong currents (which suggest a lot of strong kicking might be required) etc. If you do see risks there, increase your conservatism.

Although this is generally good advice, there is a subtle contradiction in it.

As you say in the first paragraph, the statistics suggest how an average person will probably do. An average person will probably have some of the risk factors mentioned in the second paragraph. If not, then that person would not be average. You don't have to be a perfect human specimen to follow a standard dive algorithm.

 

[Dan]

Tables are a good lo-tech reference but are limited because they cannot account for your actual dive profile and rely on pressure groups for repetitive diving, which cannot consider all tissues so make a conservative guess as to which might be controlling. If using tables for planning, it's safest to use maximum depth. But if you're looking back for a comparison with your dive computer after you've finished diving, average depth would be a closer (but less conservative) approximation.
If you're interested, why don't you download your dives into Subsurface? You can then look at the NDLs (or deco requirement) calculated by Subsurface at any point in any dive. You can also see the effect of changing gradient factors or compare Buhlmann gf with VPM-B models. What's reported by your dive computer may also be shown (depending on what your dive computer can do and what data gets downloaded).

Thanks for pointing out Subsurface. I like what it shows. It's better than the SUUNTO DM5. I'll use Subsurface as my electronic logbook from now on.