horiba GA-370, Инструкция по эксплуатации

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8 Reference
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8 Reference
This chapter describes the measuring principles, specifications, etc.
8.1 Measuring principles
Zero drift has been improved in principle for this trace gas monitor by employing the cross-flow
modulation method. Sample gas and zero gas are flowed alternately with a constant flow rate
to the cell at 1 Hz intervals by using a solenoid valve as shown in Fig. 71. This method uses
the modulation effect generated by the infrared absorption of the sample gas itself.
Infrared rays emitted from an infrared light source are injected through the measurement cell
to a detector in which the gas same as the sample gas is enclosed under certain conditions. At
this point, the infrared on the reference gas flow side reach the detector as is without being
absorbed, while on the sample gas flow side, infrared energy corresponding to the absorption
spectrum of the measurement component in the sample gas is absorbed and the amount of
transmitted light reaching the detector is reduced by just the absorbed amount. The detector
detects the difference between them.
A movable membrane in the detector is displaced by the pressure of energy absorbed in the
light receiving chamber. This displacement is detected as electrical output. Therefore, unless
there is a difference between the concentrations of the sample gas and reference gas, zero
drift does not occur because the membrane essentially does not pulsate.
Interference correction: <AS> measurement
This analyzer is equipped with a detector combining a MAIN detector (for the measurement
component) and a COMP detector (for compensation). The signal of <measurement
component A + interfering component B> is extracted by the MAIN detector, and the signal of
<interfering component B> (which may contain some of the measurement component) is
extracted by the COMP detector. These signals are amplified and then calculated by a
subtractor to extract the output of only the target <measurement component A>. This analyzer
has been developed to obtain extremely highly accurate measurement values without being
influenced by an interfering component.