In the gas industry you often need to know the installation volume of a system, i.e. the amount of gas it contains in the pipework, fitting and meters. If part of the installation is buried or inaccessible this is difficult to estimate. My concept is to estimate installation volume by measuring the change in pressure when the system volume is increased by a known amount (i.e. by opening a valve to a smaller system of known volume). I've had a go at the maths and experimentation but the results have not yet been good. The method is as follows: The SYSTEM is the meter / pipework installation whose volume we wish to know. The TESTER is a small section of pipework and a gas meter whose volume is known, which is attached to the SYSTEM by an isolation valve. 1. Ensure the TESTER is at atmospheric pressure 2. Determine atmospheric pressure 3. Ensure the SYSTEM is pressurised above atmospheric pressure 4. Close the incoming gas supply to the SYSTEM 5. Measure the absolute pressure of gas within the SYSTEM (atmospheric pressure + gauge pressure) 6. Open the isolation valve on the system to allow gas to enter the TESTER 7. Measure the absolute pressure of gas within the now-combined SYSTEM + TESTER Some figures I got: Atmospheric pressure = 1015 mbar TESTER volume = 0.030 cubic metre Absolute initial pressure of SYSTEM = 1038.4 mbar Absolute final pressure of SYSTEM + TESTER = 1032.85 I tried using the Ideal Gas Equation, but I got a SYSTEM volume of 5.6 cubic metre, which is vastly larger than the actual size. One problem I can see is that the TESTER does not contain a vacuum initially, so the number of molecules of gas increases when the SYSTEM and TESTER are combined. This means the simple proportionality of the Ideal Gas Equation won't work. This is showing me the limitations of my maths, and it's where I could really do with a little help.