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Teledyne API Model 200A NO
X
Analyzer Instruction Manual, 02246, Rev. G, DCN 5247
4 THE M200A NO
X
ANALYZER
4.1 Principle of Operation
The Teledyne API Model 200A Analyzer is designed to measure the concentration of nitric
oxide [NO], total oxides of nitrogen [NO
X
] and, by calculation, nitrogen dioxide [NO
2
]. The
instrument measures the light intensity of the chemiluminescent gas phase reaction of nitric
oxide [NO] and ozone [O
3
] as follows:
223 O*NOONO
hvNO*NO 22
The reaction of NO with ozone results in electronically excited NO
2
molecules as shown in the
first equation above. The excited NO
2
molecules release their excess energy by emitting a
photon and dropping to a lower energy level as shown in the second equation. It has been shown
that the light intensity produced is directly proportional to the [NO] concentration present.
The Analyzer samples the gas stream and measures the [NO] concentration by digitizing the
signal from the Analyzer's photomultiplier tube (PMT). A valve then routes the sample stream
through a converter containing heated molybdenum to reduce any NO
2
present to NO by the
following reaction:
315
o
C
32 MoONO3MoNO3
The Analyzer now measures the total NO
x
concentration. The [NO
x
] and [NO] values are
subtracted from each other by the built-in computer yielding the [NO
2
] concentration. The three
results [NO], [NO
x
], and [NO
2
] are then further processed and stored by the computer yielding
several instantaneous and long term averages of all three components. These readings are also
stored internally in the M200A’s data acquisition system (DAS).
The software uses an adaptive filter to accomodate rapid changes in concentration. The
algorithm monitors the rate of change in concentration for both the NO and NO
x
channels. When
a change in concentration is detected, the software changes the sample filters to rapidly respond
to the change. The filters are adjusted to minimize the errors introduced by the time delay
between the NO
x
and NO channel measurements; this assures accurate NO
2
measurements.
When the rate of change decreases, the filters are lengthened to provide good signal/noise ratio.
The parameters used to operate the adaptive filter have been tuned to match the electrical and
pneumatic characteristics of the M200A.
4-1
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