146 Theory of Operation 083730300A DCN8101
IR unaffected by N
2
in Measurement Cell
IR is affected by CO in Reference Cell
Figure 6-3. 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 CO in the gas filter wheel strips the beam of
most of the IR at 4.7μm. When the light beam is exposed to the measurement cell, the N
2
in the filter wheel does not absorb IR light. This causes a fluctuation in the intensity of
the IR light striking the photo-detector which results in the output of the detector
resembling a square wave.
6.1.2.2 THE MEASURE REFERENCE RATIO
The T300/T300M determines the amount of CO in the sample chamber by computing the
ratio between the peak of the measurement pulse (CO MEAS) and the peak of the
reference pulse (CO REF).
If no gases exist in the sample chamber that absorb light at 4.7μm, the high concentration
of CO 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 CO 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 CO MEAS to be more sensitive to the presence of CO in the sample chamber than
CO REF and the ratio between them (M/R) to move closer to 1:1 as the concentration of
CO in the sample chamber increases.
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