083730300A DCN8101 Theory of Operation 147
IR unaffected by N
2
in Measurement Cell of
the GDC Wheel and no additional CO in the
Sample Chamber
IR affected by CO in Reference Cell
with no interfering gas in the
Sample Chamber
IR shining through Measurement Cell of the
GDC Wheel is reduced by additional CO in
the Sample Chamber
IR shining through Reference Cell is
also reduced by additional CO in the
Sample Chamber, but to a lesser extent
Figure 6-4. Effect of CO in the Sample on CO MEAS & CO REF
Once the T300/T300M has computed this ratio, a look-up table is used, with
interpolation, to linearize the response of the instrument. This linearized concentration
value is combined with calibration SLOPE and OFFSET values to produce the CO
concentration which is then normalized for changes in sample pressure.
6.1.2.3 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 CO
MEAS and CO REF. In effect, the difference between the peak heights remains the
same.
IR shining through both cells is
affected equally by interfering
gas in the Sample Chamber
Figure 6-5. Effects of Interfering Gas on CO MEAS & CO REF
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