Model 300E Carbon Monoxide Analyzer
Instruction Manual
THEORY OF OPERATION
P/N 04288 Rev: A
159
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
R
ef
CO
M
eas
Figure 10-4: Affect of CO in the Sample on CO MEAS & CO REF
If no gases exist in the Sample chamber that absorb light at 4.7
µ
m, the high
concentration of CO in the gas mixture of the Reference Cell will attenuate the
intensity of the IR Beam by 20% giving a M/R Ratio 0f 1.2:1.
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.
Once the Model 300E 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.