Theory of Operation
Model 200E Instruction Manual
more details), is converted to a voltage and amplified by the preamplifier board and then
reported to the motherboard’s analog inputs.
Figure 10-3: Reaction Cell with PMT Tube
10.1.3.2. Optical Filter
Another critical component in the method by which your M200E detects chemiluminescence
is the optical filter that lies between the reaction cell and the PMT (Figure 10-2). 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 M200E will respond (Figure 10-1).
The narrow band of sensitivity allows the M200E to ignore extraneous light and radiation
that might interfere with the M200E’s measurement. For instance, some oxides of sulfur can
also undergo chemiluminescence when in contact with O
3
but emit light at shorter wave-
lengths (usually around 260 nm to 480 nm).
10.1.4. 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 M200E diverts the sample gas flow
directly to the vacuum manifold without passing the reaction cell once every minute for
about 5 seconds (Figure 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 M200E 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.
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Summary of Contents for 200E
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