Teledyne API T801 NDIR CO2 Analyzer Operation Manual Principles of Operation
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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.
SPRING
O-RINGS
FILTER
CRITICAL
FLOW
ORIFICE
REA OF
LOW
PRESSURE
AREA OF
HIGH
PRESSURE
Sonic
Shockwave
Figure 12-4: 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.
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 T801 is designed to provide a flow rate of
120 cm
3
/min.
12.4.2. PARTICULATE FILTER
The T801 Analyzer comes equipped with a 47 mm diameter, Teflon, particulate
filter with a 1 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
