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Teledyne API T803 CO2/O2 Analyzer Operation Manual Principles of Operation
211
SPRING
O-RINGS
FILTER
CRITICAL
FLOW
ORIFICE
A
REA OF
LOW
PRESSURE
AREA OF
HIGH
PRESSURE
Sonic
Shockwave
Figure 12-6: 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 T803 is designed to provide a flow rate of
120 cm
3
/min.
12.5.2. PARTICULATE FILTER
The T803 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
12.5.3. PNEUMATIC SENSORS
12.5.3.1. SAMPLE PRESSURE SENSOR
An absolute value pressure transducer plumbed to the outlet of the sample
chamber is used to measure sample pressure. The output of the sensor is used to
compensate the concentration measurement for changes in ambient air pressure.
This sensor is mounted to a printed circuit board with the sample flow sensor on
the sample chamber; see the following section and Figure 3-5.
12.5.3.2. SAMPLE FL
OW SENSOR
A thermal-mass flow sensor is used to measure the sample flow through the
analyzer. The sensor is calibrated at the factory with ambient air or N
2
, but can
be calibrated to operate with samples consisting of other gases such as O
2
. This
07276B DCN6418