Pardon my lazy cut and paste of a post I made on that other board for a similar question. The numbers/tanks are different, but the math is the same. There are other methods discussed, so the thread may be worth a peek.
https://www.scubaboard.com/community/th ... ks.411223/
OK I was hoping someone with better math skills than me (Awap) will give you a simpler method; but the rascals have abandoned us; so here's one way. Transfer a known volume into the PST tank. Divide that volume by the pressure now in the tank to give you a constant. Next, multiply the constant by the working pressure to give you the volume at working pressure.
For instance, let's say you rent/borrow/buy/steal an AL 80 which actually holds 77 cu ft. at 3000psi
77 divided by 3000 gives you the constant (0.0256 cu ft per psi) for the 80
Transfer a known amount out of the 80 and into the PST (let's say take it down to 2000)
1000 x 0.0256 = 25.6 cu ft out of the 80 and into the PST
So now, we measure the psi in the PST tank and we have say 900 lbs
25.6/900 = 0.0284 our constant for the PST tank.
0.0284 X the working pressure 3000 give us the full volume 85.2 cu ft.
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