154 Theory of Operation 083730300A DCN8101
6.4.2 OPTICAL BENCH & GFC WHEEL
Electronically, in the case of the optical bench for the T300 Analyzer, GFC Wheel and
associated components do more than simply measure the amount of CO present in the
sample chamber. A variety of other critical functions are performed here as well.
6.4.2.1 TEMPERATURE CONTROL
Because the temperature of a gas affects its density resulting in the amount of light
absorbed by that gas, it is important to reduce the effect of fluctuations in ambient
temperature on the T300’s measurement of CO for the T300 Analyzer. To accomplish
this both the temperature of 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 48°C (8 degrees above the maximum suggested ambient
operating temperature for the analyzer). A strip heater attached to the underside of the
chamber housing is the heat source. The temperature of the sample chamber is sensed by
a thermistor, also 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 68°C. A cartridge heater implanted into the heat sync on the motor is the heat
source. The temperature of the wheel/motor assembly is sensed by a thermistor also
inserted into the heat sync.
Both heaters operate off of the AC line voltage supplied to the instrument.
6.4.2.2 IR SOURCE
The light used to detect CO 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.
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