Tank pressure drops day after filling?
I took my new tank to be filled at a dive shop. It took about 10 minutes to fill. The tank gauge was 4500 on the nose when I left but the next day dropped down to 4100? Is this normal? Thanks. Bill
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Cool!!!
Just for fun, let’s estimate how hot the air in the tank was at the end of fill. We know your end fill pressure, next day pressures and let us assume normal room temperature 293 Kelvin.
std gas equation
PV = nRT
V & nR are all constant assuming you didn’t use any of the air in the tank. Only Temp and Pressure change.
That simplifies the relationship of the two days to P fill/P cooled = T fill / T cooled
4500 / 4100 = Tfill / 293
T fill = 293 * 4500 / 4100 = 321.6 kelvin
Temp in tank at fill end was 321.6 Kelvin or 48.6 celsius or 120 F.
That isn’t that hot to the touch with carbon fiber insulating you from the interior heat.
You only feel it to be somewhat warm if you touch the tank on the day of fill
That is normal. The air is hotter just after filling. Cooling overnight lowers the pressure.
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The ideal gas law, PV = nRT, works pretty well for air at 1 bar and “normal” temperatures. At 200 to 300 bar it is only a rough approximation. The van der Waals equation is a better approximation and is easy to use.
For engineering and design work there are computerized air models that give good results at high pressures.
-x356b