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Model GFC7000E Instruction Manual THEORY OF OPERATION
04584 Rev A1 155
Measurement
Cell
(Pure N
2
)
Reference Cell
(N
2
with CO
2
)
Figure 10-2: 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 in Reference Cell
Δ H
Figure 10-3: 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