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Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer
Teledyne Analytical Instruments 195
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.
SPRING
O-RINGS
FILTER
CRITICAL
FLOW
ORIFICE
A
REA OF
LOW
PRESSURE
AREA OF
HIGH
PRESSURE
Sonic
Shockwave
Figure 9-8: Flow Control Assembly & Critical Flow Orifice
The actual flow rate of gas through the orifice (volume of gas per unit of time), depends on the size and
shape of the aperture in the orifice. The larger the hole, the more gas molecules, moving at the speed of
sound, pass through the orifice. Because the flow rate of gas through the orifice is only related to the
minimum 2:1 pressure differential and not absolute pressure the flow rate of the gas is also unaffected by
degradations in pump efficiency due to age.
The critical flow orifice used in the Model GFC 7000TA is designed to provide a flow rate of 800 cm
3
/min.