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Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer
Teledyne Analytical Instruments 188
directed through multi-pass cell filled with sample gas. The sample cell uses mirrors at each end to
reflect the IR beam back and forth through the sample gas to generate a 2.5 meter absorption path (see
Figure 10–1). This length was chosen to give the analyzer maximum sensitivity to fluctuations in CO
2
density.
IR
Source
IR Beam
Sample Chamber
Band-Pass Filter
Photo-Detector
Figure 9-1: 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.
9.1.3. Gas Filter Correlation
Unfortunately, water vapor absorbs light at 4.3 µm too. To overcome the interfering effects of water vapor
the Model GFC 7000TA 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 9-2: GFC Wheel
9.1.3.1. 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).