The Measure Reference Ratio - Teledyne T300 Operation Manual

Theory of Operation

13.2.1.2. THE MEASURE REFERENCE RATIO

300
IR unaffected by N in Measurement Cell
2
IR is affected by CO in Reference Cell
M
IR
Source
R
GFC Wheel
Figure 13-3:
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 in the filter wheel does not absorb IR light. This causes a fluctuation in the
2
intensity of the IR light striking the photo-detector which results in the output of the
detector resembling a square wave.
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.
Teledyne API – Model T300/T300M CO Analyzer
Measurement Fundamentals with GFC Wheel
Δ H
Photo-Detector
06864B DCN6314