Say we have a required mass flow rate, in order to have equivalent volume flow rate, we need to consider density of the operating fluid (compressible fluid).
To measure the volume flow rate in LPM, I'm using a rotameter. I'm controlling the flow using a needle valve upstream of the rotameter. The regulated pressure upstream of the needle valve is 20 bar and the pressure downstream the rotameter is ambient (1 atm).
Should I consider the density of the operating fluid (say oxygen) relating to the upstream pressure (20 bar) or the downstream (1 bar ambient)