Teledyne GFC-7001E - Trace CO Analyzer - Manual (Page 231)

Theory of Operation Model GFC7001E Carbon Dioxide Analyzer
Teledyne Analytical Instruments 231
11.2.1.1. The GFC Wheel
A GFC Wheel is a metallic wheel into which two chambers are carved. The chambers are sealed on both sides
with material transparent to 4.7 µm IR radiation creating two airtight cavities. Each cavity is mainly filled with
composed gases. One cell is filled with pure N
2
(the measurement cell)
.
The other is filled with a combination of
N
2
and a high concentration of CO (the reference cell).
IR
Source Photo-Detector
M
R
GFC Wheel
IR unaffected by N
2
in Measurement Cell
IR is affected by CO in Reference Cell
∆ H
Figure 11-3: Measurement Fundamentals with GFC Wheel
As the GFC Wheel spins, the IR light alternately passes through the two cavities. When the beam is exposed to
the reference cell, the CO in the gas filter wheel strips the beam of most of the IR at 4.7μm. When the light
beam is exposed to the measurement cell, the N
2
in the filter wheel does not absorb IR light. This causes a
fluctuation in the intensity of the IR light striking the photo-detector which results in the output of the detector
resembling a square wave.