Max Depth vs. Cylinder Capacity

[dannobee]

I've explained the two rules in question (capacity of tank in cu ft = depth and depth*10+xxx psi to start ascending) to quite a few noobs, and it's pretty easy for the casual diver to remember.

I'm not talking about the diver who takes some initiative and studies up on gas management and dives frequently, but the casual once-or-twice per year diver who breaks out his/her "breather thingy" and "beeper thingy" just before they go on vacation.

For those types of divers, those two rules of thumb serve an easy to remember, useful purpose.

 

[MXGratefulDiver]

Interesting read ... for those of you attacking Lamont, the rule of thumb wasn't his idea. It's been around for a long time ... I've also included the same rule of thumb in my gas management article ... which was written long before Lamont's. It appears in several others that I'm aware of.

The origins of the "rule" predate my own OW class in 2001, and it was intended FOR NEW DIVERS ... folks who typically have a very high gas consumption rate and who generally don't know much more than "watch your SPG" and "end your dive with 500 psi". The purpose of the rule of thumb was to give these divers something tangible that could possibly help keep them out of trouble ... because here in Puget Sound, where AFAIK the "rule" originated, it is quite easy to get stupid deep on a simple shore dive ... and every year we have at least one fatality due to someone going deeper than they should and running out of air.

The rule was never intended to apply to more experienced divers, because those divers (a) develop lower consumption rates with experience, and (b) develop enough knowledge about how to manage their dive to make better judgment calls concerning depth and available gas supply.

So please ... keep things in context. Scuba instruction is chock full of rules of thumb. They're mostly designed to provide a simplified framework for the new diver developing basic skills to keep them out of trouble while they develop enough of an experiential context to use good judgment.

That, really, is all it is.

Oh ... and getting back to something the OP said ... you are correct. I NEVER allow my AOW students to do their deep dive on an AL80. Prior to doing the dive, we do gas consumption measurements, and I provide them a dive plan for the dive. Using their own measured gas consumption and the dive plan, I have them calculate how much gas they would need for that dive. Without exception, every student who has ever taken the class has concluded that an AL80 is too small of a cylinder for that dive. Oh, many won't use the 77.4 CF of gas contained in that cylinder ... but when they factor in how much they would need for a controlled ascent while sharing it with a buddy, they quickly realize why it's inadequate.

The minimum size cylinder we use for the AOW deep dive is an LP95 ...

... Bob (Grateful Diver)

 

[docbrown]

Thank you. Very informative post. I think the adage of "Your mileage may vary" is appropriate here. Air consumption is highly individualized and the "rules" stated here take that into account to an acceptable degree. Much as one person's tolerance of cold differs greatly from another's, (I can get by with a bioply skin when my son needs a 3mil) air consumption varies from person to person. Overall the guidelines stated are a great starting point that can be customized as a diver gains experience. Just make sure that however you plan your dive, you do so with the least common denominator. You may only use .75 cu. ft/min, but if your buddy uses 1.25, you should use his consumption for your turn calculations.

Thanks again to the author.

 

[Heavy water]

Rule of Thumb?

Today I wore a 149 cu-ft tank pumped to it's rated pressure of around 3,600 psi. First dive: quick drop to around 160 feet, shot a nice grouper, grabbed it by the gills and headed up immediately with no safety stop. Didn't check my watch or computer, but got on the boat with 3,000 psi. Took about a 25 minute surface interval.
:hanged:
. THE SCUBA POLICE ARE TRACKING YOU NOW MY GOD HOW DID WE ALL MANAGE TO STAY ALIVE.

 

[Marek K]

I NEVER allow my AOW students to do their deep dive on an AL80. [...] The minimum size cylinder we use for the AOW deep dive is an LP95 ...

What kind of profile do you do?

 

[MXGratefulDiver]

What kind of profile do you do?

Dive Plan:

• Surface swim to a buoy and descend down the buoy line to 45 fsw. Total descent time is 2 minutes.
• Follow a rope to the I-beams at 85 fsw. Time from the buoy line to the I-beams is 7 minutes.
• Spend 8 minutes at the I-beams, depth varies between 85 fsw and 100 fsw.
• Follow the rope back upslope to some pilings at a depth of 60 fsw, taking 5 minutes.
• Explore the pilings and return to the descent line. Elapsed time is 8 minutes.
• Ascend up the buoy line to safety stop depth, taking 1 minute to ascend from 45 fsw to 15 fsw.
• Do a safety stop at 15 fsw for 3 minutes.
• Ascend to the surface, taking 1 minute.

Quiz:

1. What is your expected gas consumption for this dive plan? (use RMV)
2. What is your rock bottom pressure at 100 fsw? (use SAC)

 

[Blackwood]

I submit that "guideline" would be a better choice of words than "rule".

That's probably as good a definition of "rule of thumb" in the common lexicon as any.

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.

I can only half agree with that. The vast majority of divers can't readily produce working decompression tables. Should they not be using those provided/purchased from PASI, SDI, whatever?

 

[Blackwood]

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.

I imagine you're selling yourself short.

Gas planning (i.e. what you do on the shore/boat) is one thing. It's premature and fairly passive.

Gas MANAGEMENT is an active task that must take place throughout a dive (if it doesn't, it ain't management).

With some conservative assumptions, the math can be very simple and allow you to do things like know 10 minutes into a planned 30 minute dive that you're breathing harder than accounted for and should accordingly turn earlier/decrease depth. I have a personal goal to never simply reading my SPG. Rather, I "check" it, which I'll define as looking at it with an expectation of what it's going to say.

 

[peterbj7]

The vast majority of divers can't readily produce working decompression tables. Should they not be using those provided/purchased from PASI, SDI, whatever?

I did say "simplistic" rules. No, of course they don't need to be able to replicate those tables, but they do need to understand the principles behind them. What I think is undesirable and dangerous is for people to follow a rule blindly until it leads them down a dark alley. I stick to what I said - understanding is vastly more desirable than memorising.

 

[TSandM]

peterbj7, I agree with you . . . protocols are written to permit relatively novice or minimally trained people to function safely (whether that's in diving or elsewhere). For the most part, they work. One can occasionally get into a world of trouble by not understanding when one is faced with an exception to the assumptions that underlay the original protocol. Completely understanding is far preferable to unquestioning obedience. But it is better to give someone a tool they can use 99% of the time, than to give them no tools at all, which is the way most people come out of open water.