Teledyne GFC-7001E - Trace CO Analyzer - Manual (Page 232)

Theory of Operation Model GFC7001E Carbon Dioxide Analyzer
Teledyne Analytical Instruments 232
11.2.1.2. The Measure Reference Ratio
The GFC 7001E/EM determines the amount of CO in the sample chamber by computing the ratio between the
peak of the measurement pulse (CO MEAS) and the peak of the reference pulse (CO REF).
If no gases exist in the sample chamber that absorb light at 4.7μm, the high concentration of CO in the gas
mixture of the reference cell will attenuate the intensity of the IR beam by 60% giving a M/R ratio of
approximately 2.4:1.
Adding CO to the sample chamber causes the peaks corresponding to both cells to be attenuated by a further
percentage. Since the intensity of the light passing through the measurement cell is greater, the effect of this
additional attenuation is greater. This causes CO MEAS to be more sensitive to the presence of CO in the
sample chamber than CO REF and the ratio between them (M/R) to move closer to 1:1 as the concentration of
CO in the sample chamber increases.
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 shinning 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
M/R
is reduced
CO REF
CO MEAS
Figure 11-4: Effect of CO in the Sample on CO MEAS & CO REF
Once the GFC 7001E/EM 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.