Teledyne gfc 7000ta operation manual Gas Filter Correlation, GFC Wheel

Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer

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 CO2 density.

IR

Source

IR Beam

Band-Pass Filter

Sample Chamber

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 N2)

Reference Cell

(N2 with CO2)

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 N2 (the measure cell). The other is filled with a combination of N2 and a high concentration of CO2 (the reference cell).

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