If I remember my A Level physics correctly, you have to use mass, as the volume will change with temperature and pressure. I belive the equation is:

(P1 * V1) (P2 * V2)
--------- = ---------
T1 T2

Where P1 is the initial pressure in bars
Where V1 is the initial volume in cubic metres
Where T1 is the initial temperature in K
And P2 is the second pressure in bars
V2 is the second volume in cubic metres
and T2 is the second temp in K.

Basically, it's pressure multiplied by volume, then divided by the temperature.

If the temperature changes (30C at ground, -50C at 37,000ft) the volume of the fuel will change.

I think this is Charles' Law.

This was some years back, so please feel free to correct me.

(Hmm, can't seem to make the equation come out right, the T2 should be underneath the P2 and V2)

[This message has been edited by Dave_C (edited 10-02-2003).]