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Originally Posted by Alpha
Not necessarily.
You have a HPA tank at 3000 PSI and its 68 cubic inches.
You have a scuba at 136 cubic inches and 3000 PSI
You will be able to fill your HPA tank to 3000 PSI twice. Beucase your taking 68ci from the scuba, and putting it in the HPA tank.
Now a 136 cubic inch scuba is unrealistic and probably doesn't exist, but its just an example. But oyu get what I'm syaing?
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No, that is incorrect. You don't take 68 ci of air at 3000 psi. You need to have some grasp over the ideal gas laws to understand this precisely, but I'll try and explain it.
The ideal gas law is pv = nrt: pressure x volume = number of mols x gas constant x temperature.
What happens is you take a certain number of molecules of air and allow them to enter into your hpa tank. Based on the gas laws, that number of molecules of air at a certain temperature and volume, will be contained under a certain pressure. So you don't take 68 ci of 3000 psi air, you take a certain number of mols of air, the volume is set at 68 ci, the temperature should be the same as the temperature it is being stored at, and it gives a resultant pressure.
The problem with using a scuba tank though, is that you can't just pump as much as you want into the tank, you just connect the scuba tank to your hpa tank, and the air goes from an area of high pressure, the scuba tank, to an area of low pressure, your empty hpa tank. It just balances out, it doesn't pump in, so now you have the same number of mols of air contained in a slightly larger container than when it was just the scuba tank, because your hpa tank has become part of the system. So everytime you fill your hpa tank, you decrease the pressure in the scuba tank slightly because it has less mols of air in the same size tank, so every fill is less and less psi than the previous fill until finally you can't get a decent fill off your scuba tank.
This is what air compressers are for, and why they cost so much, they make every fill a complete fill.
In response to the initial question, I don't believe that they make 3500 psi tanks, but if you fill a 4500 psi tank up to 3500 psi it will be fine. I'm not sure what would happen if you connected a 3000 psi tank to a 3500 psi fill station, because I don't know what sort of safety measures they implement in order to prevent over filling, but I would imagine there would be something in place.