# Air breathing apparatus with pressure gauge and cylinder

I have got self contained air breathing apparatus with pressure gauge and cylinder. Cylinder has marking that it's 6 ltrs. When I open valve pressure gauge shows me 250 bars. Can I calculate the overall volume of air by multiplying volume of cylinder by actual pressure? So in my case it would be 6 $\times$ 250 = 1500 ltrs. I've got nothing in manual, concerning this formulae, neither I heard about it. But it fits my experience pretty nice. Is it right to calculate like this? Thank you.

Well. I will explain, giving more details. I work with air cylinders and Breathing Apparatus for fire fighting. They are similar to those divers use. I have requirement, that all of the cylinders should be filled by not less than 1200 ltrs of air. But I have only pressure gauge. The problem is how can I be sure, that cylinder has 1200 ltrs. I need to convert somehow ltrs to bars. When I had 8ltrs cylinders, the pressure gauge had marking on 150 bars that it's enough. So I decided, that it would be 1200 ltrs. So I can calculate volume of air by multiplying pressure by gage and capacity (volume) of cylinder. I just wonder, may be it's just a coincidence and there is no such a formulae. That's why I ask to clarify me please. Am I right?

• You mean the volume of the gas at standard pressure (i.e., 1 bar)? May 21 '17 at 11:30
• Take a look please, I edited my post and added some details May 21 '17 at 16:50

This is a simple application of the Ideal Gas Law $pV=nRT$. The pressures and volumes of a fixed amount $n$ of gas in two different states, but at the same temperature $T$, are related by
$pV=p_0 V_0$.