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Teledyne API Model 200AU NO
X
Analyzer Instruction Manual, 02293, Rev. F
4-1
4 THE M200AU NO
X
ANALYZER
4.1 Principle of Operation
The Teledyne API Model 200AU 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
x
present to NO by the
following reaction:
315
o
C
3x
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. In the
third measurement phase, the instrument measures sample gas which has been mixed with ozone
outside of the reaction cell. This Pre-Reactor allows the measurement of any hydrocarbon
interferents present in the sample gas stream. 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.
The software uses an adaptive filter to accommodate 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 better signal/noise ratio.
The parameters used to operate the adaptive filter have been tuned to match the electrical and
pneumatic characteristics of the M200AU.