The optical bench has a thermostat that maintains a constant operating temperature of 50 °C. A
feedback loop is used to regulate the optical bench temperature. As shown in Figure 4-1, two
thermistors, located in the source and detector housings, measure the present temperature. The
thermistors are monitored as part of the control loop to determine corrections necessary in the
thermal balance. Two heating elements are the sources of thermal energy into the source and
detector housing. The optical path is in mechanical contact with the source and detector hous-
ing and thereby achieves thermal equilibrium.
The bench requires approximately 10 minutes to achieve the specified thermal temperature. A
longer period of approximately 20 minutes is required to bring the performance of the detection
system to within 1 to 2% of reading. As shown in Figure 4-1 on the previous page, the detector
housing has a pressure transducer integrated into the housing design. Part of the CO
2
con-
centration calculation depends on the pressure observed in the optical path, measured with an
in-line pressure transducer. Many parameters can affect the pressure and thus the concentration
reading. The processing center in the analyzer reads the pressure reading as part of its data col-
lection task and uses this information in the concentration calculation. The gas flow enters the
source housing, passes down the optical path and exits at the detector housing. The maximum
flow rate for the analyzer is approximately 1 liter/min.
Section 4
4-4 Section 4. Theory of Operation
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