Trying to understand my air consumption

[high/baric]

One significant thing people often forget about things that affect your gas consumption calculations is change in temperature. Where I dive all diving is basically drysuit diving. If you fill up a tank with a portable compressor on the sunny deck of a boat and shortly after equip the tank and jump into +5C water (sorry, I use metric), the pressure inside the tank can drop significantly just because of the drop in temperature. If you check your starting pressure before going into the water (as most people do I would imagine) and do a short dive with low total gas consumption, your values can be way off. I believe this is also the reason for a lot of people thinking that the first breaths you take from a tank somehow miraculously use massive amounts of gas. So, in the future, when you have more material (dives) to work with and calculating SAC becomes more current, I suggest you take your starting pressure readings right after decent or at least after having the tank submerged for a while.

Just my 2 cents.

 

[tbone1004]

water temp doesn't effect SAC, it is expressed as a function of pressure and the pressure is normalized after the gas reaches ambient, and if you check the temp right before you get in, it should be close enough, maybe change 10bar max if there is a big differential but you have to wait for the temps to cool off.

First off, your sac numbers have to be right, which they aren't, your tanks hold 77.4 cf of gas, it's not much, but it does change them a bit.

.72
.77
.81
.46
.49 *make sure to change atmospheres from 34 to 33ft*
.44
.58

So to me it looks like your SAC rate is fine and I wouldn't worry about it. It's much better than most peoples. I'd obviously not use a bungeed 55lb wing as that is creating drag and likely having a negative effect on your weighting and balance, but it's not the end of the world. The cavern numbers are high, but not abnormal for first time in an overhead, so I wouldn't worry about it. Get your own plate, with an appropriately sized wing, and you'll be in the .4-.5 range which is as good as anyone can realistically expect

 

[high/baric]

I think you may have missed my point there, although it could be because of the way I presented it. Yes, it does not affect the calculation, which is why most people may have the false idea that it does not affect the results in any way. For example, I've done about 30 dives in tropical conditions over the past couple of weeks and often saw that my SPG was showing 200 bar when I picked the tank up from the sandy beach or deck of the boat. However, after going below the surface and checking the SPG again after a few breaths it often showed about 190 bar. So 5-10 bar is not a huge difference if you dive until the tank is almost empty, but the OP listed several dives with low total air consumption. So if I had 140 bar in the tank after the dive, still in the water (obs!), and were using 200 bar as the starting pressure instead of 190, I would get a number that is 20% too high. Again probably not significant in the large scale of things (after all you usually dive until a relatively empty tank), but since the OP seemed to be interested in carefully monitoring his SAC I just wanted to point out something he may not have thought of. From my experience, people do not find this intuitive and of the fellow divers that I have talked to about it not a single one had even considered taking it into account in their logs (although, to be fair, only few even went as far as to ever calculate their SAC rates). Point of the post: take the values for your calculations in similar conditions, because changes in temperature will affect the gas pressure, more or less.

Edit: Actually it did say it in a rather misleading way in my earlier post, edited that a bit now.

 

[Diver0001]

Edit - posted my calculations above, seems I'm kind of all over the place, but gradually getting lower rates. Thanks again!

The numbers look fine and actually look fairly consistent to me. The three higher dives were cavern dives and the excitement of diving in this environment might have something to do with that. SAC rates under .4 are not overly common among men. You're a small guy so it might drop to that level with more experience but there is no reason to be concerned about where you're at.

R..

 

[high/baric]

Also, although I think you probably have already thought of this, but remember not to mix dives where there has been more than a slight current with dives without current, as it will either give you a really high (swimming against the current) or quite low (swimming or coasting along the current) rate.