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THEORY OF OPERATION Model 360E Instruction Manual
IR
Source
IR Beam
Sample Chamber
Band-Pass Filter
Photo-Detector
Figure 10-1: Measurement Fundamentals
Upon exiting the sample cell, the beam shines through a band-pass filter that allows only light at a wavelength of
4.3 µm to pass. Finally, the beam strikes a solid-state photo-detector that converts the light signal into a
modulated voltage signal representing the attenuated intensity of the beam.
10.1.3. Gas Filter Correlation
Unfortunately, water vapor absorbs light at 4.3 µm too. To overcome the interfering effects of water vapor the
Model 360E adds another component to the IR light path called a gas filter correlation (GFC) wheel (see Figure
10-2).
Measurement Cell
(Pure N
2
)
Reference Cell
(N
2
with CO
2
)
Figure 10-2: GFC Wheel
10.1.3.1. 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).
164 05232 Rev B3