? cubic feet air in a tank at 3500psi vrs 3000psi hot

[nice-diver]

I am going on a liveaboard in october. The Truth out of Santa Barbara.

If I bring my HP 120 3442psi tank, and it get filled with the onboard compressor that only fills to 3000psi, most likely hot. What will be the volume in the tank???

I am looking for an equation or an answer.


thanks

[thelawgoddess]

equation: ideal gas law (wikipedia) says PV = nRT

it's actually just simple algebra, but you need to know your tank's ratings. (if you know what make of tank you have, you should be able to find this information online.) roughly:

actual pressure of tank
----------------------------- x rated capacity of tank = actual volume in tank
rated service pressure

(i hope i got that right!)

[thelawgoddess]

i forgot to note that the "rough" equation assumes a constant temperature. i'm not sure what the estimated loss of pressure would be going from a hot tank to cold water ...

[Grateful Diver]

A 120 with a working pressure of 3442 psi would have roughly 3.48 cubic feet of gas for every 100 psi of pressure (120 CF / 3442 psi = .0348 CF per psi).

Therefore at 3,000 psi it would hold 104.6 cubic feet of gas ... if you don't consider variations due to temperature changes.

You can do a rough calculation for any pressure on that tank by multiplying 3.5 cubic feet of gas for every 100 psi of pressure.

... Bob (Grateful Diver)

[Nwbrewer]

Lawgoddess' first equation will tell you how the temperature effects your tank volume.

Bob's equation will tell you how much volume you have in the tank assuming that both fills are at the standard tank temperature (it's around room temp I believe, somewhere around 70 deg F?? One of the more knowledgable should be able to help me out.)

Anyway, you should be able to then figure what your pressure drop will be by PV = nRT.


Jake

[Burntchef]

i just did a charter and of course the fills are hot ones especially for a hp tank, my spg would read 3500 ish but once i hit the water it would drop at least 300 easy. all my buddies were using lp tanks and had much better fills even after dropping in.

my guess is the commpressor on the boats just arent as big as a shops, they are filling multiple tanks off one whip, and they have a bunch of tanks to fill in about a hour depending on the si.


go with lp.

[CaptnJack]

3000psi hot typically cools to about 2700. Don't ask me for the math, but experience has shown that in this pressure range a 10% loss in volume is typical. So using Grateful Diver's math above gives about 93cf of useful gas for your dives. (27 x 3.48) For some reason the CA boats have limited to non-existant HP pumping capabilies.

[thelawgoddess]

nice to know that the general expected drop is about 300psi. (we always do our dives with a turn pressure in mind, so by the time we hit that the tank's long cooled ...)