Critical Flow Orifice; Locations And Descriptions Of Critical Flow Orifices; Figure 6-6. Flow Control Assembly & Critical Flow Orifice - Teledyne T200 User Manual

CRITICAL FLOW ORIFICE

The most important component of each flow control assembly is the critical flow orifice.
Critical flow orifices are a simple means 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 through the orifice, a pressure differential is created. This pressure differential,
created by the analyzer's external 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.
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 holes, the more gas
molecules (moving at the speed of sound) pass through the orifice.
In addition to controlling the gas flow rates into the reaction cell, the two critical flow
orifices at the inlets of the reaction cell also maintain an under-pressure inside it, effectively
reducing the number of molecules in the chamber and the corresponding incidence of third
body quenching (see Section 6.1.5.2), and thereby increasing the chemiluminescence yield.
The analyzer reaches its peak sensitivity at about 2 in-Hg-A, below which the
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sensitivity drops due to there being too few molecules present and a corresponding
decrease in chemiluminescence.

LOCATIONS AND DESCRIPTIONS OF CRITICAL FLOW ORIFICES

Several critical flow orifices (Figure 6-7) are used to create and maintain the proper flow
rate of gas through its various components. (Please note that Figure 6-7 represents the
standard configuration and is provided for reference).
083730200A DCN7962
Figure 6-6. Flow Control Assembly & Critical Flow Orifice
Teledyne API T200 NO/NO /NO Analyzer with NumaView™ Software
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