We had done six days of snorkeling all around the BVI prior to the dive. Both Kev and I were pretty impressed with how different things look once you are down 50 feet, versus on the surface.  The equipment we rented was pretty nice, and my gauge looked something like this -

The top gauge is your depth, the bottom gauge is your air pressure. My maximum depth on the dive was 55 feet. Before even putting the tanks on, my brother and I confirmed that each of us had 3000 psi. 3000 is the max psi a “normal” rated SCUBA tank can hold. Given that your air supply is limited, breathing control is really important while diving. 

On the surface, you are at one ATM. The standard atmosphere (symbol: atm) is a unit of pressure and is defined as being equal to 101.325 kPa. Another way to think about this is: a 1 inch column of air as tall as the atmosphere, would weigh 14.7 pounds.

Water weighs considerably more than air does, so it can exert much more pressure. It only takes a 1 inch column of sea water 33 feet tall to weigh 14.7 pounds. This means that at a depth of 33 feet deep in the ocean, there is a total pressure of 29.4 pounds per square inch (psi). This would be 2 ATMs of pressure.

Alright, here are your lungs breathing air at 1 ATM, and the rest of your body’s cells absorbing the normal (by volume) 78.08% nitrogen, 20.95% oxygen that compose “air”.

At 55 feet we are close to 3x the pressure experienced at the surface. The pressure causes the gas particles to be compressed much closes together, so that every normal breath you take contains 3x the normal concentration of nitrogen and oxygen. It looks like this in your lungs:

As you continue to breath this compressed air, the tissues in your body and your blood absorb the concentrated nitrogen. 

When underwater the nitrogen is in your blood as microscopic bubbles that pass harmlessly through your body. The problem is that when you head back to the surface, these microscopic bubbles expand as the pressure decreases. If you have too much nitrogen in your system, these bubbles expand inside you and give you decompression sickness, aka the Bends. The Bends can kill you if the bubbles form in your brain, heart, or thickly in your blood stream.

To avoid the Bends, divers use a dive chart, which takes into account the time you are under and the depth at which you are diving. These two factors really determine how much nitrogen your body is absorbing. Here is a PADI dive chart which indicates the time you can spend at each depth before requiring a decompression stop on your way back to the surface.

You can see by this chart that between 50 and 60 feet we’d have a maximum dive time of somewhere around 1 hour. We were probably only under 45 minutes because my brother was facing two problems which caused him to use his air much faster than me. 

First, Kev did not have enough weight on his weight belt. A diver a buoyant with a tank of compressed air on his back. To compensate, we wear weight belts – in the photo below you can see the red weight belt, and the weight on it. Looping the belt through it self like Kev has done is a small danger. The belts are designed to be released with one hand and slip off which would send you rocketing towards the surface in an emergency. We noticed it and unlooped his and mine before we actually went down.

Because Kev did not have enough weight he was Positively Buoyant. This meant he had a tendency to float upwards all the time, requiring him to counteract this with a small amount of downward swimming. This caused his muscles to be used more, which in turn required heavier breathing. 

Kev also got cold on the dive. For a number of reasons, at different levels of cold, the body increases breathing to generate more heat via shivering. This probably was not as much of a factor as the buoyancy because the tropical waters are never really that cold, but it probably did add to his usage somewhat. He had on a tshirt versus my rashguard.