I grew up on the water. I sailed dinghies as a kid and later moved into offshore yacht racing. I’m a decent swimmer and did lots of snorkelling, so the underwater world was known to me. As a young adult, I started competing in triathlons.
In my early twenties, many of my friends were already divers, but the stories they told centred on the crayfish they caught. Not being a fan of crustacean consumption, it didn’t capture my imagination. I have to thank my wife, Sandra, for getting me started. She suggested we do an open water diver course. The idea of doing anything with Sandra appealed, so we signed up.
Fast forward fourteen years, we’re still married and I’m still diving. Along the way, I discovered a few things: I breathe a lot of gas, I am comfortable diving alone and I love underwater photography. These three things have a big influence on my diving choices.
A diver, quietly swimming at the surface, will breathe at a certain rate. This has a few names, with surface air consumption (SAC) and respiratory minute volume (RMV) being the most commonly used. (RMV also has a more specific medico-definition.) Simply put, it is the volume of air a diver breathes, per minute. When I started diving, my SAC was around twenty-eight litres per minute (LPM). These days it’s around nineteen.
Instructors teach new divers not to worry about air consumption. It’s more important to breathe properly to prevent hypercapnia (CO2 retention) and pulmonary barotraumas (lung expansion injuries). But when someone dives with others and is consistently the first to reach minimum gas pressures, it’s natural to wonder how to improve. I was that diver.
SAC usually decreases with experience – mine did – until it reaches a physiological limit, around which it stays. This final SAC varies greatly between individuals and I suspect genetics play a role. Having reached my SAC plateau, I found I still consumed my gas supply faster than others did, and I was still the one to end a dive. I didn’t enjoy curtailing my friends’ fun. My solution was simple – carry more gas!
I tried a few variations on the more-gas theme. I started with bailout bottles, trying different sizes. This made me comfortable with using almost all the air in my tank, but introduced new issues without solving the problem. Before long I realised I wanted a second tank and moved up to a set of twins.