Teledyne Measurement Principle, Chemiluminescence Creation in the 9110T Reaction Cell

Model 9110T NOx Analyzer Principles of Operation

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 (NOX, the sum of NO and NO2) and nitrogen dioxide (NO2) 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 (O3), 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 NO2 in the sample gas to NO which, along with the NO present in the sample is reported as NOX. NO2 is calculated as the difference between NOX 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 NO2 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 NOX 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 (O3). This reaction is a two-step process:

∙In the first step, one molecule of NO and one molecule of O3 collide and chemically react to produce one molecule of oxygen (O2) and one molecule of nitrogen dioxide (NO2). Some of the NO2 molecules created by this reaction retain excess energy from the collision and exist in an excited state, where one of the electrons of the NO2 molecule resides in a higher energy state than normal (denoted by an asterisk in the following equation).

Equation 13-1

NO + O3 →NO2* +O2

∙The second step occurs because the laws of thermodynamics require that systems seek the lowest stable energy state available, therefore the excited NO2 molecule quickly returns to its ground state, releasing the excess energy. This release takes the form of a quantum of light (hν). The distribution of wavelengths for these quanta range between 600 and 3000 nm, with a peak at about 1200 nm.

Equation 13-2

Teledyne Analytical Instruments 319