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.
In fluid dynamics, mass flow rates can be controlled by adjusting orifice size or upstream pressure in choked flow situations. To achieve choked flow in oxygen at a temperature of 20°C, the upstream-to-downstream pressure ratio must exceed 1.893. At standard atmospheric conditions, this requires an upstream pressure greater than 1.013 × 1.893 = 1.918 bar.