Teledyne 9110TH - Nitrogen Oxides Analyzer - Manual (Page 276)

Model 9110TH NOx Analyzer Principles of Operation
Teledyne Analytical Instruments 256
8. PRINCIPLES OF OPERATION
The 9110TH/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 (chemiluminescence). 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 9110TH/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 (indicated
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 ) with wavelengths between 600 and
3000 nm, with a peak at about 1200 nm (Equation 9-2, Figure 8-1).