For a given gas in choked flow, the mass flow rate may be controlled by setting the orifice size or the upstream pressure. To produce a choked flow in oxygen, the absolute pressure ratio of upstream and downstream gas must exceed 1.893 at 20°C. At normal atmospheric pressure this requires an upstream pressure of more than 1.013 × 1.893 = 1.918 bar.
For a given gas in choked flow, the mass flow rate can be controlled by adjusting the orifice size or upstream pressure. To achieve choked flow in oxygen, the absolute pressure ratio of upstream to downstream must exceed 1.893 at 20°C. At standard atmospheric pressure, this necessitates an upstream pressure greater than 1.013 × 1.893 = 1.918 bar.