First, I am a brand new diver. I did my first non cert dive in open water yesterday. I'm a big dude, 310lbs 6'3", diving cold water OC. Wetsuit.
I went with a Steel HP120 yesterday, and kept track of my psi and bottom time and average depth. Bottom time was 56mins, average depth was 15ft, beginning pressure was 3500 or tad below. Ended my dive with 800psi.
My insta-buddy remarked after the dive that I have insanely efficient air consumption to stay down that long on a single tank and still have 800 ending the dive. I threw my numbers into the SAC calculator and came up with
You consumed 92.1 cubic feet of air.
Your SAC rate is 32.4 PSI per minute.
Your RMV rate is 1.131 cubic feet per minute.
I just don't know what those numbers specifically mean. Did my calculations get screwed up? Or are those numbers good?
I had to translate this to metric to make any sense of it. If Google didn't lie to me, 1.13cft per minute translate to 31 litres per minute which is actually a very high RMV. When I started, my RMV was around 20-25 litres per minute though I have the "benefit" of being a short, fat woman. You don't need to worry about your RMV too much right now.
When we begin to dive, our air consumption is fairly high simply because we are new to diving so the excitement and maybe a little anxiety cause us to breathe more rapidly than we do when we are very comfortable. In addition to this, your trim, buoyancy and propulsion techniques won't yet be on point. If your trim is off, i.e., you are not perfectly horizontal and are perfectly streamlined with your equipment, the drag of the water against you will increase your air consumption. If your buoyancy is off and you spend a lot of time inflating and deflating your BCD, you will lose air this way. Your pressure gauge does not differentiate between whether you have breathed the air or whether you used it to inflate your BCD. Your propulsion techniques should be chosen according to circumstances. If you are just calmly diving along a reef without any currents, and there's no need to rush, slow, strong frogkicks will save you some air compared to constant flutter kicks. Flutter kicks can be more appropriate against currents or where speed is required. So perfecting your propulsion technique will absolutely save you some air. The same goes for turning around. If you're doing a basic reef dive, i.e., 20min left-hand shoulder to go, 20min right-hand shoulder to come back, how do you turn around? If you swim in a circle to turn, learn how to helicopter turn. It's quick, easy and on the spot. What do you do if you're diving behind someone and suddenly they stop in mid-air? Do you need to flail your arms, do you need to dive in a little circle around them til they keep moving? Those will cost you air, too. If you learn how to hover, you can conserve air that way. Hovering is also really useful if you need to stay still to look at a fish, a turtle, a coral, etc. And finally, also very important, if you are facing a coral full of clownfish and you're done looking, where do you go to make room for the next diver? Everyone will be trying to get a view of the clownfish, so there's divers to your left, your right and probably above you (maybe even below you...). That's right, you go backwards! Learn how to back kick early on, that way you can avoid chaotically swimming sideways through everyone, trying not to bump into other divers looking at the same clownfish.
These are just tips and thoughts for improving your RMV by addressing factors under your control. Obviously environmental aspects such as currents will play a role but you cannot control the currents, only your response to them. If I were you, I'd keep track of my RMV even as a beginner diver. If you ever get to a point where you plan your dives independently, knowing your RMV in different conditions and depths will be very valuable to your planning.
