Tuesday, March 10, 2009

Mixed Results

One of the biggest changes in diving since I started almost 15 years ago is the common adoption of mixed-gas diving. It used to be that the tank on your back held compressed air, and while that is still sometimes true, on most of these tropical dive boats the tanks are mostly filled with Nitrox now.

"Nitrox" is a generic term for "enriched air" which is air with more oxygen added. Oxygen is generally very good for your body. We've all see tired athletes breathing oxygen to recover, and various kinds of patients breathing it under a doctor's care.

Divers have tried lots of modifications of breathing gas over the years. Ultimately there are two (or three) issues they are trying to solve:
  • Air contains 79% nitrogen (versus 21% oxygen), and nitrogen bubbles can cause "the bends." The less nitrogen, the better.
  • Oxygen at high pressure can be toxic. So not too much of that in the mix, especially as you dive deeper.
  • Breathing too much pressurized oxygen over the course of a day can also be toxic (but in a different way).
I'll spare you all the science behind all this. Suffice it to say that it's desirable to displace some of the nitrogen in breathing gas with another gas. Under many conditions, oxygen is a good choice, but if one is diving very deep (and therefore under very high pressure), that could be deadly, so an inert gas such as helium or argon is generally used instead.

I remember vividly seeing film of divers years ago in a bathysphere breathing a gas with lots of helium added. The resulting "helium voice" was quite memorable. There is still "heliox" in use, but divers more commonly use the heavier (but also inert) argon now.

Fairly recently, recreational divers started using Nitrox (air with additional oxygen), which for most diving provides a margin of safety and comfort.

The drawback for me is that one needs a special dive computer to handle the necessary calculations for allowable depth and time given a particular mix of gases. Most modern dive computers handle this pretty elegantly, but my rather outdated one requires that I set it for the percentage of oxygen in the Nitrox for every single dive. And if I should forget, it defaults to assuming I'm using a 50% oxygen gas for my next dive, rather than the more reasonable assumption that I'll be using whatever I used the last time (usually between 30 and 32% oxygen).

So of course, I have now twice forgotten to set the computer before a dive on this trip, and it has been a huge hassle. I think I've handled it well and responsibly, by occasionally lying to the computer and telling it I was breathing plain air instead of Nitrox, so as to balance out the calculations. But I should not have to do this.

Before I do any more serious diving, I will probably need to invest in a new dive computer. Not that I mind new toys (especially computers!), but I basically like the one I have, but for this one thing. Annoying.

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