fnerg wrote:So never having done this, or even having a whip, if you're filling a smaller tank from a bigger tank, I'm guessing the resulting pressure would be something like
(tank pressure 1 - tank pressure 2)/((1- (tank size 1/tank size 2))/2)
So transfilling a 19 cf tank at 500psi from a 120 cf tank at 3500psi would give you a pressure closer to 3500psi in both tanks?
I just think about things like this sometimes.
I’ll attempt to answer the original question, assuming constant temperature:
Say we have two tanks, pressures P1 and P2, empty volume V1 and V2. The volume of gas in each tank is P1.V1 and P2.V2, respectively (pressures in ATM, but you’ll see that PSI work just the same). The volume of gas after equalization is Pe.Ve = P1.V1 + P2.V2 and we know that Ve = V1 + V2 (same number of moles).
Solves Pe = (P1.V1 + P2.V2) / (V1 + V2)
As you can see the units do not matter as long as 0 PSI == 0 ATM, so this works with tank factors.
Say 1 is a 19 cu.ft aluminum tank, V1 = 19/3000 = .006333, and P1 = 500 PSI
Say 2 is a 120 cu.ft HP tank, V2 = 110/3500 = .040619, and P2 = 3500 PSI
Results in Pe = 3032 PSI.
Now the big tank will be cooler and the small tank hotter because of the transfill, so unless you let the temperatures equalize, there will be a small difference. You will also lose a bit of gas in the transfill whip.
