SAC Rates

[ereediver]

If this has been asked pardon the repeat, but I could not find anything in the search. Well to the point. I have been trying to keep track of my SAC rates to get an idea of how long my tank "SHOULD" last. I have a general Idea after tracking 28 dives of various levels. My SAC rate averages 20 to 26 PSI per min. depending on depth. Now given 3000 PSI in a tank, If I divide that by my rate, 3000 / 26 I get 115...... Now I get nowhere near 115 minutes which I take to mean I am missing a step in the process. Any math wizzes out there care to lend a hand?

 

[jracca]

Surface air consumption SAC
Means you would get 115 minutes at the surface, I assume your dives aren't
done totally at the surface so hence your times would be shorter.

FYI, PSI per minute is not a good air consumption measure unless
you always use the same size tank.

 

[ClayJar]

Using a surface consumption number and the amount in your cylinder, you can find how long your cylinder will last on the surface. To convert that to how much time it'll last at any given depth, you indirectly use the ideal gas laws.

Basically, multiply your time by the (absolute) pressure on the surface, and divide by the (absolute) pressure at the relevant depth (which would be your dive's average depth, if you were doing this after the fact).

At 33', you'll only get half the time you would on the surface. At 100', you'll get a quarter as long as on the surface. (The numbers work just like volumes.)

 

[Dash]

Try this site. Nice online calculator and has the formulas I believe.

http://www.spearfishing.org/bruces_tips/java/sac.html

 

[The Kraken]

OK, an aluminum 80 (77.4 cf)
Given a residual psi of 500psi, based on a square profile and an AVERAGE depth of dive, this is the dive time you can expect at the respective depths:

SAC rate = 0.70 50' = 34; 60' = 30; 70' = 27; 80' = 25; 90' = 23; 100' = 21
SAC rate = 0.60 50' = 40; 60' = 35; 70' = 32; 80' = 29; 90' = 27; 100' = 25
SAC rate = 0.50 50' = 48; 60' = 43; 70' = 38; 80' = 35; 90' = 32; 100' = 30
SAC rate = 0.40 50' = 60; 60' = 53; 70' = 48; 80' = 44; 90' = 40; 100' = 37

Keep in mind that these figures are approximate.

But, at least, it can give you something from which to start.

the K

 

[ArcticDiver]

Send a PM to NWGrateful diver. He has generously put together an excellent instructional paper on this subject. I feel confident he will share.

One word of caution:
Don't use your SAC/Gas Consumption Rate/etc as a comparative measure of your diving competence. It is used exclusively for two purposes: 1)To make sure you, and your buddy if you have one, have enough gas to do the planned dive. 2)For gas matching so you and your buddy know who has what gas and whose gas supply controls the planned dive.

Remember to have fun!

 

[The Kraken]

Well put, Arctic !!!!

A SAC rate is use for planning !!! It's not something you work with as you're diving.

the K

 

[ereediver]

Yes I intend to use it for dive planning. especially when it comes to turn marks. I realize my rates should get better with time. As it is I am probably at a high mark so I want to get a good idea when I need head back to the line. I am not really near the NDLs for the most part so it comes down to air. So far an aluminum 80 is my tank. When I get this down I will work on cubic feet per minute.

 

[Meng_Tze]

Well put, Arctic !!!!

A SAC rate is use for planning !!! It's not something you work with as you're diving.

the K

What about instances where you think: "Hmm, that looks cool over there. I want to go have a look. Uhh, do I have enough gas and time for that? I have 13 mins NDL left. It takes 3 mins. to surface so I could go out 5 mins and 5 mins back. I use about 11 cft at this depth/5 mins..... so I need 22 cft for that.... I have a little over 30cft left... so yes, lets go and see that abyss. I'll just take 4 mins out"

No?

 

[The Kraken]

What about instances where you think: "Hmm, that looks cool over there. I want to go have a look. Uhh, do I have enough gas and time for that? I have 13 mins NDL left. It takes 3 mins. to surface so I could go out 5 mins and 5 mins back. I use about 11 cft at this depth/5 mins..... so I need 22 cft for that.... I have a little over 30cft left... so yes, lets go and see that abyss. I'll just take 4 mins out"

No?

Well, by the time ya stopped an figgered all of that out, ya'd be outta gas any nohow !!!

the K