Remember the first rule of scuba diving that you were taught in your basic open-water class? I believe it goes something like: “Keep breathing!” Simple advice and unarguably the best advice possible for any diver, not just those entering the sport for the first time.
For example, the same first rule is true for technical diving. Gas management 101 starts off by stating something like: “Always have a sufficient volume of appropriate gas to breathe throughout the whole dive!”
More wordy—and there are other nuances to a gas plan such as having gas to share with your buddy—but the message to the tech crowd is essentially the same as it is to the new diver. And unless I’ve missed a memo somehow, that message applies to all technical divers without exception.
Given all that, what’s difficult to understand, is why some folks seem to lose the plot when they strap on a rebreather… even more baffling is when they strap on a rebreather and then swim into a cave while seeming to ignore the primary directive… always have something to breathe.
You may already know about rebreathers, and you may also be a cave diver. But for those of you who are only one or the other—or neither—here is the Coles Notes version of your CCR cave class. A rebreather offers divers the ability to get a long, long way from fresh air without much effort. Unlike an open-circuit cave diver who generally has to carry many stage bottles of gas to extend her foray into the deepest regions of a cave, way back from the exit, a CCR diver can push many hundreds even thousands of meters without making any allowances at all.
Here’s one reason why. On a rebreather, the exhaled gas is recycled and the carbon dioxide is removed by a little chemistry set carried in the unit’s scrubber. Apart from a few litres of diluent gas used up now and again, all the gas that needs to be added regularly to “the loop” (the breathing gas going round and round in the unit) is the oxygen metabolized by the diver as she swims. A working average for this is about 1.5 litres per minute, and this does not significantly change with depth. In other words, a three-litre bottle charged to 200 bar with oxygen can last for up to 400 minutes.
What that means is that if we were to say that the average cave diver on an easy outing swims at a speed of between 15 to 20 meters a minute, that volume of oxygen could translate into more than 6,000 metres of distance round-trip!
Now here’s the problem. Everything on that six kilo-meter journey might be fine as long as the diver’s CCR continued to function as it should. But what if it did not? (...)