Particulate Filter; Figure 6-4. Flow Control Assembly & Critical Flow Orifice - Teledyne T400 User Manual

The critical flow orifice is the most important component of this flow control assembly 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 through 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 6-4. 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. 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.
6.2.3 P F
ARTICULATE ILTER
The T400 Ozone Analyzer comes equipped with a 47 mm diameter, Teflon, particulate
filter with a 5 micron pore size. The filter is accessible through the front panel, which
folds down to allow access, and should be changed according to the suggested
maintenance schedule described in Table 5-1.
083730400A DCN8130 Teledyne API – Model T400 O Analyzer 145
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