Do you still calculate pressure groups if you use a computer?

[emttim]

Most computers (though Charlie will hopefully correct me if I'm wrong) track 16 compartments.

They assume that compression and decompression is like a standard exponential growth and decay problem, and that different tissues have different half lives. So a 16 compartment model is tracking 16 half lives.

Now it's really up to you what you consider clean. Do you consider a 1% load to be fully decompressed? If so, somewhere between 7 and 8 halflives you'll be clean.

But what if you don't consider yourself to be fully decompressed until the load is 0.05%? Well that will take 12 halflives. So in an hour, that 5-minute compartment is clean, but a 2-hour compartment will take all day.

So there are a lot of assumptions at play. How much is fully loaded? How much is fully unloaded. What are reasonable halflives? Is 2-hours too much even for the dense, non-vascular tissues like bone?



I think that most models consider full saturation/desaturation after five cycles, though some might be four, some might be six. If you google ZHL-16B, you can probably find out the basis for what most computers are assuming, what their compartments are, what they consider to be the maximum allowable pressure gradient, etc..

If I recall correctly (from Walter's post), PADI's RDP table tracks 14 compartments (5, 10, 20, 30, 40, 60, 80, 100, 120, 160, 200, 240, 360, and 480 minutes with associated depths) for the dive portion, but chooses a single compartment (60-minutes) for surface interval/repet calculations.

Hmm, very informative response. Thanks for your input, I'll have to look up the specifics on my own dive computer for how it calculates its NDLs.

 

[Jandee]

I dive with 2 dive computers. My primary computer reads very conservatively, my second one is there just in case. I'm not really into having 2 of everything but when I replaced my old one with the new one I thought I might as well use both. Having the second computer came in handy on a long dive trip when the battery went dead on my primary even though I always use new batteries before a dive trip. I was able to finish the week with my back-up.

 

[Bottomfeeder Brodie]

i never fill out the tables...like you said....computer log and tables are on different planets.....

 

[muddiver]

I dive with 2 dive computers. My primary computer reads very conservatively, my second one is there just in case. I'm not really into having 2 of everything but when I replaced my old one with the new one I thought I might as well use both. Having the second computer came in handy on a long dive trip when the battery went dead on my primary even though I always use new batteries before a dive trip. I was able to finish the week with my back-up.

I do the samr thing, and it paid off recently when my NEW dive computer stopped working.

 

[bfw]

No need to log them, but as for calculating and keeping track of them, let me put it this way. I have 16 years developing software and firmware similar to what's in your dive computer, some of it even more critical to human safety, in devices subject to far greater regulatory scrutiny than a dive computer, and, based on that perspective, my approach to dive computers is the same as Reagan's approach to the Soviets - trust, but verify.