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Model GFC7000E Instruction Manual THEORY OF OPERATION
04584 Rev A1 154
10.1.2. Measurement Fundamentals
In the most basic terms, the Model GFC7000E uses a high energy heated element to generate a
beam of broad-band IR light with a known intensity (measured during Instrument calibration.
This beam is 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 Chambe
r
Band-Pass
Filter
Photo-Detecto
r
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 GFC7000E adds another component to the IR light path called a gas filter
correlation (GFC) wheel (see Figure 10-2).