Calculating refills from a higher pressure tank

[Brett Hatch]

The specific question comes from paintball, which is the reason for the smaller 13 cubic inch tank, but I figured that the math behind it relates equally well to scuba

I think I discovered the source of my confusion. The manufacturer of the 97 cf tank says "97 cubic ft (or 550 cubic inches)": (Air Tanks For Sale Discounts for PCP (Pre-Compressed-Pneumatic) Air Gun by AirTanksForSale )

Holy cow, no wonder there is confusion -- that website is a godawful nightmare

 

[WetSEAL]

FIFY

Not sure what the standard way to describe this is, but seems to me that if the volume is 550 cubic inches or 0.31 cubic feet, then by definition it always holds 0.31 cubic feet of compressed air (though that's not very meaningful because pressure is not specifiied), but more specifically it can hold up to 97 cubic feet of air that is at atmospheric pressure. ie, it can hold up to 97 cubic feet of uncompressed air

 

[Storker]

Use the tanks' water volume. The pressures will equalize, and then all you have to do is to multiply water volume with pressure.

This is one of the many reasons I love our (metric) way of expressing tank capacities. Water volume times pressure is surface volume.

 

[Aviyes]

Try this calculator for airgun/paintball tanks and calculating number of fills off a larger tank.
Airgun Calculators by Lloyd Sikes

 

[ATJ]

Use metric! Far far easier!

 

[ATJ]

This is one of the many reasons I love our (metric) way of expressing tank capacities. Water volume times pressure is surface volume.

Approximately... air doesn't compress linearly but is close enough up to 200 bar for most uses. Between 200 and 300 bar it is quite a lot off.

But yes, metric is so much easier.

 

[lermontov]

Use metric! Far far easier!

 

[Storker]

Approximately... air doesn't compress linearly but is close enough up to 200 bar for most uses. Between 200 and 300 bar it is quite a lot off.

I know. And if "quite a lot" means "up to about 10%", I agree.

I use these fudge factors:
200 bar: pretty accurate
232 bar: close enough for government work
300 bar: subtract 10%