Teledyne GFC-7000T - Trace CO2 Analyzer - manual - Manual (Page 208)

Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer
Teledyne Analytical Instruments 190
IR unaffected by N
2
in Measurement Cell of
the GFC Wheel and no additional CO
2
in
the Sample Chamber
IR affected by CO
2
in Reference Cell
with no interfering gas in the Sample
Chamber
IR shinning through Measurement Cell of
the GFC Wheel is reduced by additional
CO
2
in the Sample Chamber
IR shining through Reference Cell is
also reduced by additional CO
2
in the
Sample Chamber, but to a lesser extent
M/R
is reduced
CO2 REF
CO2 MEAS
Figure 9-4: Affect of CO2 in the Sample on CO2 MEAS & CO2 REF
Once the Model GFC 7000TA 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
2
concentration which is then normalized for changes in
sample pressure.
9.1.4. 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.