Confusion regarding SAC rates and SAC rate computers

[Ice9]

So I'm reading the SAC discussion below and I noticed something odd:
My numbers look nothing like what others are posting.

Looking at my trusty NAUI SAC Rate computer, I'm seeing my numbers in the range of 30-40. Hmm. What does that mean? I'm the Uber Hoover?

I think this thing uses a calculation that looks like this:

ATA = (average depth / 33) +1
DELTA = starting PSI - ending PSI
TIME = Dive time in minutes

SAC = (DELTA / TIME) / ATA

Just looking at one dive and I get a SAC rate of "38." Is that 38 PSI per minute? Seeing numbers like .61 in the other post makes me think something is terribly wrong with this SAC Rate computer or there is a mismatch somewhere.

What am I missing here?

 

[shakeybrainsurgeon]

I believe the 0.61 is cubic feet per minute, whereas you are measuring psi/minute... converting the two requires a knowledge of the tank geometry

 

[Ice9]

Hmm.. How would you convert to cu ft/min with an AL80 rated for 3000psi?

 

[Blackwood]

Aye. As was said, the computer is figuring psi/min. Since your computer doesn't know the rated volume or pressure of your tank, it can not display cfm.

 

[Ice9]

So if you knew you had an AL80 rated for 3000PSI and a delta of 2048PSI, how would you convert to cu ft/min?

 

[Blackwood]

Hmm.. How would you convert to cu ft/min with an AL80 rated for 3000psi?

Figure you get 78 cubic feet at 3000 PSI, so each PSI represents 0.026 cubic feet.

PSI/min * CF/PSI = CF/min

 

[shakeybrainsurgeon]

A sac rate of 38 psi/min on an al80 works out to just under 1 cfm I think

 

[Ice9]

OMG!

(38 * 78) / 3000 = .988

.988 = HOOVER SUPER-SUCK 3000

Hmm... It would seem that I have an air consumption problem.....

EEEEK!!!!

 

[SparticleBrane]

SAC is in cf/min/ata. So you need cubic feet, the time of the dive in minutes, and the average depth in atmospheres absolute.

3,000psi / 77.4cf (Al80) = 38.76 psi/cf
You used 2,048 psi, so that's (2,048psi/(38.76psi/cf)) = 52.83cf used during the dive.
You didn't tell us how long the dive was, nor the average depth. Using the information you've provided this is as much as we can help. To get your SAC, take 52.83cf and divide it by the length of your dive in minutes, then take that quantity and divide it by your average depth in ata.

 

[SparticleBrane]

OMG!

(38 * 78) / 3000 = .988

.988 = HOOVER SUPER-SUCK 3000

Hmm... It would seem that I have an air consumption problem.....

EEEEK!!!!

It would seem that you're not calculating it correctly.