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Principles of Operation
265
8. PRINCIPLES OF OPERATION
The T200H/M Nitrogen Oxides Analyzer is a microprocessor controlled instrument that
determines the concentration of nitric oxide (NO), total nitrogen oxides (NO
X
, the sum of
NO and NO
2
) and nitrogen dioxide (NO
2
) in a sample gas drawn through the instrument.
It requires that sample and calibration gases are supplied at ambient atmospheric pressure
in order to establish a constant gas flow through the reaction cell where the sample gas is
exposed to ozone (O
3
), initiating a chemical reaction that gives off light (chemilumines-
cence). The instrument measures the amount of chemiluminescence to determine the
amount of NO in the sample gas. A catalytic-reactive converter converts any NO
2
in the
sample gas to NO, which is then – including the NO in the sample gas – is then reported
as NO
X
. NO
2
is calculated as the difference between NO
X
and NO.
Calibration of the instrument is performed in software and usually does not require
physical adjustments to the instrument. During calibration, the microprocessor measures
the sensor output signal when gases with known amounts of NO or NO
2
are supplied and
stores these results in memory. The microprocessor uses these calibration values along
with the signal from the sample gas and data of the current temperature and pressure of
the gas to calculate a final NO
X
concentration.
The concentration values and the original information from which it was calculated are
stored in the unit’s internal data acquisition system (DAS Section 4.7.2) and are reported
to the user through a vacuum fluorescence display or several output ports.
8.1. MEASUREMENT PRINCIPLE
8.1.1. CHEMILUMINESCENCE
The principle of the T200H/M’s measurement method is the detection of chemilumi-
nescence, which occurs when nitrogen oxide (NO) reacts with ozone (O
3
)
.
This reaction is
a two-step process. In the first step, one molecule of NO and one molecule of O
3
collide
and chemically react to produce one molecule of oxygen (O
2
) and one molecule of
nitrogen dioxide (NO
2
). Some of the NO
2
retains a certain amount of excess energy from
the collision and, hence, remains in an excited state, which means that one of the
electrons of the NO
2
molecule resides in a higher energy state than is normal (ded by an
asterisk in Equation 8-1).
2 2 3
+
→
+ O NO O NO
*
Equation 8-1
Thermodynamics requires that systems seek the lowest stable energy state, hence, the
NO
2
molecule quickly returns to its ground state in a subsequent step, releasing the excess
energy in form of a quantum of light ( h
ν
) with wavelengths between 600 and 3000 nm,
with a peak at about 1200 nm (( Equation 8-2), Figure 8-1).
07270D DCN7141