Calculating efficiency

[TSandM]

I understand and applaud your wish to become more efficient in the water. It's a good goal. But you do recognize that your air consumption while diving will and SHOULD never approach your resting surface SAC rate, right? It would be far more realistic to try to identify a surface activity with a similar energy expenditure to the demands of a well-executed dive, and compare to that. But I have no idea how you would determine what the surface activity comparable to a well-performed dive in your local conditions would be.

I think the best you can do is to figure your SAC rate for a fairly reproducible dive, if you have one. For example, we have a very commonly dived local site, where you can describe a pretty typical tour. If you do that dive repeatedly, you can compare the trends in air consumption over time, or with gear or technique changes. Since no two dives are precisely the same, you will need a good set of data points to have reasonably meaningful information, but if you dive frequently, that's not too hard to do. I dive these local sites often, so it was easy for me to see that one of my first training dives using a reel caused a 50% increase in my SAC rate!

 

[dumpsterDiver]

I personally think that your air consumption while watching TV is pretty much irrelevant. Playing "the game" of trying to use as little air as possible while diving makes about as much sense as seeing how fast you can ascend from a dive without getting a headache.

Both activities are potentially dangerous and really are not consistent with "safe" diving. You need to use as much air as you need, trying to use even a tiny bit less than you "need" can result in CO2 headaches and can make narcosis much worse and it is really not a worthwhile goal (in my opinion).

You also have to remember that if you are trying to compare watching TV versus diving,,, a big difference is thermal stress, even with a good exposure suit, your metabolism is almost surely accelerated to some extent to preserve your core temperature while diving (at most temperatures anyway).

Figuring your SAC is an important tool... for planning... and if you are smart about your planning.. you are going to round your SAC rate up to a reasonably conservative value for you and for the expected dive conditions... activity level, type of gear, temperature etc...can cause big changes in air use.

Striving for some "numeric efficiency" in this manner, makes little sense to me. There is way more fun things to do underwater than trying to use the absolute minimum amount of gas.

 

[Will_Evo]

Bubbletrubble-Ah I see! RMV is independent of the tank being used and thankfully isn't too much harder to convert. I hadn't even considered that different tanks would produce different results. Thanks much. I am seeing what everyone is saying...I suppose I went into this simply wanting a percentage, and the only way I can see of comparing the whole dive to your optimal breathing rate is by testing your optimal rate. I suppose I could take this optimal breathing rate at 60ft, but its just going to be corrected back to surface pressure anyway.

Charlie99- I understand and actually before I read this post was doing those crude estimates on how long I could stay at a depth given my SAC. Like I said above, I went into this just wanting a comparison, and I see how sitting in a chair isn't exactly useful. Unfortunately I don't have the resources yet to do this kind of test so its going to have to wait.

TSandM- No no I understand that my breathing underwater shouldn't reach a resting SAC, its just that in order to have some type of percentage, Ive got to have a starting point.

dumpsterDiver-Thanks for the reply, I am on board with the surface lazyboy SAC being rather useless to compare a dive to and please understand I am not trying to improve my SAC, I'm still new and am focusing on other things...like diving haha Me and friend just got bored at work and started crunching numbers and thought to ourselves this would be cool, or that would be useful and even though most if it has led nowhere, I've learned a ton just researching it all.

Again thanks everyone for the helpful replies. Some things I have taken from this thread are :

-For comparisons sake(dive to dive or trending), I need to use RMV instead of SAC because it is independent of the tank used
-Taking my SAC sitting in a lazy boy isn't going to be very useful considering the factors that go into diving as compared to just sitting.
-I can use my SAC in the predive process to give myself an idea of how much time I can spend underwater. (Assuming I am using the same tank as the one used to find my SAC)
-The world of dive calculations is a terrible place...haha by that I mean before we found an article to show us how to calculate SAC(Something simple) we spent the better part of two hours trying to calculate it but kept falling short on how to correct for depth. I applaud those who make dive computer algorithms haha pretty intense stuff.
-Having a percentage isn't going to be very useful, rather just a number to gawk at. This point in a nutshell is me saying, SAC and RMV alone will be more useful and I understand why.

-Evo

 

[Q1988]

This is how we did it.

Dove to 2 atm in fresh water, about 66 feet and paced at a casual swim. Marked the time and the tank pressure and swam for 5 minutes. Marked the time and the tank pressure again.

At the surface determined the cubic feet consumed from the tank size (we all have different steels at different pressures) and with that number divided by the time and then by 3 for the conversion to SAC. The 3 comes from 2 atm from the depth and 1 at the surface.

Most of us were between 0.64 and 0.68 SAC in cubic feet.