The more I go diving, the more I want to know about what’s actually going on in the tank. I’m no expert, far from it. I’m just the guy in the tank, sucking oxygen at 2 ATA, or, 2 atmospheres, whatever that means.

So, I set out to try to figure out what it all means.

Air in the atmosphere exerts 14.7 psi. That means that a one inch square column of air, the height of the atmosphere (about 50 miles) weighs 14.7 pounds. That unit of measure is called an “atmosphere” and the pressure of one square inch (described above) becomes one atmosphere. If it were possible to double the height of the atmosphere to about 100 miles, the weight of the 1 square inch column of air would weigh 29.14 psi and the unit would be two atmospheres. Of course, that isn’t possible but it explains what the units mean.

Conversely, when you rise above sea level, the weight of the air is reduced as the air gets thinner. You experience the thinner air as you climb to altitude in an airplane, even in a car climbing a high hill or a mountain.

Air is relatively light but water is much heavier. One cubic foot of water weighs 62.5 pounds while one cubic foot of air weighs only 1/12 pound. The force exerted by water, at depth, increases much faster than air. At the surface, water exerts 1 ATA but as we go below the surface, the pressure increases quickly. The force of 2 ATA is reached at a depth of 33 feet.

When diving, the most impact on the body is within the first 15 feet of the dive. In those first few feet, a diver experiences the squeeze of pressure on body parts with air - intestines, lungs, ear canals - as they are reduced to half their normal size.

In the hyberbaric chamber, air is compressed by mechanical means (a compressor) rather than water. The controls are simple - there is a valve that allows compressed air into the chamber and one that releases the compressed air from the chamber. Across the top of the chamber are several valves that regulate the air flow.That’s it - that’s really all there is to it.

The tech turns on the oxygen flow to my helmet, turns on the air flow from the compressor, closes the valve and shuts the hatch behind me. Nature takes over and does the rest.

In the chamber, I feel the same thing the diver feels in those first 15 feet, especially in my ears. You feel the same changes in pressure in an airplane.

As my “dive” starts, the door to the chamber almost slams shut from the compressed air that is already filling the chamber. I always have two plastic water bottles in the chamber with me, one full and one empty. The empty bottle collapses, to about half its normal size, almost immediately as the pressure increases. I used to loosen the cap and let it inflate, but now I let it collapse to use as a pressure guage. The full bottle doesn’t collapse as quickly because water does not compress, however, I do open that one periodically to balance the pressure. (I don’t need my water bottle bursting at the start of a dive.) What happens to the plastic bottles is also happening to me and I mostly feel it in my ears. Lots of yawning, swallowing and otherwise trying to balance the pressure in my ears.

Once past that “first 15 feet” the dive becomes easier and the same is true in the chamber. In fact, those last few minutes of reaching the specified pressure of 2 ATA is comparibly uneventful. Conversely, when the chamber is depressurized, the initial change in pressure is obvious to my ears, then it is relatively easy until the last few minutes of depressurization.

It is difficult for my tech to get the hatch open. Even the slightest bit of pressure holds it snugly closed, and even the oxygen flow has to be turned off to help balance the pressure enough to allow the hatch to open. It opens with a “WHOOSH!” and I can feel the pressure drop, especially in the my ears, when the air rushes out of the chamber. The water bottles feel it, too, as the empty comes back to full size. The one with water hisses like a Coke can when you open it.

Tomorrow, I cross the halfway point of my sojourn.