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Model 200E Instruction Manual Theory of Operation
10. THEORY OF OPERATION
The M200E 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 (iDAS Section 6.10.2) and are reported
to the user through a vacuum fluorescence display or several output ports.
10.1. Measurement Principle
10.1.1. Chemiluminescence
The principle of the M200E’s measurement method is the detection of chemiluminescence,
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 (denoted by an asterisk in Equation 10-1).
Eq 10-1
2
*
2 3
O NO O NO
+ → +
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 10-2, Figure 10-1).
Eq 10-2
ν
h NO NO
+ →
2
*
2
All things being constant, the relationship between the amount of NO present in the
reaction cell and the amount of light emitted from the reaction is very linear. More NO
produces more light, which can be measured with a light-sensitive sensor in the near-
infrared spectrum (Figure 10-1). In order to maximize the yield of reaction (1), the M200E
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