THEORY OF OPERATION
Model 300E Carbon Monoxide Analyzer
Instruction Manual
158
P/N 04288 Rev: A
A GFC Wheel is a metallic wheel into which two chambers are carved. The
chambers are sealed on both sides with material transparent to 4.7µm 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 (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 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 in the gas filter wheel
strips the beam of most of the IR at 4.7
µ
m. 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 (See
Figure 10-3) that results in the output of the detector resembling a square wave.
The Model 300E determines the amount of CO in the sample chamber by computing
the ratio between the peak of the Measurement pulse (
CO MEAS
) and the peak of
the Reference Pulse (
C0 REF
).