Universal Microsystems
High precision, restrictive flow orifices for
gas flow control

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  Principles of gas flow through sharp edged orifices
The basic behavior of gas flow through precision flow orifices can be seen in the accompanying figure for Nitrogen gas flow through a 144 Ám▓ orifice versus inlet pressure. Two curves are shown, one with an exit pressure to atmosphere and one to a vacuum. In both cases if the pressure is high enough the flow is linear with pressure. However, in the case of the atmosphere outlet the flow begins to fall off around 29 psia and eventually goes to zero at 14.7 psia.

The reason for this behavior is due to the velocity of the gas through the orifice which rises with increased flow until it reaches the speed of sound at the so called critical pressure ratio. In turn this critcal ratio is determined by the ratio of the gas heat capacity at constant pressure to heat capacity at constant volume.

The equations for sonic and subsonic gas flow are given below.
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