Non-dispersive UV absorption photometry (NDUV) uses the electronic transitions of molecules excited by the radiation absorption of certain gases. In comparison to IR absorption, these excitations are associated with higher cross sections (→ absorption coefficient α), so that the proportion of radiation absorption is greater. The NDUV technique is therefore particularly well suited for trace analysis in the ppm range. Furthermore, this type of gas analysis is not affected by disturbing water vapor concentrations. For the first time, Wi.Tec-Sensorik GmbH has been able to offer OEM-modules suitable for serial production (ULTRA.sens®) on this technological basis.
In the photometer, the radiation of the LED is imaged with a UV lens, so that a parallel beam path is formed. This radiation is split in a subsequent beam splitter into a measurement and reference path. The cuvette length (L) is chosen between 5 mm and 250 mm, depending on the gas concentration. At the end of the measuring cuvette, the radiation is then directed with another UV lens onto a high-sensitivity UV detector, which converts the radiation into a measuring voltage. The radiation absorption in the cuvette is then a measure of the gas concentration (c) according to the Lambert-Beer law:
With increasing pressure (p) and decreasing temperature (T), the proportion of radiation absorption in the cuvette increases.
UV LED technology
By using the AlGaN-LED technology spectral ranges from 360nm to 230nm can be covered completely. In order to extend the life of the sensitive UV LEDs, a special control electronics (smartPOWER) was developed with which the expected lifetimes could be extended by a factor of> 10. Under these operating conditions, lifetimes of the sensitive UV radiation sources of> 2 years are possible for the first time. This is an important prerequisite for use in continuously operating gas detection systems. Furthermore, the UV LED can be modulated with almost any frequency, so that even rapid changes in the gas concentration can be detected. Response times of t90% <100 milliseconds can therefore be achieved without difficulty.
The physical structure consists of a photometer with two receiving detectors. The reference detector continuously monitors the radiation intensity of the UV-LED and thus serves for the calculation of aging effects and temperature influences. The measurement detector detects the selective radiation absorption in a 100 mm long cuvette, which is provided with a special coating on the inside to suppress disturbing hang-up effects. The control of the UV-LED and the evaluation of the detector signals is done in a powerful microprocessor electronics, which is located below the photometer.
The LEDs required for the different gas measurements are selected by a spectral analysis and then assigned to the respective measurement task. This measure eliminates the use of optical filters for spectral limitation to the respective gas type.