Why Even Air-Breathing Divers Should Analyze Their Tanks

All nitrox divers are taught to analyze the contents of their tanks before using them, but now airbreathing divers should be doing it, too. With nitrox fills the norm on dive boats, on tap there and in dive shops, a new hazard lurks -- when tanks of air are filled alongside tanks of nitrox, and some divers may pick the incorrect one. Tanks are not always clearly or correctly labelled.

Air which is actually nitrox 21 (meaning it is composed of approximately 21 percent oxygen and 78 percent nitrogen) , is recommended for a maximum safe depth of 185ft. Nitrox 32, one of the most common mixes for recreational diving, is limited to 111ft. It has higher partial-pressure amounts of oxygen, which can subject a diver to the risks of oxygen toxicity. If a diver mistakenly takes a tank of nitrox, thinking it is air, he may go too deep and suffers a life-threatening spasm (depending on an individual's susceptibility) with the likelihood of drowning. Which means we should all analyze the contents of every tank before we go diving.

Any dive operation that provides nitrox should have a portable analyzer available. You can simply adjust it to 20.9 percent, then hold it over a tank's valve that you've opened sufficiently to supply a gentle flow of gas, and wait until the display stabilizes. If it's air in the tank, the display will still read 20.9 percent oxygen.

Do you always know what you're breathing when you dive? Have you or someone on the boat ever picked the wrong tank and if so, what happened? We'd like to know. Write to BenDDavison@undercurrent.org, tell us your story, and include your town and state.