150 Theory of Operation 083730300A DCN8101
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 6-8. 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 the gas
molecules move at the speed of sound and pass through the orifice. Because the flow rate
of gas through the orifice is only related to the minimum 2:1 pressure differential and not
absolute pressure, the flow rate of the gas is also unaffected by degradations in pump
efficiency due to age.
The critical flow orifice used in the T300/T300M is designed to provide a flow rate of
800 cc/min.
6.3.2 PARTICULATE FILTER
The T300/T300M 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.
6.3.3 PNEUMATIC SENSORS
There are two pneumatic sensors: one each to measure sample pressure and flow.
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