These instruments utilize a highly efficient discharge ozonizer. This allows the use of dry air instead of O2, without the risk of turning N2 in the air in to NO, as is the case with units incorporating a corona discharge ozonizer. It provides known, accurate amounts of NO2 for converter efficiency testing and fully meets the applicable sections of US-EPA (Title 40) Part 86.332-79 standards.
Meeting the Requirement
Chemiluminescent NOx analyzers rely upon the efficiency of the NO2 to NO converter in the analyzer. 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 US Environmental Protection Agency stipulates that such converters must be checked before the analyzer is initially used, and thereafter at weekly intervals, to ensure that they have an efficiency of at least 90%. The Signal NOxGen3 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 NOxGen3 provides no NO from air, in contrast to the high voltage corona discharge technique.
Stability against mains power variations, and the consequent effects on NO2 concentrations, is ensured by the incorporation of a voltage stabilized electronic circuit feeding pulses to a high voltage transformer.
Operation of the NOxGen3
The instrument is fed with nitric oxide and oxygen, the latter being subjected to a high energy lamp that 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 in nitric oxide concentration. If, for example, this concentration falls by 400vpm, then 400vpm 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 analyzer converter, the NO2 should be converted back to NO. From the results obtained the converter efficiency can be determined.
The NOxGen3 output is simply and effectively controlled by a two-decade thumbwheel switch adjustable from 00 to 99, and a three-position ozone level switch for low, medium and high settings. The electronic circuitry is so designed that there is a nonlinear relationship between NO2 output and the thumbwheel switch setting, thus allowing very fine adjustments of ozone outputs to be made at low levels.
The Signal NOxGen3 is suitable for regular use with all types of chemiluminescent analyzers.
- Gas Supply Requirements: Nitric oxide in nitrogen, oxygen or dry air
- Span Gas Flow Rate: 6 L/min
- Power Supply: 230V (± 13%) 50Hz
- Fittings: ¼" stainless steel
- Weight: 6.6Kg
- Dimensions: 280mm width, 175mm height, 245mm depth (3U 19-inch rack module)
- Variation of NO2 Output with Voltage: ± 5ºC
- Additional NO from N2: When air is used with ozonizer, no more than 0.5ppm is generated from the nitrogen in the air.
NOxGen3 can be fitted with options for remote control, as used in automated calibration systems. It is available in the following three configurations.
- Model 861 - NOxGen3A - standard unit (sans any remote control features)
- Model 862 - NOxGen3R - remotely-controlled (via TTL inputs) unit
- Model 863 - NOxGen3M - microprocessor-controlled unit with full remote control via serial communications
The NO2 to NO Converter (theory of operation)