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Teledyne API Model T360/T360M Operation Manual Principles of Operation
197
7.1.4. Interference and Signal to Noise Rejection
If an interfering gas, such as H
2
O vapor is introduced into the sample chamber,
the spectrum of the IR beam is changed in a way that is identical for both the
reference and the measurement cells, but without changing the ratio between the
peak heights of CO2 MEAS and CO2 REF. In effect, the difference between
the peak heights remains the same.
IR shining through both cells is effected
equally by interfering gas in the Sample
Chamber
M/R
is Shifted
Figure 7-5: Effects of Interfering Gas on CO2 MEAS & CO2 REF
Thus, the difference in the peak heights and the resulting M/R ratio is only due to
CO
2
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 CO
2
.
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
CO
2
REF
CO
2
MEAS
Figure 7-6: Chopped IR Signal
07272B DCN6552