Gas Management Questions?

[TSandM]

There are arguments on both sides, Guy. Safety stops are optional, but less so as the dive becomes deeper and longer (note the PADI "mandatory" safety stops from certain pressure groups!). If I had a dive buddy clog a dip tube and come to me for gas, I'd rather do the stops, if my buddy is calm. If he isn't, I may have trouble just keeping the ascent rate under control! I plan for enough gas for the stops, and either take a bigger tank, or muti-level the dive so that I'm always within limits. It's not that hard to do, nor does it impact one's dive much, except in the case of deep, square profiles done on single tanks. Errol Kalayci has an eloquent post here on SB about doing the reef dives off West Palm Beach (which are often deep) on a single tank -- the numbers are daunting.

One of our local divers has a nice rock bottom calculator HERE. Or you can work your way through the problem -- figure one minute at max depth to sort out the emergency, and then whatever ascent profile you usually use (the calculator uses the GUE profile).

 

[Akimbo]

… The DAN statistics show that deaths from running out of gas out number DCS deaths by more than 50 to 1.

I believe that those stats are based on a presumed COD by the ME who fails to find evidence of the actual cause. Obviously if the body is hopelessly tangled in fishing net or trapped under wreckage the presumed COD is becomes probable… but those are in the minority.

There are a lot of circumstances where bodies are recovered with otherwise functional empty cylinders, but the root cause is unknown. Did that person lose consciousness for some other reason but continue automatic respiration stimulated by CO2? Was there an undetected medical condition where they lost consciousness, the regulator fell out of their mouth and free-flowed until empty?

We have all read of cases where bodies were found in shallow water, say less than 60', and there are no signs of attempting to surface — no dropped weights, an empty BC, and no signs of struggle. That is inconsistent with my observations of OOA divers where a panicked attempt to surface causing barotrauma is a more probable outcome. I don’t care how hard you impress the importance of safety stops and controlled ascents; that takes a backseat to breathing every time.

This is not to diminish the importance of not running out of breathable gas, but I believe we are missing some other causes when looking at a 50:1 ratio. There is still a lot to learn and we shouldn’t suspend looking for answers that actually make sense.

 

[boulderjohn]

This is not to diminish the importance of not running out of breathable gas, but I believe we are missing some other causes when looking at a 50:1 ratio. There is still a lot to learn and we shouldn’t suspend looking for answers that actually make sense.

I think an important factor in that presumed ratio is simply that DCS has become so very rare. If you read the annual DAN fatality reports, you will see very, very few cases of DCS leading to fatality.

 

[Akimbo]

I think an important factor in that presumed ratio is simply that DCS has become so very rare. If you read the annual DAN fatality reports, you will see very, very few cases of DCS leading to fatality.

DCS fatalities have always been very rare outside of extreme cases of explosive decompression accidents, often caused by operational oversights rather than inadequate gas planning. This is especially true today with the high availability of treatment chambers and medications.

Unfortunately, far too many accident reports are inconclusive regarding the sequence of events that lead to diver fatalities. COD (Cause of Death) doesn’t necessarily mean the same things to a Medical Examiner and people trying to prevent future deaths. My reading of many accident reports has lead me to believe that blaming inadequate gas supplies or separation from a buddy is far too simplistic and misses solutions that can actually improve safety.

Understandably the evidence is simply unavailable in many cases. Recovering a body from depth often creates barotrauma from macro levels through gross tissue destruction. It doesn’t take much time immersed in sea water to destroy biochemical indicators. Sometimes all there is a hypoxic body, with water in the lung, an empty cylinder, and no detected medical abnormalities. That doesn’t mean that we should suspend our knowledge of what really happens when divers run out of air.

 

[guyharrisonphoto]

There are arguments on both sides, Guy. Safety stops are optional, but less so as the dive becomes deeper and longer (note the PADI "mandatory" safety stops from certain pressure groups!). If I had a dive buddy clog a dip tube and come to me for gas, I'd rather do the stops, if my buddy is calm. If he isn't, I may have trouble just keeping the ascent rate under control! I plan for enough gas for the stops, and either take a bigger tank, or muti-level the dive so that I'm always within limits. It's not that hard to do, nor does it impact one's dive much, except in the case of deep, square profiles done on single tanks. Errol Kalayci has an eloquent post here on SB about doing the reef dives off West Palm Beach (which are often deep) on a single tank -- the numbers are daunting.

One of our local divers has a nice rock bottom calculator HERE. Or you can work your way through the problem -- figure one minute at max depth to sort out the emergency, and then whatever ascent profile you usually use (the calculator uses the GUE profile).

Thanks very much for that link. I had found a site that had the formulas and did the calculations for a 130' dive on a 130' high pressure single, allowing for 4 minutes of mandatory decompression. My "rock bottom" number came out to 2000 psi. When I used the plug-in formula, it gave the exact same number! At least I could work the formulas correctly. So, 1500 psi bottom time . . . not very long for sure.

Can you give the link to Errol's post? West Palm Beach is where I do most of my diving.

Thanks

 

[TSandM]

Found the thread: http://www.scubaboard.com/forums/ad...ngle-80s-smart-choice-deeper-than-60-fsw.html

 

[sambolino44]

I'm glad to see another discussion of rock bottom; I think it should be more widely known. One of my early dives (#71) was the Blue Hole in Belize, 130 ft on an AL 80. A group of eight with one (maybe two?) DMs. It didn't seem reckless at the time. Then I read NWGD's website about rock bottom, then I took the NAUI deep diver cert course from him. I made two Excel spreadsheets, one to track and calculate my SAC, one to calculate rock bottom. I record my beginning and ending PSI on (almost) every dive. I now use a rule of thumb of never diving deeper in feet than my cylinder holds in cubic feet, and we always calculate rock bottom for anything deeper than 80 ft. The only time we have "individual" rock bottoms is when we have different cylinders (we have 3 100s and 1 80), but that almost never happens anymore.

My calculation of RB includes a deep stop (half max depth), a 3-minute safety stop, and a 500 PSI reserve. So, those things are on the conservative side. However, it's also based on 1.5 times our normal SAC, and that's something that could be variable enough to throw the whole calculation off. I bet two stressed, near-panic divers in a current may get nearer two or three times our normal SAC. There's also the "one minute to address the problem", which, in a real situation, would probably disappear before you knew it.

But to me, what's more important than getting it down to some specific number, is that the exercise of doing the calculation (even with my spreadsheet I go over every step of the calculation, not just entering an input and getting my RB) gives me an awareness of what's going on that I didn't have before. And the more times I do it, the greater that awareness gets. So, during the dive, even if I'm not actively thinking about it every second, it's never far from my mind. And with that rock bottom number right there on my slate, even if I get "confused" I have a reference to go back to.

Speaking of slates, we will often put two RB numbers, for instance 100 ft and 60 ft, on there.