083730300A DCN8101 Theory of Operation 145
Figure 6-1. Measurement Fundamentals
Upon exiting the sample cell, the beam shines through a band-pass filter that allows only
light at a wavelength of 4.7 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.
6.1.2 GAS FILTER CORRELATION
Unfortunately, water vapor absorbs light at 4.7 µm too. To overcome the interfering
effects of water vapor the T300/T300M adds another component to the IR light path
called a Gas Filter Correlation (GFC) Wheel.
Measurement Cell
(Pure N
2
)
Reference Cell
(N
2
with CO)
Figure 6-2. GFC Wheel
6.1.2.1 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).
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