True Tank Volume

[reo]

I just bought an XS Scuba HP Steel 119 tank. I've looked in a few places and have found the true capacity at 3442 PSI to be either 119 cu ft or 123 cu ft, depending on whose chart you use. Which of these is correct?

Also, is tank pressure PSIA or PSIG?

Thanks,

Ron

 

[JohnB47]

Tank pressure is PSIG. When your tank is empty, at the surface, your gauge reads 0 but there is still air in the tank at 14.7 PSIA. Of course, the difference between 3442 PSIG and 3456.7 PSIA is not significant.

I don't know what the actual volume of the X8-119 is, or why there is a discrepancy in the numbers, but here is a guess. If you measure the volume of the unpressurized (empty) tank and multiply by the rated pressure in bar, you get the theoretical capacity of the tank. However, when you actually pressurize the tank, the walls of the tank will stretch slightly, so that the internal volume increases, and the actual capacity will be greater than the theoretically calculated value.