Teledyne GFC-7000T - Trace CO2 Analyzer - manual - Manual (Page 219)

Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer
Teledyne Analytical Instruments 201
9.3.3.3. 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 rate 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.
IR Detection Ring
Segment Sensor Ring
M/R Sensor Ring
KEY:
Detection Beam shining
through MEASUREMENT
side of GFC Wheel
Detection Beam shining
through REFERENCE side
of GFC Wheel
Figure 9-11: GFC Light Mask
M/R Sensor
This emitter/detector assembly that produces a signal that shines through a portion of the mask that
allows light to pass for half of a full revolution of the wheel. The resulting light signal tells the analyzer
whether the IR beam is shining through the measurement or the reference side of the GFC wheel.
Segment Sensor
Light from this emitter/detector pair shines through a portion of the mask that is divided into the same
number of segments as the IR detector ring. It is used by the synchronous / demodulation circuitry of the
analyzer to latch onto the most stable part of each measurement and reference IR pulse.