900 RRX wrote:A 10 litre capacity cylinder will hold x amount of breatherble air at 220 Bar. What would the difference be if the Bar pressure was raised to 270.
Roughly 270/220 times as much. But you are reaching pressures where the behavior deviates from ideal gas law, and you need to consider compressibility factor or some other model for behavior.
(Make sure it is rated for 270 Bar or it might not hold any air.)