SAC Rates

[ctscubadiver]

Question is: How to properly calculate SAC Rate. example: 70'dive, 38 min dive, 3000psi starting pressure, 500psi ending pressure using alum 80 tank. Thanks.

 

[Rick Murchison]

Most of the major Open Water agencies teach SAC (Surface Air Consumption) rate or SCR (Surface Consumption Rate) based on a standard Aluminum 80, and calculate the answer in psi per minute (surface). In the example given, (assuming you arrived at 70' with 3000psi and departed 70' with 500psi) at 70 feet, you are at 3.1 Atmospheres, and so will consume air at 3.1 times the rate you would at the surface. Since you used 2500 psi in 38 minutes, that's 65.8 psi/minute, which yeilds a SAC rate of 65.8/3.1, or 21.2 psi/minute at the surface.
In technical diving, because we use many different size tanks, it is much more useful to know how much actual air we use, so we take that 21.2 psi/min in a standard AL 80 and convert it to CFM (Cubic Feet per Minute) - in this case, 1 CF in an Aluminum 80 takes 38.75 psi, so our SCR is 0.55 CFM.
Using that number, for example, I can calculate that a 20 minute bottom time at 100 feet would use 4(Atmospheres) times 20 (minutes) times 21.2 psi (SCR) - about 1700 psi from a standard AL 80, or 4 X 20 X .55CFM (SCR) - about 44 CF. Note that by using the 44 CF I can easily apply that to any size tank, while the 1700psi is only meaningful for the Aluminum 80.
Rick

 

[Warhammer]

Here's an article that explains all you'll ever want to know about SAC and the some.

http://www.ivydene1.demon.co.uk/dive/gascalc.htm

 

[Rick Murchison]

Caution:
When calculating CF, if you use the article cited, the author makes a basic mistake by assuming a standard 80 holds 80 CF at its working pressure of 3000 psi. It does not - holds 77.4 CF at 3000, 80 at 3100 (assuming the tank's at 15C/59F of course).
Rick

 

[ctscubadiver]

Thanks for the SAC info. Very helpful

 

[Warhammer]

Originally posted by Rick Murchison
Caution:
When calculating CF, if you use the article cited, the author makes a basic mistake by assuming a standard 80 holds 80 CF at its working pressure of 3000 psi. It does not - holds 77.4 CF at 3000, 80 at 3100 (assuming the tank's at 15C/59F of course).
Rick

What mistake? The article doesn't say which 80ft3 cylinder, it just says an 80ft3 cylinder. There are several 80ft cylinders that hold 80ft3 of air. To name a few the Luxfer S808 aluminum, the Pressed Steel LP80, Faber HP80, and the Pressed Steel HP80.

 

[Rick Murchison]

Warhammer, you are correct in saying that there are several different 80's - and some do hold 80. But caution is advised because the article didn't specify which 80, and the most common by far - indeed the only one you're likely to run into as a rental tank anywhere in the States or Caribbean, is the 77.4. I don't have numbers - only 30+ years diving all over the world, and I don't believe I've ever seen an 80 other than the standard 77.4 offered as rental equipment to a recreational diver.
Rick

 

[Warhammer]

That's true, the author should have used a different example or pointed out that you should use the tanks actual capacity for calculations. But he probably assumed folks were aware of their tanks capacity. As far as rental tanks go, correct again, the vast majority are, as you say, the standard 77.4cf AL80s. But I've found a few places that have HP80s for rent. My wife, who is small, always asks and sometimes we get lucky.