2000-11 Out The Oxygen Window

By Karl Shreeves

This is really, really cool: For any depth, depth plus 33, times one minus one, divided by .79, minus 33 always equals -33.

Practically poetry. Brings a tear to your eye, I bet. That’s the oxygen window—the phenomenon that gets your freezing butt out of the water sooner when you’re decompressing following a push to upwards of 300 feet in depth in Cannonball Cave, Missouri.

Okay, I hear some of the less initiated yelling, “Depth what plus pi times what the atomic square root what of negative what? How does that keep you warm? Hang on and back the tek truck up!” Wisdom comes to those who listen, Grasshopper.

Rewind: Go back to your open water diver and rudimentary enriched air nitrox course. Remember that your time underwater without required decompression stops is limited by how much nitrogen (or helium when tek diving with special mix) dissolves into your body tissues. You will recall that as you ascend, the nitrogen dissolves out of your body, and the shallower you are, the faster it goes. If you have too much nitrogen and ascend too shallow too soon, the gas may form bubbles, and that’s what can cause decompression sickness (DCS). The no stop limits are the times at each depth based on how long you can stay and still ascend directly to the surface without a substantial risk of DCS.

Jump to nitrox. Enriched air nitrox has more oxygen and less nitrogen than air does, so you absorb nitrogen more slowly at a given depth. That’s why it provides longer no stop limits and/or shorter decompressions compared to air. To use enriched air nitrox with air tables, you use an Equivalent Air Depth (EAD), which is simply an adjusted shallower depth that equates the enriched air’s nitrogen pressure with the depth where you’d hit the same nitrogen pressure using air.

You don’t get something for nothing, though, and increasing your oxygen to cut your nitrogen is no exception. If your oxygen partial pressure exceeds 1.4 to 1.6 ATA, you run the risk of an oxygen “hit”—a convulsion that makes you black out and drown. (Something you want to avoid.) So, the more oxygen in the mix, the shallower you hit 1.4 to 1.6 ATA, which negates diving deep where you’d get the most from long no stop times and/or shorter hang times.

But even though you can’t use high oxygen blends or pure oxygen when you’re deep, you can use it on the way back. As it turns out, high oxygen blends and pure oxygen are great for decompressing. Tek divers switch to higher oxygen, and eventually pure oxygen, as they ascend shallow enough to do so—and that accelerates their decompression compared to using the same gas blend all the way up.

Take a tek diver coming back from 160 feet using air. The diver makes several decompression stops until arriving at 70 feet and switches to EAN50 (50 percent oxygen, 50 percent nitrogen), which is the deepest you can safely use EAN50.

What happens? The Equivalent Air Depth formula (the poetic math we started this article with) says using EAN50 at 70 feet is the same as using air at 32 feet with respect to nitrogen pressure. Does this pose a DCS risk? No—because bubble formation results depend on the actual depth. So, nitrogen exits the diver’s body as if being about 40 feet shallower, but without actually going that shallow and risking major bubble trouble.

The nitrogen is said to be “going out the oxygen window,” which is the basis for accelerated decompression. Special dive tables and dive computers allow tek divers to shorten their decompression time based on switching to nitrox when they ascend.

The biggest oxygen window comes from 100 percent oxygen. The deepest you can use it is 20 feet, but consider your EAD—the depth at which you have the same nitrogen pressure using air. Using pure oxygen at any depth, the EAD equals -33 feet—an impossible depth with air unless you go into space (without a space suit). What this means is that when you’re using 100 percent oxygen at 20 feet, you’re off-loading excess nitrogen faster than if you were at the surface.

True, oxygen has its own concerns: special equipment and handling requirements (things burn more easily in high oxygen environments), central nervous system toxicity, pulmonary toxicity and other issues among them. These are why you need special training and certification for diving with enriched air nitrox, for decompression diving and for diving with special mixes like trimix.

But now you understand why enriched air nitrox and oxygen decompression are such big deals to tekkies. It’s far more efficient and reduces your in-water exposure time, so you don’t freeze off your…finer parts. Oxygen decompression is more reliable, all other things being equal, so it’s not only shorter, but carries less DCS risk. In exchange for learning to manage oxygen issues, you can say that much of your DCS risk…goes out the window.

Karl Shreeves is vice president, Technical Development for DSAT and PADI.