Which short steel tank??? help!

[Scubakevdm]

Actually I have a related question about HP tanks vs. LP tanks that I've been too embarassed to ask, but it seems like we have lots of shingle-board physicists on here so I'll ask away. I own HP steel 100s (3450psi), but last weekend I was diving with a buddy who rented LP steel 100s (2400psi). So I know for my tank exactly what PSI to turn around with on a given dive. How do I correlate a turn-around pressure of, say 1500psi on my HP tank with that of my buddies LP tank of the same volume? Is it simply 1500/3450 * 2400 = 1045 psi?

If so can I assume that a SAC rate on an LP100 will be about 70% of that for an HP100 (for a given diver on a given dive, etc.)?

Your math for the turnaround looks good but the sac should be the same for LP or HP.

 

[DA Aquamaster]

Your math for the turnaround looks good but the sac should be the same for LP or HP.

Not neccesarily, it's more confusing than that.

"SAC" is traditionally defined as surface air consumption in psi per minute and is consequently very tank dependent. If you change tanks, you also change the psi per minute that you will be using.

RMV on the other hand is by definition done in units of volume like cu ft/min or lpm. So it is independent of tank size and service pressure and the same unit can be used regardless of the tanks used.

Where it gets confusing is that many divers and at least one tech traning agency, use the term SAC when they are actually referring to RMV.

So if you are talking about your SAC as being in psi per minute, you are correct, it will have to be adjusted to a suitable psi/min value for another style of tank by converting the figure to cfm (an RMV) and then reducing it again to get a suitable psi/min figure for the differently sized or serviced pressured tank. If you are talking about a "SAC" in cu ft/min, then it will remain the same regardless of tank size or service pressure.