Model GFC7000E Instruction Manual THEORY OF OPERATION
04584 Rev A1 157
calibration SLOPE and OFFSET values to produce the CO2 concentration which is then normalized
for changes in sample pressure.
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 CO2 MEAS & CO2 REF
If an interfering gas, such as H2O 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.
Thus, the difference in the peak heights and the resulting M/R ratio is only due to CO2 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 CO2.
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 bein
g
chopped by the GFC
Wheel Screen
CO REF
CO
MEAS
Figure 10-6: Chopped IR Signal