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Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer
Teledyne Analytical Instruments 200
9.3.3. Optical Bench & GFC Wheel
Electronically, the Model GFC 7000TA’s optical bench, GFC wheel and associated components do more
than simply measure the amount of CO
2
present in the sample chamber. A variety of other critical
functions are performed here as well.
9.3.3.1. Temperature Control
Because the temperature of a gas affects its density and therefore the amount of light absorbed by that
gas it is important to reduce the effect of fluctuations in ambient temperature on the Model GFC 7000TA’s
measurement of CO
2
. To accomplish this the temperature of both the sample chamber and the GFC
Wheel are maintained at constant temperatures above their normal operating ranges.
Bench Temperature: To minimize the effects of ambient temperature variations on the sample
measurement, the sample chamber is heated to 48C (8 degrees above the maximum suggested ambient
operating temperature for the analyzer). The heat source is a strip heater attached to the underside of
the chamber housing. The temperature of the sample chamber is sensed by a thermistor attached to the
sample chamber housing.
Wheel Temperature: To minimize the effects of temperature variations caused by the near proximity of
the IR Source to the GFC wheel on the gases contained in the wheel, it is also raised to a high
temperature level. Because the IR Source itself is very hot, the set point for this heat circuit is 68C. The
heat source is a cartridge heater implanted into the heat sync on the motor. The temperature of the
wheel/motor assembly is sensed by a thermistor inserted into the heat sync.
Both heaters operate off of the AC line voltage supplied to the instrument.
9.3.3.2. IR Source
The light used to detect CO
2
in the sample chamber is generated by an element heated to approximately
1100
o
C producing infrared radiation across a broad band. This radiation is optically filtered after it has
passed through the GFC Wheel and the sample chamber and just before it reaches the photo-detector to
eliminate all black body radiation and other extraneous IR emitted by the various components of those
components.