Chapter 8 Theory of Operation
If a time constant other than 10 seconds is set, the program takes the average over the
proper number of values. Thus if a time constant of 60 seconds is set, the program will
retrieve the past six measurements, take the average, and output this information to the
display and analog outputs.
Note, however, that a new update still occurs every ten seconds, with the latest computed
concentration being added to the average, and the earliest one being dropped. Thus every
ten seconds, there will be an updated average of the past six current computed
concentrations.
SUBASSEMBLIES
Optical Bench
The optical bench is of the White Cell design. The use of the White Cell multipass
optical bench allows one to achieve a long path length, with a large acceptance angle, in a
small physical package. The bench has been designed for easy disassembly for cleaning.
The source, detector, correlation wheel, and chopper motor mount rigidly to the bench.
No realignment should be necessary after routine cleaning.
Correlation Wheel and Chopper Motor
The correlation wheel consists of two hemispherical cells, one filled with CO and the
other with N
2
. Integral with the correlation wheel is the chopper pattern necessary to
produce the high frequency (360 Hz) chop necessary for the infrared detector. The
correlation wheel is rotated by a synchronous motor.
Infrared (IR) Source
The infrared source is a special wire wound resistor. It is heated by passing a highly
regulated dc voltage through the resistor. Replacement, when necessary, is
straightforward.
8-5
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