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Teledyne API - Model 200EH/EM Operation Manual Theory of Operation
199
GAS FLOW
CONTROL
ASSEMBLIES
SAMPLE
PRESSURE
SENSOR
VACUUM
PRESSURE
SENSOR
Figure 10-10-9: Location of Gas Flow Control Assemblies for M200EH with Second Span Point Option 52
NOTE:
Location of flow control assemblies in the M200EH/EM with zero/span option 50
installed are the same as shown in Figures 10-6 and 10-7.
10.3.3.1. Critical Flow Orifice
The most important component of the flow control assemblies is the critical flow orifice.
Critical flow orifices are a remarkably simple way to regulate stable gas flow rates. They operate without moving
parts by taking advantage of the laws of fluid dynamics. By restricting the flow of gas though the orifice, a
pressure differential is created. This pressure differential combined with the action of the analyzer’s pump draws
the gas through the orifice.
As the pressure on the downstream side of the orifice (the pump side) continues to drop, the speed that the gas
flows though the orifice continues to rise. Once the ratio of upstream pressure to downstream pressure is
greater than 2:1, the velocity of the gas through the orifice reaches the speed of sound. As long as that ratio
stays at least 2:1 the gas flow rate is unaffected by any fluctuations, surges, or changes in downstream pressure
because such variations only travel at the speed of sound themselves and are therefore cancelled out by the
sonic shockwave at the downstream exit of the critical flow orifice.
04521C (DCN5731)