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Theory of Operation Model GFC7001E Carbon Dioxide Analyzer
Teledyne Analytical Instruments 240
11.5.3. OPTICAL BENCH & GFC WHEEL
Electronically, in the case of the optical bench for the GFC 7001E 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.
11.5.3.1. 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 GFC 7001E’s measurement of CO
for the GFC 7001E 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 48C (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 68C. 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.
11.5.3.2. 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.
11.5.3.3. GFC Wheel
A synchronous AC motor turns the GFC Wheel motor. For analyzers operating on 60Hz line power this motor
turns at 1800 rpm. For those operating on 50Hz line power the spin rate is 1500 rpm. The actual spin rate is
unimportant within a large range since a phase lock loop circuit is used to generate timing pulses for signal
processing.
In order to accurately interpret the fluctuations of the IR beam after it has passed through the sample gas, the
GFC Wheel several other timing signals are produced by other photo emitters/detectors. These devices consist
of a combination LED and detector mounted so that the light emitted by the LED shines through the same mask
on the GFC Wheel that chops the IR beam.