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The UV photometry is based on the absorption of radiation in the spectral range between 200nm and 400nm. In this area, some important technical gases have a pronounced absorption band. The advantage of this type of gas analysis is that the measurement is not disturbed by steam and carbon dioxide. Furthermore these absorption bands show a high absorption behavior so that very low gas concentrations (<< ppm) can also be detected reliably. In the Wi.Tec photometers of the ULTRA.sens® series UV light-emitting diodes are preferably used which emission wavelengths correspond spectrally to the respective absorption band. This means that no additional optical spectrometers or filter elements are required. This type of UV photometry is known as the non-dispersive UV method, also called NDUV.
The radiation of the UV-LED is splitted into a measuring and reference path via a beam splitter. The reference beam arrives a detector directly which converts it into a reference voltage value. With this reference signal aging effects of the UV-LED can be almost completely compensated. The measuring beam enters the sample cell in which the radiation is absorbed by the gases located there. The absorption behavior is recorded by the measuring detector and is used to calculate the gas concentration within the measuring cuvette. The ULTRA.sens® is designed in such a way that radiation components from several UV LEDs can also be coupled into the photometer. This means several gases can be determined simultaneously by the ULTRA.sens®.
The measurement of nitrogen monoxide (NO) requires a selective UV radiation source in the spectral range around 226nm. For this purpose a gas discharge lamp filled with NO is used, which emits precisely the radiation that is required for the NO measurement. In this context, one speaks of resonance absorption. The process also referred to as UVRAS (UV resonance absorption spectrometry).