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Model 9110T NOx Analyzer Principles of Operation
Teledyne Analytical Instruments 319
13. PRINCIPLES OF OPERATION
The 9110T 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 be 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 (hv).
The instrument measures the amount of chemiluminescence to determine the amount of NO in the
sample gas.
A catalytic-reactive converter converts NO
2
in the sample gas to NO which, along with the NO
present in the sample is 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 7) and are reported to the user through a vacuum
fluorescence display or several output ports.
13.1. MEASUREMENT PRINCIPLE
13.1.1. CHEMILUMINESCENCE CREATION IN THE 9110T REACTION CELL
The 9110T’s measures the amount of NO present in a gas by detecting the chemiluminescence which
occurs when nitrogen oxide (NO) is exposed to 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
molecules created by this reaction retain excess energy from the collision and exist in an excited
state, where one of the electrons of the NO
2
molecule resides in a higher energy state than normal
(denoted by an asterisk in the following equation).
Equation 13-1
2
*
23
ONOONO
The second step occurs because the laws of thermodynamics require that systems seek the lowest
stable energy state available, therefore the excited NO
2
molecule quickly returns to its ground state,
releasing the excess energy. This release takes the form of a quantum of light (h). The distribution
of wavelengths for these quanta range between 600 and 3000 nm, with a peak at about 1200 nm.
Equation 13-2