Teledyne T200 Operation Manual page 296

Principles of Operation
13.2.2.1. CRITICAL FLOW ORIFICE
13.2.2.2. LOCATIONS AND DESCRIPTIONS OF CRITICAL FLOW ORIFICES INSIDE THE T200
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Teledyne API – T200 NO/NO2/NOx Analyzer Operation Manual
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 hole, 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 13.1.5.2) and therefore increasing the
chemiluminescence yield.
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The T200 reaches its peak sensitivity at about 2 in-Hg-A, below which the sensitivity
drops due to there being too few molecules present and a corresponding decrease
in chemiluminescence.
The T200 uses several of the following critical flow orifices (Figure 13-7) to create and
maintain the proper flow rate of gas through its various components. (Please note that
Figure 13-7 represents the standard configuration and is provided for reference).
06858E DCN7057