Teledyne T360, Operation Manual, Page: 199 / 358

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