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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 

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.   

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). 

(Pure N

2

)

 

(N

2

 with CO

2

)

 

 

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).