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

[emttim]

You can. But who's to say that the series of formulas is right? Unlike the gas simple gas properties manipulated in basic nitrox calculations, deco is theoretical. No one really knows what causes gas to bubble. Your tables, the wheel, a computer, probably figure something like 1.58ATA gradient. But there's nothing hard and fast about it.

Why are you wary of the 'red zone' but not of the 'green zone'? It just seems to me that if you take exception to the numbers you're getting out of a computer, you shouldn't be using that computer.

True, but I can see where the formulas are derived from because I've had a year of general chemistry which included gas compositions, partial pressures, gas laws, etc. so I know that how the formulas are set up makes intuitive sense at least to me...it's not a matter of someone telling me it works and me just blindly saying "ok!".

To add to that, as part of my equipment specialty segment of my AOW class my dive instructor gave us several articles, published papers, etc. on the theoretical models used by dive computers, the mathematics behind their use, and the explanation of why it's not black & white but necessarily a gray area between silent bubbles that exhibit no visible symptoms and bubbles that do present with symptoms and can be considered a "hit" so I'm very aware of the fact that it's not a black and white matter to begin with.

It's not that I take exception to the numbers given to me by my computer anyway....it's the general knowledge that in pretty much anything, getting into the red zone or red line is usually a bad thing. Would you red line the RPM in your car? How about driving through an intersection when the light is red? It's not a guarantee that red = bad but the possibility that red may mean be cautious that prompted my question.

I'm not asking whether the red zone on a dive computer's nitrogen absorption display is an insta-DCS zone...I'm asking whether it's a good idea to stay out of the red zone to reduce chances of DCS or not. I fail to see where that suddenly translates into me not trusting my computer and how I shouldn't dive with it; that has little if anything to do with my question. However, at this point, I don't think I'm going to get a particularly helpful answer so I'll leave it at that since I don't want to waste anyone's time arguing semantics, interpretations or otherwise hijacking this thread. I'll just ask elsewhere for opinions and thank you for the input so far.

Fair 'nuff. I was probably wrong, and that's beside the point anyway.

Agreed.

 

[Blackwood]

Thanks for sharing, I need to mull that around a bit.

Please do. I'd like to hear your thoughts.

FYI, I just came across this document on in water recompression. Interesting read so far:

http://www.google.com/url?sa=t&ct=r...KH3O0bYrfLYDdxDUA&sig2=Ph9dDssyqy2Q1BdhU-6oQQ

 

[Blackwood]

I fail to see where that suddenly translates into me not trusting my computer and how I shouldn't dive with it; that has little if anything to do with my question.

Philosophically, that's just how I interpret it.

I'm all about discussion rather than simple answers. However, if that's what you're after...

I'm asking whether it's a good idea to stay out of the red zone to reduce chances of DCS or not.

Sure, but with a caveat.



Is it a good idea for me to use a Louisville Slugger to increase my chances of hitting a home run?

 

[emttim]

Philosophically, that's just how I interpret it.

I'm all about discussion rather than simple answers. However, if that's what you're after...

Well, I've never been big into philosophy...too much pondering and not enough action for my tastes. I'm all about discussion too when I can contribute meaningfully to it. Fact of the matter is that you probably know more about decompression theory than me so not much I can contribute to the discussion at hand.

Sure, but with a caveat.

Is it a good idea for me to use a Louisville Slugger to increase my chances of hitting a home run?

Perhaps a good idea, but probably unnecessary, which answers my original question if I'm interpreting your question correctly.

 

[Blackwood]

Perhaps a good idea, but probably unnecessary, which answers my original question if I'm interpreting your question correctly.

What I meant was: staying "green" to reduce the chances of a DCS hit is a good idea only if staying "green" actually reduces the chances of a DCS hit. My suggestion is that it may not.

That said, it may be a good idea for other reasons too: gas consumption, temperature exposure, oxygen exposure, etc..

 

[Squishy]

While the procedure that Thal has posted is more accurate, I have a much simpler but almost as effective dead-computer plan.

Assuming that I have stayed within no decompression limits, I simply assume that I exited the previous dive at the highest pressure group on the PADI table, pressure group Z. The penalty for overestimating my N2 loading is a bit of extension of the surface interval. For example, if my true pressure group is Q, my surface interval will be 29 minutes longer (the time it takes to go from Z to Q) than if I had more precise information available.

There are some W,X,Y,Z rules that require an extended surface interval of either 1 hour or 3 hours in a series of 3 or more dives, but one can only truly reach these pressure groups with relatively shallow dives much longer than I can achieve with a single tank, even with 0.4cfm SAC, so I know that I can safely assume that those rules are not invoked in my typical dives.

That seems like a reasonably safe approach in the event of a computer failure. I am still pretty new to this and my inclination would probably be to call it a day if my computer failed. If I were on a dive vacation, though, this seems to give a reasonable option to salvage some dive time on the trip.

Interesting discussion! I appreciate all of the opinions.

-Mike-

 

[Thalassamania]

Please do. I'd like to hear your thoughts.

FYI, I just came across this document on in water recompression. Interesting read so far:

http://www.google.com/url?sa=t&ct=res&cd=1&url=http%3A%2F%2Fwww.thaiwreckdiver.com%2Ftechnical_diving_articles%2Fin_water_decompression_treatmentx.doc&ei=b348SJ6vMZCg8gTU7Pm3BA&usg=AFQjCNEPj-peP0Q8zKH3O0bYrfLYDdxDUA&sig2=Ph9dDssyqy2Q1BdhU-6oQQ

I'm familiar with it. Rich's article was published as "The Case for In-Water Recompression" in aquaCorps, No. 11:35-46, back when I was a member of the Editorial Board.

 

[mruseless]

Nope, I stopped calculating pressure groups about my 16th dive, because I kept going over the tables and I didn't see the point. I don't carry a backup computer either, although that may change since I just bought a new computer of the same brand. My plan if my computer fails is to abort the dive, no dives for 24 hours, then continue on a rental computer or tables.

Don't really know if that's a good plan or not.

 

[emttim]

What I meant was: staying "green" to reduce the chances of a DCS hit is a good idea only if staying "green" actually reduces the chances of a DCS hit. My suggestion is that it may not.

That said, it may be a good idea for other reasons too: gas consumption, temperature exposure, oxygen exposure, etc..

Good point. Outside of a possible dive injury, unless the diver is using a mixed gas such as nitrox, I see no other reason to stay within a certain range as far as nitrogen absorption goes unless the diver is interested in having a shorter surface interval before getting back in the water, which of course would require the diver to be in a lesser "pressure group" or whatever you want to refer to the amount of nitrogen loading on a computer dive.

Now if there is oxygen exposure to consider as well, that's one thing, but that's why I'm aware some divers set their O2 limits on their computers to 1.6 ata, but since the grey area for oxygen toxicity and CNS convulsions begins at around 1.6 ata, I'll happily reduce my dive time a little by setting my O2 limit at 1.4 ata. Based on my experience with nitrox calculations so far, even at 1.4 ata, you have more than enough bottom time.

Nope, I stopped calculating pressure groups about my 16th dive, because I kept going over the tables and I didn't see the point. I don't carry a backup computer either, although that may change since I just bought a new computer of the same brand. My plan if my computer fails is to abort the dive, no dives for 24 hours, then continue on a rental computer or tables.

Don't really know if that's a good plan or not.

I would imagine 24 hours would be enough time for nitrogen loading to clear down to nothing, since regardless of how much nitrogen you theoretically have in your blood at any given time after diving, isn't it always 24 hours and then you can fly? I'd assume if you can fly after 24 hours, you can consider your nitrogen loading back down to pre-dive levels, so I'd say it sounds like a good plan to me.

I'm interested to see what the more experienced people on SB have to say about this though.

 

[Blackwood]

I would imagine 24 hours would be enough time for nitrogen loading to clear down to nothing, since regardless of how much nitrogen you theoretically have in your blood at any given time after diving, isn't it always 24 hours and then you can fly? I'd assume if you can fly after 24 hours, you can consider your nitrogen loading back down to pre-dive levels, so I'd say it sounds like a good plan to me.

I'm interested to see what the more experienced people on SB have to say about this though.

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.