Max Depth vs. Cylinder Capacity

[peterbj7]

Simplistic rules and formulae can be invaluable aides-memoire, but only that. If you can't readily work through the reasoning that produced the rule/formula you should NOT be using it.

There is far too much of a tendency these days in dive training, at all levels, to teach people to remember rather than to understand. It isn't only PADI and the like - as far back as ten years ago I criticised the then training manual of IANTD for exactly this. WHY do so many people say "I don't do maths" when all it is is simple logic. You might just as well say "I don't think logically". There are people in this thread who have said as much.

 

[lamont]

Keep in mind I wrote that article as self-psychotherapy after having dragged a dead diver to a dock who was diving to 100 feet on an Al80 and ran OOG.

I can also think of another 2 fatalities around of here of similar issues with OOG at 100 feet, plus there's been all kinds of near misses.

If you understand the limitations of that particular rule of thumb and want to violate it, by all means go right ahead, it wasn't directed you.

And I actually don't think that AOWs to 100 feet around here should be taught using Al80s. I'd like to see dive shops using larger steels for their AOW courses (and teaching some kind of halfway useful gas management). The "nobody could get AOW certified" argument isn't a good counter-example to me. Yes, I think nobody (up here anyway) should get AOW certified on an Al80.

A lot of the rest of the comments in this thread are basically trying to read that section of the document like its a legal document and taking it apart word by word and sentence by sentence, generally out of context with the rest of it. I don't know where all the discussion of overheads and technical diving comes from -- it seems obviously aimed at recreational diving and technical divers should have better tools in their toolchest -- bringing that issue up seems to be deliberately being obtuse for the sake of argument. And I stated, what I thought was extremely clearly:

"inexperienced divers (100 dives or less) doing coldwater dives to 100 fsw on Al80s are what this rule is squarely aimed at preventing."

Also, credit for that rule technically goes to Bob Bailey, I ripped it off from him.

And is it a "rule" or a "guideline"? I'm not interested in that level of navel gazing. If you don't see any sense to it, its pointless to try to clarify it at that level.

As far as the depth*10+300 rule, I explicitly mention a depth*10+100 rule of thumb for larger steel tanks right above it. The depth*10+300 rule works reasonably well for Al80-sized tanks in the 60-100 foot range. It is based on all the rest of the computations in that article, so if it seems too conservative then you probably have an issue with computing rock bottom using 2.0 cu ft / min gas sharing with a 3 minute stop + 30 fpm ascent + 1 minute bottom time -- if you disagree with that, we're at an impasse.

I also went on a dive as early as yesterday when we broke the Al80 deeper than 77 feet rule, and with a relatively new diver. But they had a clear rock bottom of 1800 psi and we spent about 5 minutes at depth, so it was more of a touch-and-go -- plus we had bigger tanks, including one set of doubles available in the water, with technical divers that I knew could donate gas, and the diver in question I've done s-drills with before. The pre-dive briefing included both the rock bottom pressures and the expectation of only 5 minutes at depth.

 

[lamont]

Simplistic rules and formulae can be invaluable aides-memoire, but only that. If you can't readily work through the reasoning that produced the rule/formula you should NOT be using it.

There is far too much of a tendency these days in dive training, at all levels, to teach people to remember rather than to understand. It isn't only PADI and the like - as far back as ten years ago I criticised the then training manual of IANTD for exactly this. WHY do so many people say "I don't do maths" when all it is is simple logic. You might just as well say "I don't think logically". There are people in this thread who have said as much.

The "be on the boat with 500 psi" rule, though, is ridiculous and that is what >90% of the divers seem to be taught.

After having dealt with an OOG fatality that would have been entirely preventable with *anything* better, I'll take the depth*10+300 rule as being simple enough to teach to anyone with bad math skills, and improve their chances of survival.

I'd love it if everyone in the world could do green's functions solutions to the partial differential equations, tensor calculus, and understand non-Abelian guage theory, but clearly not everyone is going to be that good at math.

 

[lamont]

And, if you want to write up a better article on gas management, by all means set yourself up with a blog and post it.

It actually takes a fairly large amount of effort to produce something like that.

Takes very little effort to make posts on scubaboard to pick nits over it.

So, put up or shut up. Where are your gas management guidelines for recreational divers?

 

[Akimbo]

…If you understand the limitations of that particular rule of thumb and want to violate it, by all means go right ahead, it wasn't directed you...

I hope that my comments were not taken as criticism of your thorough and well written article:

http://www.scriptkiddie.org/diving/rockbottom.html

My concern is that a small part may be used out of context or beyond your well stated constraints.

... Unfortunately, rules of thumb like Lamont's are too often taken out of context and become the end of the story rather than just the beginning...

My intent was to condemn the blind misuse of any rule, guideline, or whatever you choose to call it.

 

[sea2summit]

I took 2 years of post calculus, (decades ago), so I'm pretty good at math, but when I'm at 120' I'm hard pressed to add 4+4 much less multiply 10 x's my depth and add 300. What I can do though is remember, a maximum depth that we plan to go to and a turn around pressure.

Here's the tricky, your supposed to do it before you get in the water. If you're gonna go way into it you do your buddies too, calculating reserve in case they have to help you then you have your turn pressures. I know crazy.

 

[lamont]

My concern is that a small part may be used out of context or beyond your well stated constraints.

Well, that's always a risk.

Part of the reason why there's a whole kitchen full of recipes in my article is to present it in context. If instructors are presenting gas management rules, they should present it in context. Everything in general should be taken in context, and we culturally have a problem with facts taken out of context. You've got to start teaching people at some point, though.

I used "psi * 10 + 300" yesterday to present rock bottom to a new diver, probably without sufficient context, because I can't run through that whole article in a single pre-dive briefing. Since the divers in question are coming back and diving with us consistently, I can go further next dive and start to explain the larger context next time.

 

[lamont]

Here's the tricky, your supposed to do it before you get in the water. If you're gonna go way into it you do your buddies too, calculating reserve in case they have to help you then you have your turn pressures. I know crazy.

You also don't have a single reserve pressure, though.

If you plan a 90 foot dive and go to 110 feet, you have a different reserve pressure at 110 feet. If you plan a 110 foot dive and only ascend to 70 feet you've got another reserve pressure at that depth as well. Even at 20 feet you've got a rock bottom that you should ascend at, which isn't the turn-around reserve pressure, but you need to be able to look at your SPG and decide to end the dive due to gas at 20 feet.

Yesterday we spent 10 minutes swimming around at 10 feet videoing a bunch of big schools of fry we found near the end of the dive. I had less gas that point than my rock bottom for the whole dive, and had an absolute minimum gas reserve that would have caused my to thumb the dive if I hit that level.

You don't have an MGR for the dive, you have an MGR for that level on that dive, and really you can have multiple pressures -- at least two of them, the "turn the dive and swim back upslope" turn pressure and the "we need to directly ascend now" MGR.

Gas management isn't just memorizing one number per dive.

 

[sea2summit]

You also don't have a single reserve pressure, though.

No argument from me, I've got like 8 posts in this thread. Getting a little old trying to tell people to learn their SAC and actually calculate what they need.

 

[Akimbo]

Well, that's always a risk.

Part of the reason why there's a whole kitchen full of recipes in my article is to present it in context. If instructors are presenting gas management rules, they should present it in context. Everything in general should be taken in context, and we culturally have a problem with facts taken out of context. You've got to start teaching people at some point, though…

It is not my impression that anyone on this thread disagrees. I fear that my words are being interpreted in a way that was not intended.

…I used "psi * 10 + 300" yesterday to present rock bottom to a new diver, probably without sufficient context, because I can't run through that whole article in a single pre-dive briefing. Since the divers in question are coming back and diving with us consistently, I can go further next dive and start to explain the larger context next time.

I don’t think anyone on this thread had any concern that you might misapply your rule. I don't understand how you could have written the following if there was fundamental disagreement:

Warning
These rules of thumb are not the whole story. They are simplified as much as possible to make them more widely accessable. As with all massively oversimlified rules they may not apply well in every circumstance and are necessarily aimed at the worst case of beginning divers and harsher conditions. They are hopefully simple enough for all divers to be able to employ with minimal math and should be able to be recalled even through the haze of narcosis.

I think you did a nice job and would hate to see your work maligned because someone improperly applies the formula.