Model 360E Instruction Manual THEORY OF OPERATION

IR

Source Photo-Detector

M

R

GFC Wheel

IR unaffected by N2 in Measurement Cell
IR IS affected by CO in Reference Cell
Δ H
Figure 10-3: Measurement Fundamentals with GFC Wheel
As the GFC wheel spins, the IR light alternately passes through the two cavities. When the beam is exposed to
the reference cell, the CO2 in the gas filter wheel strips the beam of most of the IR at 4.3μm. When the light
beam is exposed to the measurement cell, the N2 in the filter wheel does not absorb IR light. This results in a
fluctuation in the intensity of the IR light striking the photo-detector (See Figure 10-3) that results in the output of
the detector resembling a square wave.
10.1.3.2. The Measure Reference Ratio
The Model 360E determines the amount of CO2 in the sample chamber by computing the ratio between the peak
of the measurement pulse (CO2 MEAS) and the peak of the reference pulse (CO2 REF).
If no gases exist in the sample chamber that absorb light at 4.3μm, the high concentration of CO2 in the gas
mixture of the reference cell will attenuate the intensity of the IR beam by 60% giving a M/R ratio of
approximately 2.4:1.
Adding CO2 to the sample chamber causes the peaks corresponding to both cells to be attenuated by a further
percentage. Since the intensity of the light passing through the measurement cell is greater, the effect of this
additional attenuation is greater. This causes CO2 MEAS to be more sensitive to the presence of CO2 in the
sample chamber than CO2 REF and the ratio between them (M/R) to move closer to 1:1 as the concentration of
CO2 in the sample chamber increases.
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