If air were a perfect gas, the free volume of gas in a cylinder at constant temperature would be equal to P x V
where P is the tank pressure in bars
V is the cylinder capacity

If the temperature changes, the volume would be equal to P x V x T1 / T2
where T1 is the temperature in °Kelvin at the temperature the gas is used
where T2 is the temperature in °Kelvin at the temperature the gas is stored
(°Kelvin =°C + 273)

However air is not a perfect gas, so the compressibility factor has been measured and this must be included for accurate results. The volume is
P x V x (T1 / T2) x (Z1 / Z2)
Where Z1 is the factor at which the gas is used
Where Z2 is the factor at which the gas is stored

The graph below gives the factors at different temperatures and pressures. 77°C is included because air at low pressures has a compressibility factor of 1 at that temperature. Therefore it is following ideal gas laws at 77°C and low pressures