T400 Ozone Analyzer Operator’s Manual Theory of Operation
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13.2.2. FLOW RATE CONTROL
To maintain a constant flow rate of the sample gas through the instrument, the Model
T400 uses a special flow control assembly located downstream from the absorption tube
and in the exhaust gas line just before the pump (see Figure 10-7). This assembly
consists of:
A critical flow orifice.
Two o-rings: Located just before and after the critical flow orifice, the o-rings seal
the gap between the walls of assembly housing and the critical flow orifice.
A spring: Applies mechanical force needed to form the seal between the o-rings, the
critical flow orifice and the assembly housing.
Figure 13-4: Flow Control Assembly & Critical Flow Orifice
13.2.2.1. 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.
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. Using this critical
flow orifice design extends the useful life of the pump. Once the pump degrades to the
point where the sample to vacuum pressure ratio is less than 2:1, a critical flow rate can
no longer be maintained.
