Model 360E Instruction Manual THEORY OF OPERATION
Thus, the difference in the peak heights and the resulting M/R ratio is only due to CO2 and not to interfering
gases. In this way, Gas filter correlation rejects the effects of interfering gases and so that the analyzer
responds only to the presence of CO2.
To improve the signal-to-noise performance of the IR photo-detector, the GFC wheel also incorporates an optical
mask that chops the IR beam into alternating pulses of light and dark at six times the frequency of the
measure/reference signal. This limits the detection bandwidth helping to reject interfering signals from outside
this bandwidth improving the signal to noise ratio.
The IR Signal as the Photo-Detector sees it
after being chopped by the GFC Wheel
Screen
CO2 REF
CO2 MEAS
Figure 10-6: Chopped IR Signal
10.1.4.1. Ambient CO2 Interference Rejection
CO2 absorbs IR light very well. So well that even the narrow volume of ambient air between the IR source and
the sample chamber is enough to alter the analyzer’s measured concentration of CO2. Also, ambient air, which
averages around 350 ppm to 400 ppm, will vary significantly over the course of the day. The ambient CO2
concentration can rise as high as 1 000 ppm during the time of the day when people are present. It can fluctuate
300 ppm as the photosynthesis of plant life in the nearby area increases during the day and decreases at
night.
The basic design of the M360E rejects most of this interference at a 100:1 ratio; however this still can allow
small fluctuations in CO2 concentration during the course of the day. To completely remove all effects of
ambient CO2 from the analyzer’s measurement of CO2, dried air, scrubbed of all CO2 is pumped into the GFC
wheel housing to purge all ambient CO2 (see Figure 10-7)
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