Dive computers and DCS

[dmaziuk]

I too think that, unless/until proven otherwise, repeated exposure to high-grade VGE can't be good for you. I'm willing to accept the invisible bubbles as a measure of decompression stress even for that reason alone, not to mention the little detail about them being actually measurable. This, however, spun off of the usual whining about NDL numbers (previous post edited accordingly). My take on that: if you are in the position to routinely overstay the NDL: do a deco course and plan your gas for deco dives. NDL problem solved and you'll learn a few useful things.

If one knows one's at risk then one knows one need to adjust one's profiles accordingly (studiously avoiding "you"s . If I know I'll rash after 31 minutes at 81 feet, I'm not staying there even if my computer gives me an hour to NDL. But that's the opposite of "too conservative" that usually starts these threads.

 

[stuartv]

if you are in the position to routinely overstay the NDL: do a deco course and plan your gas for deco dives.

Or, if you are using a computer that is known to be "conservative", you could save yourself the $1500 or so you'd spend on a deco course tuition and required equipment and just buy a more liberal computer. There's still nobody that I've seen say that the DSAT computers are unsafe...

Of note: If you are staying down for your whole NDL, then taking a deco course might not help you anyway. If your normal diving is going out on recreational charter boats, they're probably not going to be very happy with you if you do a deco dive from a boat where everyone else is doing NDL dives. Not to mention the need to either dive doubles or carry a pony bottle (redundant gas supply being considered mandatory when doing a deco dive). OTOH, they're not likely to say anything if you dive to the NDL of a DSAT computer. And, tech charters are more expensive and harder to arrange.

 

[dmaziuk]

If your normal diving is going out on recreational charter boats, they're probably not going to be very happy with you if you do a deco dive from a boat where everyone else is doing NDL dives. Not to mention the need to either dive doubles or carry a pony bottle (redundant gas supply being considered mandatory when doing a deco dive). OTOH, they're not likely to say anything if you dive to the NDL of a DSAT computer.

Which is why I said "if you're in the position to routinely overstay your NDL". And I don't think DM'll be happy with you if he said "first one to 500 psi or 60 minutes" at the briefing and you tried to go for 90 minutes because that's what your DSAT NDL is and you have gills. Now, if you hire your own private boats and dive 120 doubles, then I expect you can afford a deco course too.

 

[stuartv]

Which is why I said "if you're in the position to routinely overstay your NDL". And I don't think DM'll be happy with you if he said "first one to 500 psi or 60 minutes" at the briefing and you tried to go for 90 minutes because that's what your DSAT NDL is and you have gills. Now, if you hire your own private boats and dive 120 doubles, then I expect you can afford a deco course too.

I have not had any trouble finding dive operators that let everyone dive their tank. I dove with Kona Diving Company in Hawaii in April. That is what they do and they will rent 100s or 120s if you want.

 

[scubadada]

Which is why I said "if you're in the position to routinely overstay your NDL". And I don't think DM'll be happy with you if he said "first one to 500 psi or 60 minutes" at the briefing and you tried to go for 90 minutes because that's what your DSAT NDL is and you have gills. Now, if you hire your own private boats and dive 120 doubles, then I expect you can afford a deco course too.

Hi dmaziuk,

I'm not sure where you dive. I do most of my diving in Boynton Beach and use 3 different operators. As I do not have my own tanks in Florida, I rent AL80s. I do have a reasonable RMV and I do dive the DSAT algorithm. On the wreck of the Castor and on a couple of the deep reef dives, I'm limited by my NDL and generally surface with no deco. On many of the shallower reef dives, I compromise and dive for a total run time of 65-75 minutes to assist the operator in staying on schedule. I think many divers do essentially the same thing and it works out well for all of us. This is recreational diving at its best.

Good diving, Craig

 

[dmaziuk]

This April on Roatan there was someone on our boat who regularly surfaced with 2000 psi after the 60-minute dives. On a single Al80. The rest of the group came up at or below 1000 and I really don't think the op would've let her just "dive her tank". Especially since 60 minutes runtime was set by the DM at pre-dive briefing. "Compromise to assist the operator" is an interesting way of phrasing it, but yeah, that is what she did.

 

[scubadada]

This April on Roatan there was someone on our boat who regularly surfaced with 2000 psi after the 60-minute dives. On a single Al80. The rest of the group came up at or below 1000 and I really don't think the op would've let her just "dive her tank". Especially since 60 minutes runtime was set by the DM at pre-dive briefing. "Compromise to assist the operator" is an interesting way of phrasing it, but yeah, that is what she did.

That's what it is, a compromise, I often come up with 1000-1200 psi, I could easily have 20 minutes or more of run time.

 

[dmaziuk]

"Compromise" as in "if I keep them out here for another 20 minutes they almost certainly won't sail away and leave me to swim back home on my own, but I'll definitely not get another ride on this boat ever. And with nobody they know either."

Anyway, the relevance here is that if I come up with NDL 99 on my scary conservative leonardo and you come up with NDL 999 on your bern-feeling liberal oceanic, do I get less "risk of DCS"? I don't think so. So in the context of liberal vs conservative dive computer algorithm, DCS risk is only relevant if you're taking all your dives up to the limits of what your computer considers "safe". Otherwise, meh.

 

[stuartv]

Anyway, the relevance here is that if I come up with NDL 99 on my scary conservative leonardo and you come up with NDL 999 on your bern-feeling liberal oceanic, do I get less "risk of DCS"? I don't think so. So in the context of liberal vs conservative dive computer algorithm, DCS risk is only relevant if you're taking all your dives up to the limits of what your computer considers "safe". Otherwise, meh.

So, your gist is that nobody dives to their NDL anyway (or at least, not on a regular basis), so worrying about which computer gives a longer NDL is a waste of time? And, those that do regularly dive to their NDL should not worry about having a liberal computer? Instead, they should spend a BIG chunk of time and money to become certified tech divers, then spend more money to dive so they can do their dives as tech dives, instead of rec dives?

It's nice (or sad, I suppose, depending on whether you're a cup full or empty person) that your dives are rarely limited by NDL. But, I don't think I'm particularly unusual in my diving. And most of my fun diving is NC wrecks, where the depths are such that I almost always stay down right up to my NDL (on a liberal computer) and still get out with at LEAST 900 psi in my tank (of Nitrox). Especially on the first dive of a 2-tank charter. The second tank depends. If it's one of the shallower wrecks (say, 60' to the sand), then I'll usually get low on air before I hit my NDL.

 

[scubadada]

Stuart's diving pattern is not at all unusual. @dmaziuk spends a lot of his time laughing on his back rather than diving