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Teledyne API – Model T700 Dynamic Dilution Calibrator Principles of Operation
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10.2.1.3. Critical Flow Orifices
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 calibrator’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.
Figure 10-2: 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.
With a nominal pressure of 10 in-Hg-A in the sample/reaction cell, the necessary ratio of
reaction cell pressure to pump vacuum pressure of 2:1 is exceeded and accommodating a
wide range of variability in atmospheric pressure and accounting for pump degradation.
This extends the useful life of the pump. Once the pump degrades to the point where the
sample and vacuum pressures is less than 2:1, a critical flow rate can no longer be
maintained.
06873B DCN6388