By Steve Lewis
I can’t recall which Charles Dickens character I am misquoting—I believe it is Ebenezer Scrooge from the novella A Christmas Carol — but the gist is this: “Income one pound; expenses 19 shillings and 11 pence; outcome bliss. Income one pound; expenses one pound and one penny; outcome misery.” Which essentially translates into, “don’t spend more than you earn”. 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 money, nevertheless.
Surprisingly, it’s also really, really sound advice if we apply the principle to our gas supply when diving: save a little for a rainy day. It’s a great way to manage breathing gas.
Now at this point, it’s a pretty safe assumption that most of us have heard of the cave diver’s 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 go pear-shaped and you need extra time to sort things out. Not only is the Rule of Thirds the go-to gas management protocol for cave diving, it’s the widely accepted protocol for most forms of technical diving, too. I’d 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.
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 (that’s 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, “I paid for a complete fill and intend to use it.”
In those cases, a reasonable modification to the standard Rule of Thirds would be for a sport diver to use a sensible portion of her contingency gas (the remaining third of her starting pressure) for putzing around near the ascent line or some other safe, known exit. That way she can use the gas she paid for but will be close enough to the exit to be considered “safe” if something nasty happens as gas reserves approach ‘seeds and stems’ 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 or—the odds-on favourite—a crappy experience that scares the smiles out of everyone. There’s 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 diver—especially anyone planning to dive between 100 and 130 feet (30 and 40m)—to carry their own “something’s hit the fan” gas in a totally redundant system. If it were up to me to write agency standards, I’d push that any diver venturing below 100 feet (30m) 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, we’d still be left with the discussions over how to carry it, how often to practice using it, and how much gas that redundant system should have in it.
I’m 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 case you’re 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.
As a point 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 using 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 I’ll quote the Buddha. He said, “Enough is too much.” And as wonderfully informative and instructive as that direction is to our outlook on life and First-World consumerism, it should be painfully obvious that the Buddha never dived a lick in his whole life. When it comes to gas planning, enough is the absolute minimum.
Let’s say we have an experienced diver bumbling around at just about 130 feet (40m) on a little shipwreck somewhere off a beautiful tropical island. Everything is fantastic for this diver and his buddy. They start their dive at the pointy end of the wreck and after a moderate swim for five minutes make it to the blunt end and drop ten feet (3m) to check out the prop and rudder. At this point, something goes off the tracks and our experienced diver’s regulator first stage misbehaves. Unfortunate, but no big deal; he can bail out of the dive and share his equally experienced buddy’s gas via a longhose, so they head for the surface.
They make it, but there’s not enough gas for them to swim back the way 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 (40m) than it was on their long-ago open-water checkout dives from 50 feet (15m); which was, incidentally, the last time either of them practiced that particular skill. They are 100 feet (30m) from the dive boat tied up to the mooring line way over there at the shipwreck’s pointy end.
So let’s rethink the situation. How much gas would be enough for the unlucky diver to swim back to the ascent line five minutes away, make his way 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 boat?
We could run through the calculations…. 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 make 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.
My advice is to do the math before your next dive below 100 feet (30m) and then think about the ramifications of ignoring what the numbers tell you. Following that exercise, I am sure that you’ll agree that the Buddha’s advice about enough being too much, simply doesn’t cut it. And that Ebenezer Scrooge should have been a Divemaster.
Steve Lewis is an author, cave diver, life coach 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.
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