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Theory of Operation Teledyne API - Model 200EH/EM Operation Manual
192
Figure 10-10-3: Reaction Cell with PMT Tube
10.2.2. OPTICAL FILTER
Another critical component in the method by which your M200EH/EM detects chemiluminescence is the optical
filter that lies between the reaction cell and the PMT (Figure: 10-3). This filter is a high pass filter that is only
transparent to wavelengths of light above 645 nm. In conjunction with the response characteristics of the PMT,
this filter creates a very narrow window of wavelengths of light to which the M200EH/EM will respond (Figure 10-
10-1
).
The narrow band of sensiti
vity allows the M200EH/EM to ignore extraneous light and radiation that might
interfere with the M200EH/EM’s measurement. For instance, some oxides of sulfur can also undergo
chemiluminescence when in contact with O
3
but emit light at shorter wavelengths (usually around 260 nm to 480
nm).
10.2.3. AUTO ZERO
Inherent in the operation of any PMT is a certain amount of noise. This is due to a variety of factors such as
black body infrared radiation given off by the metal components of the reaction cell, unit to unit variations in the
PMT units and even the constant universal background radiation that surrounds us at all times. In order to
reduce this amount of noise and offset, the PMT is kept at a constant 7° C (45° F) by a thermo-electric cooler
(TEC).
While this intrinsic noise and offset is significantly reduced by cooling the PMT, it is not eradicated. To
determine how much noise remains, the M200EH/EM diverts the sample gas flow directly to the vacuum
manifold without passing the reaction cell once every minute for about 5 seconds (Figure 10-10-4). During this
time, only O
3
is present in the reaction cell, effectively turning off the chemiluminescence reaction. Once the
chamber is completely dark, the M200EH/EM records the output of the PMT and keeps a running average of
these
AZERO values. This average offset value is subtracted from the raw PMT readings while the instrument
is measuring NO and NO
X
to arrive at a auto-zero corrected reading.
04521C (DCN5731)