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2.00
INTRODUCTION
Chemiluminescent NOx analysers, such as Signal’s own Model 4000 Series; rely
upon the efficiency of the NO2 to NO converter in the analyser. The converter
enables a measurement to be made of the total oxides of nitrogen in the sample
stream (NOx = NO + NO2).
The 1979 Heavy Duty Register of the U.S. Environmental Protection Agency
stipulates that such converters must be checked before the analyser is initially
used, and thereafter at weekly intervals, to ensure that they have an efficiency of
at least 90%. The Signal NOXGEN III provides known, accurate amounts of NO2
for converter efficiency testing, and fully meets the EPA requirements.
The instrument is compact, inexpensive and provides a precision of control and
repeatability of results that were unobtainable with the previous generation of
converter testers. Ozone is generated by a high energy lamp using an electronic
pulse, which can be varied to adjust the Ozone level. The Noxgen III produces no
NO from air, in contrast to the high voltage corona discharge technique.
Stability against mains power variations, and the consequent effects of NO2
concentrations, is ensured by the incorporation of a voltage stabilised electronic
circuit, feeding pulses to a high voltage transformer.
2.01
PRINCIPLES OF OPERATION
The instrument is fed with nitric oxide and oxygen, the latter being subjected to a
high-energy lamp, which partially converts the oxygen to ozone. The
oxygen/ozone mixture is passed to the nitric oxide flow, and the nitric oxide is
instantly converted by the ozone to give nitrogen dioxide. The reaction between
the remaining oxygen and nitric oxide again produces nitrogen dioxide, but this
reaction is so slow that it does not need to be taken into account for test
purposes.
The amount of nitrogen dioxide generated is determined by measuring the fall of
nitric oxide concentration. If, for example, this concentration falls by 400 vpm,
then 400 vpm nitrogen dioxide has correspondingly been generated, as the
oxidation is a 1 : 1 molecular reaction. The concentration of nitrogen dioxide is
therefore directly proportional to the concentration of ozone generated. When
connected to the analyser converter, the NO2, should be converted back to NO.
From the results obtained the converter efficiency can be determined.