Teledyne M300EM operation manual The Measure Reference Ratio

Theory of Operation Teledyne API – Technical Manual - Model 300E Family CO Analyzers

11.2.1.2.The Measure Reference Ratio

The M300E/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 N2 in Measurement Cell of the GDC Wheel and no additional CO in the Sample Chamber

CO MEAS

CO REF

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

M/R

is reduced

IR shining through Reference Cell is also reduced by additional CO in the Sample Chamber, but to a lesser extent

Figure 11-4: Effect of CO in the Sample on CO MEAS & CO REF

Once the M300E/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.

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