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Teledyne API Model T360/T360M Operation Manual Principles of Operation
195
7.1.3.1. 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 4.3 µm IR
radiation creating two airtight cavities. Each cavity is filled with specially
composed gases. One cell is filled with pure N
2
(the measure cell)
.
The other is
filled with a combination of N
2
and a high concentration of CO
2
(the reference
cell).
IR
Source
Photo-Detector
M
R
GFC Wheel
IR unaffected by N
2
in Measurement Cell
IR IS affected by CO
2
in Reference Cell
Δ H
Figure 7-3: 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
2
in the gas filter wheel
strips the beam of most of the IR at 4.3μm. When the light beam is exposed to
the measurement cell, the N
2
in the filter wheel does not absorb IR light. This
results in a fluctuation in the intensity of the IR light striking the photo-detector
(Figure 7-3) that results in the output of the detector rese
mbling a square wave
such as that shown in Figure 7-4.
07272B DCN6552