Teledyne 300E, Инструкция по эксплуатации, Страница: 160 / 253

THEORY OF OPERATION Model 300E Carbon Monoxide Analyzer
Instruction Manual
160 P/N 04288 Rev: A
IR shining through both cells is
effected equally by interfering gas
in the Sample Chamber
M/R
is Shifted
Figure 10-5: Effects of Interfering Gas on CO MEAS & CO REF
If an interfering gas, such as CO
2
or 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
CO MEAS and CO REF . In effect, the difference between the
peak heights remains the same.
Thus, the difference in the peak heights and the resulting M/R ratio is only due to
CO 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.
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 ration.
The IR Signal as the Photo-Detector
sees it after being chopped by the GFC
Wheel Screen
CO REF
CO MEAS
Figure 10-6: Chopped IR Signal