By Steve Lewis
IA canat recall which Charles Dickens character I am misquotingaI believe it is Ebenezer Scrooge from the novella AA ChristmasA CarolA a but the gist is this: aIncome one pound; expenditures 19 shillings and 11 pence; outcome bliss. Income one pound; expenditures one pound and one penny; outcome misery.a Which basically translates into, adonat expend more than you earna. Simple advice, which might be a little more difficult to follow in the Christmas season when the credit card usually takes a bashing, but a good way to manage your fund, nevertheless.A
Surprisingly, itas also actually, genuinely voice advice if we apply the principle to our gas furnish when diving: save a little for a rainy day. Itas a great way to manage breathing gas.
Now at this phase, itas a fairly safe assumption that most of us have heard of the cave diveras Rule of Thirds: one third of your starting gas volume for the first half of the dive( swimming in ), another third for the second half( swimming out ), and one third as a reserve, just in case things run pear-shaped and you need extra time to sort things out. Not merely is the Rule of Thirds the go-to gas management protocol for cave diving, itas the widely accepted protocol for most forms of technical diving, too. Iad argue as well, that, with a slight modification, the Rule of Thirds is the safest plan for the majority of non-technical single-cylinder dives.A
Now, when you suggest to the average open-water sport diver that they finish their dive with around 1000 psi of gas left over from their starting 3000 psi fill( thatas around 70 bar from a 210 bar fill ), they will push back. The majority of them are most likely to say that following such a plan is too way conservative. Their argument is the ubiquitous, close to universal, aI paid for a complete fill and intend to use it.aA
In those cases, a reasonable modification to the standard Rule of Thirds would be for a athletic diver to use a sensible portion of her contingency gas( the remaining third of her starting pressure) for putzing around near the ascending line or some other safe, known exit. That style she can use the gas she paid for but will be close enough to the exit to be considered asafea if something nasty happens as gas reserves approach aseeds and stemsa level.
This approach would result in fewer emergency flights to the surface with nothing left in the tank but hope, and thereby rolling the dice with DCS, embolism orathe odds-on favouriteaa crappy experience that scares the smiles out of everyone. Thereas no industry body tracking these events, but surely the come-to-Jesus experiences that occur because of running out of gas at depth result in many, many diver dropouts.
So it follows then that a much better strategy would be for the single-tank diveraespecially anyone planning to dive between 100 and 130 feet( 30 and 40 m) ato carry their own asomethingas hit the fana gas in a totally redundant system. If it were up to me to write agency standards, Iad push that any diver venturing below 100 feet( 30 m) with only one first stage should be carrying a redundant gas source with its own regulator.
And of course, even if everyone agreed with me on that score, wead still be left with the discussions over how to carry it, how often to practice employing it, and how much gas that redundant system should have in it.
Iam a cave diver and a wobbly-kneed deep diver. I start to tremble at relatively shallow depths on the rare occasions I dive open-circuit with a single tank.( In occurrence youare interested, you are unlikely to see me in a single tank and BCD any place deeper than a swimming pool .) I have been conditioned to feel uncomfortable relying on a single life-support system. Consequently, my approach to the guidelines of diving a redundant gas-supply is very conservative.A
As a phase of interest, my answers to the trio of questions in the paragraph before last are: Sidemount so the fiddley bits like the valve, first stage, SPG and handwheel are visible and easily reached; practice employing it every couple of dives until deploying the back-up in an emergency becomes second nature; have much more gas than you think you need.
On that last point, humour me for a moment and work with me through the arithmetic to discover how much gas is enough.
I quoted Scrooge; now Iall quote the Buddha. He said, aEnough is too much.a And as wonderfully informative and instructive as that direction is to our outlook on life and First-World consumerism, it is desirable to painfully obvious that the Buddha never dived a lick in his whole life. When it comes to gas planning, enough is the absolute minimum.
Letas say we have an experienced diver bumbling around at just about 130 feet( 40 m) on a little shipwreck somewhere off a beautiful tropical island. Everything is fantastic for this diver and his buddy. They commencing their dive at the pointy aim of the wreck and after a moderate swim for five minutes make it to the blunt end and drop-off ten feet( 3m) to check out the prop and rudder. At this phase, something “re going away” the ways and our experienced diveras regulator first stage misbehaves. Unfortunate, but no big deal; he can bail out of the dive and share his equally experienced buddyas gas via a longhose, so they head for the surface.
They make it, but thereas not enough gas for them to swim back the style they came or make a safety stop and once on the surface are surprised to learn that sharing air and ascending is a great deal more difficult from 130 feet( 40 m) than it was on their long-ago open-water checkout dives from 50 feet( 15 m ); which was, incidentally, the last time either of them practised that particular ability. They are 100 feet( 30 m) from the dive boat tied up to the mooring line way over there at the shipwreckas pointy end.
So letas rethink the situation. How much gas would be enough for the unlucky diver to swim back to the ascent line five minutes away, induce his route up to 20 feet( 6m) or so to take a five-minute safety stop, and then to surface close to the dive platform on the nice comfortable dive barge?
We could work through the calculationsa |. You know the drill. Resting consumption rate of N-litres per minute multiplied by a factor for the depth( 5 bar/ ata) and multipled again to account for the five minutes it will take to swim back to the ascent line, and then an added volume to allow for the ascent and the very much required safety stop somewhere on the way back to the surface. My calculations induce that about 40 cubic feet( a little more than 1100 litres) of gas. And that volume does not account for a panicked diver burning through extra gas or taking extra time to sort things out under stress.A
My advice is to do the math before your next diving below 100 feet( 30 m) and then think about the ramifications of dismissing what the numbers tell you. Following that exercise, I am sure that youall agree that the Buddhaas advice about enough being too much, simply doesnat cut it. And that Ebenezer Scrooge should have been a Divemaster.A
Steve Lewis is an author, cave diver, life coach-and-four and a Fellow of the Royal Canadian Geographical Society. Ironically, in his day-job he actually DOES help to write diving standards as the Director Diver Training for RAID International.