Posted by Ken Kurtis on February 02, 2001 at 18:36:48:
In Reply to: Never Get A Short Fill Again - Filling Stations Please Note! posted by Bill Johnson on February 01, 2001 at 13:23:33:
If you're goin g to use the "piv-nert" formula, you've got to remember that the temps must be calculated in degrees Rankin, not Fahrenheit. To convert, you simply take the ºF and add 460.
So going from 70ºF to 130ºF may appear to be an 86% change, but when you convert it to ºRankin to properly use the formula, it's going from 530ºR to 590ºR, which is a change of only 11%. Practically speaking, it means that a tank hot-filled to 3000psi should cool down to about 2670psi. A tank hot-filled to 3300psi (over-filling to compensate for the presurre loss due to cooling) will end up around 2937psi.
However, if you fill the tank in water (submerged almost to the neck), and fill slowly (300-500psi/minute), you should get a negligible temperature change. Even if the tank got to 90ºF (550ºR) it would mean that it would cool by about 3%, or down to 2910psi. Filling it to 3100psi in water would yield around 3000psi after cool-down (like when you jump into the much-cooler ocean).
Ken Kurtis
NAUI Instr. #5936
Co-owner, Reef Seekers Dive Co.
Beverly Hills, Ca.