There has been a shift in position. When Murray and I first learned to scuba dive, we both used 80 cu ft tanks. Being new divers we used tons of air due to being overweighted, kicking too much and a lack of buoyancy control. Murray used way more air than me, so he swam about 7 to 10 feet above me as he would use less air at a shallower depth. As our dive skills improved, his air consumption decreased and we were a able to dive side by side.
A few years ago, while diving with the Kona Dive Company, one of the dive masters asked me how much air I was coming up with after a long dive. Of course, I didn’t actually know as I never looked as I knew that I had lots of air left in my tank. After the next dive I mentioned to the fellow that I came up with 1,500 psi (a half tank) so he recommended that I use a 63 cu ft tank. A 63 cu ft tank still has 3,000 psi, same as an 80 cu ft tank, but it has less volume. So each breath uses more volume of air. Since I came up with 1,500 psi with an 80, I should be just fine using a 63. I did a couple of test dives with the small tank and my air consumption matched Murray’s almost perfectly. Sold! I have been using a 63, whenever they are available, since then.
We have come to realize that at depth I use more air than Murray. In the shallows I use less air than Murray. In current I use less air than Murray. On a good dive profile of going deep first and then working our way shallower and shallower, by the time we are finished the dive, we have the same amount of air left in our tanks.
When we arrive on the Bilikiki, we are assured they have small tanks. We are told they have small tanks and are 10 litres. We could not make the conversion to cu ft until we hit WIFI land.
Right away we notice that my air consumption is greater than Murray’s and, on some dives, limits our dive time. What is going on? Is the tank actually smaller than a 63 cu ft tank? Are we spending more time at depth where I use more air? Am I still overweighted? Has Murray’s breathing improved enough to cause that difference? What are we going to do about this?
I start to purposely stay a little above Murray when we are at depths below 60 ft. I drop one more pound of weight so I do not put as much air in my BCD at depth. We try not to spend too much time at depths below 60 ft, which suits me fine as I like the shallows better. I think about my breathing. Up until now I have never had to think about my breath, about how much air I take in and what part of my lungs I am breathing out of as I have always had more than enough air available. That is the hard skill to learn, but I practise and practise, and have to take gulps of air every so often.
We manage my air consumption and still enjoy dives that are from 45 minutes to 95 minutes, depending on the currents and depths. It is a good exercise for me and I improve my diving skills.
PS The calculation from a 10 litre tank to cu ft is more complicated than I have patience right now, so I will let you know what I find out once I get home.