how do you work out how many litres are in a bottle

[s410man]

9.4 litre bottle filled to 137 bar

how many litres are in this

thanks

 

[rtbcomp]

Atmospheric pressure is 1 bar, volume is inversely proportional to pressure for a given temperature, so 9.4 litres of gas at 137 bar = 137 x 9.4 litres at 1 bar.

P is pressure, V = volume, T = temperature (deg absolute) k = constant

In the bottle PV = kT,
At atmospheric pressure pv = kT

so PV = pv ie 137 x 9.4 = 1 x v

so vol (v) at 1 bar = 137 x 9.4

 

[Sid]

You don't get all of that though.

 

[Luke]

The above is sound maths for a comparison.

You'll not get the last 9.4 Litres, unless you suck it out and leave a vacuum!

Practically, the regulator and pipework restrictions mean you'll end up with a bar or two in the cylinder when you've not got enough gas flow to weld with, but the maths is still useful for comparing cylinders/suppliers.

 

[backpurge]

Most major gas companies fit a positive pressure valve into the main cylinder neck valve which means cylinders cannot be totally drained, leaving some pressure in the cylinder ensures no ingress of contamination/moisture into the cylinder

 

[rtbcomp]

I suppose a more accurate calculation would be to use the pressure difference between empty and full rather than assuming the bottle empties to atmospheric pressure?