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Teledyne API - Model 200EH/EM Operation Manual Theory of Operation
191
Figure 10-10-2: NO
2
Conversion Principle
By converting the NO
2
in the sample gas into NO, the analyzer can measure the total NO
X
(NO+NO
2
) content of
the sample gas. By switching the NO
2
converter in and out of the sample gas stream every 6 - 10 seconds, the
M200EH/EM analyzer is able to quasi-continuously measure both the NO and the total NO
X
content.
The NO
2
concentration, finally, is not measured but calculated by simply subtracting the known NO content of
the sample gas from the known NO
X
content.
10.2. CHEMILUMINESCENCE DETECTION
10.2.1. THE PHOTO MULTIPLIER TUBE
The M200EH/EM uses a photo-multiplier tube (PMT) to detect the amount of light created by the NO and O
3
reaction in the reaction cell.
A PMT is typically a vacuum tube containing a variety of specially designed electrodes. Photons enter the PMT
and strike a negatively charged photo cathode causing it to emit electrons. These electrons are accelerated by
an applied high voltage and multiply through a sequence of such acceleration steps (dynodes) until a useable
current signal is generated. This current increases or decreases with the amount of detected light (Section
10.4.3 for more details), is converted to a voltage and amplified by the preamplifier board and then reported to
the motherboard’s analog inputs.
04521C (DCN5731)