Swagelok Co. Guaranteed Flow Rate from a Flow Calibrated Orifice. A pressure regulator plus a flow-calibrated orifice is an excellent method of controlling the flow of a gas. An orifice can be flow calibrated and used over a range of flow rates by varying the inlet pressure. The flow rate is directly proportional to the absolute pressure in the critical flow range if all other factors are held constant. Critical flow occurs when the high side pressure is two times the low side pressure or larger.
Critical flow occurs when the velocity through the orifice equals the speed of sound.
The flow formula can be used to determine the flow rates for gases other than air and conditions other than critical flow. As you increase the pressure linearly, the flow increases linearly, assuming the orifice is calibrated to run within critical flow conditions. Using needle valves and mass flow controllers are not economical approaches for linear flow control.