A little of out your line, but . . .

[serambin]

When calculating SAC rate, is there an easy way to convert PSI to Cubic feet? For example, if your tank pressure drops 700 psi in 20 minutes at 40 feet, can this be converted to cubic feet of air consummed?

Is the relationship linear? For example, if you go from 3000 psi to 2300 psi, have you used the same amount of air as going from 1400 to 700 psi?

Thanks,

Stan

 

[Frank O]

When calculating SAC rate, is there an easy way to convert PSI to Cubic feet? For example, if your tank pressure drops 700 psi in 20 minutes at 40 feet, can this be converted to cubic feet of air consummed?

Is the relationship linear? For example, if you go from 3000 psi to 2300 psi, have you used the same amount of air as going from 1400 to 700 psi?

Hi Stan,

I don't know if you'd consider this easy, but the way I'd figure it would be like this (assuming you had an 80-c.f. tank that takes a fill to 3000 psi -- substitute other numbers for other tanks):

700/3000 x 80 = 18.67 c.f. used

Yes, the relationship is linear --- 700 psi is 23.3% of that 3000-psi tank, no matter whether you breathe it at the beginning, middle or end.

 

[Charlie99]

When calculating SAC rate, is there an easy way to convert PSI to Cubic feet? For example, if your tank pressure drops 700 psi in 20 minutes at 40 feet, can this be converted to cubic feet of air consummed?

Is the relationship linear? For example, if you go from 3000 psi to 2300 psi, have you used the same amount of air as going from 1400 to 700 psi?

Thanks,

Stan

The pressure to volume relationship is essential linear. Definitely close enough for SAC purposes, although gas blenders may use some correction factors.

Frank O gave you the basic way to get cubic feet of gas used.

For easier calculation of expected gas usage at different depths, air consumption is normalized back to what it would be on the surface, hence the term SURFACE air consumption.

At 33' the absolute pressure is 2ata and you deplete the tank at twice the rate as on the surface. At 99', you use up the tank 4 times faster. Use the correction factor of (Depth in feet + 33)/33 or (depth in meters + 10)/10 to compensate for depth.

Putting it all together, if we assume that you went from 3000-2300 psi on an AL80(which is really 77.4cu ft at 3000) at 40' ----

(3000-2300)/3000 * 77.4cu ft * 33'/(40'+33') / 20min = .41cfm


(startpsi - endpsi)/ratedpsi * ratedvolume * 33'/(depth+33')/divetime = SAC

For the depth, you have to use the average depth for the period you measure gas consumption. So either do it when at a constant depth, use a computer that give you average depth, or note your depth throughout the dive and average it before calculating SAC.

 

[serambin]

The pressure to volume relationship is essential linear. Definitely close enough for SAC purposes, although gas blenders may use some correction factors.

Frank O gave you the basic way to get cubic feet of gas used.

For easier calculation of expected gas usage at different depths, air consumption is normalized back to what it would be on the surface, hence the term SURFACE air consumption.

At 33' the absolute pressure is 2ata and you deplete the tank at twice the rate as on the surface. At 99', you use up the tank 4 times faster. Use the correction factor of (Depth in feet + 33)/33 or (depth in meters + 10)/10 to compensate for depth.

Putting it all together, if we assume that you went from 3000-2300 psi on an AL80(which is really 77.4cu ft at 3000) at 40' ----

(3000-2300)/3000 * 77.4cu ft * 33'/(40'+33') / 20min = .41cfm


(startpsi - endpsi)/ratedpsi * ratedvolume * 33'/(depth+33')/divetime = SAC

For the depth, you have to use the average depth for the period you measure gas consumption. So either do it when at a constant depth, use a computer that give you average depth, or note your depth throughout the dive and average it before calculating SAC.

Perfect!

Stan