Teledyne GFC-7001T - Trace CO Analyzer - Manual (Page 292)

Theory of Operation Teledyne API – Model T300/T300M CO Analyzer
Teledyne Analytical Instruments 292
IR
Source Photo-Detector
M
R
GFC Wheel
IR unaffected by N
2
in Measurement Cell
IR is affected by CO in Reference Cell
Δ H
Figure 13-3: 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.
13.2.1.2. THE MEASURE REFERENCE RATIO
The GFC7001T/GFC7001TM 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.