SAC rate recorded as PSI how to change to CF?

[Akcharger]

The Perdix records SAC as PSI per minute. No mater what I do I CAN NOT get the PSI per minute SAC to come close to the gas used.
for example: Start pressure 2816, End 1650 gas used 1166 psi for a 33 min dive. Using the recorded SAC of 10.75psi per min...which is only 363 psi??
So what am I doing wrong or Can I simply use some formula to change it to Cubic feet per minute

I have a LOT of dives to show accurate SAC rates but it's all this PSI stuff...any suggestions?

- 160 CF gas (double 80's)

Thanks in advance!

 

[Akcharger]

Wait, Googled it...then remembered

Tank size/working PSI then multiplied by PSI SAC so I got .5866

thanks anyway

 

[tursiops]

deleted

 

[Robert H. Diver]

Wait, Googled it...then remembered

thanks anyway

What did you come up with?

 

[inquis]

The SAC of 10.75 psi/min is also per atmosphere. You need to multiply by the pressure (e.g., 2 atm at 33 ft) to get the consumption (in psi) at depth from SAC.

To convert from psi -- which is specific to your tank -- to cubic feet, multiply by 1 in the form of (160 cuft / 3000 psi), and the psi units cancel, leaving cuft in the numerator. (Rated volume divided by rated pressure) For example, 10 psi/min is about 0.5 cuft/min for those tanks.

With units of cuft/min, this is known as RMV (respiratory minute volume).

Because it's easier to remember/compute, many divers will base this "tank factor" on a 100 psi basis. Two AL80s have a factor of 5.16 cuft/hundred psi (2*77.4/3000*100). (FWIW, AL80s only hold 77.4 cuft, not 80 cuft.) Typically this is rounded to 5 cuft/hundred psi for easier mental computation. If you burn 200 psi from your double 80s, that's about 10 cuft gone.

 

[Akcharger]

Robert - SAC rate of .58666 rounded up to be conservative of .6 at 120 feet
inquest - copy all...thanks for your reply!

 

[scubadada]

AL80s? 77.4 cu ft @3000 psi

RMV = 10.75 psi/min x 154.8 cu ft/3000 psi = 0.55 cu ft/min

 

[tursiops]

SAC rate of .58666 rounded up to be conservative of .6 at 120 feet

No, not quite; that is 0.6 cuft/min at the surface...it is a Surface Air Consumption rate.
At 120 ft your consumption rate would be (120/33 + 1) times larger, or 2.8 cuft/min.

 

[scubadada]

No, not quite; that is 0.6 cuft/min at the surface...it is a Surface Air Consumption rate.
At 120 ft your consumption rate would be (120/33 + 1) times larger, or 2.8 cuft/min.

SAC and RMV are both at 1 atm. I thought that is what @Akcharger wanted. Yes, you are correct, the RMV must be corrected for depth if one is doing gas planning for a dive.

 

[Belzelbub]

SAC and RMV are both at 1 atm. I thought that is what @Akcharger wanted. Yes, you are correct, the RMV must be corrected for depth if one is doing gas planning for a dive.

It sounds like he was looking to get consumption to match SAC/RMV, which will only be true at the surface.

The Perdix records SAC as PSI per minute. No mater what I do I CAN NOT get the PSI per minute SAC to come close to the gas used.
for example: Start pressure 2816, End 1650 gas used 1166 psi for a 33 min dive. Using the recorded SAC of 10.75psi per min...which is only 363 psi??

You'd need the average depth factored in to turn SAC or RMV into consumption. Assuming your SAC was correct, you'd only need 363 PSI if that were breathed at the surface. Since it wasn't, a higher consumption rate is expected. In round numbers, your actual consumption is a little over 3X your SAC. So average depth for that dive was somewhere around 3ATM or 66', which makes sense given the max depth of 120'.