Non-dispersive UV absorption photometry (NDUV) utilizes the electronic transitions of molecules stimulated by the radiation absorption of certain gases. In comparison to IR absorption, these stimulations are afflicted with higher cross-sections (→ absorption coefficient α), so that the proportion of radiation absorption is greater. The NDUV technique is therefore particularly suitable for trace analysis in the ppm range. Furthermore, this kind of gas analysis is not influenced by interfering water vapor concentrations.
Basing on this technology, Wi.Tec-Sensorik GmbH is now able to offer serial OEM modules (ULTRA.sens®) for the first time.
Basic structure of the ULTRA.sens-method
Reference: Wiegleb, G.: Gasmesstechnik in Theorie und Praxis (Kap. 8.3), Springer Vieweg 2016
In the photometer, the radiation of the LED is imaged with a UV lens. As a result a parallel beam path is created. This radiation is split into a measuring and reference path in a subsequent beam splitter. 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 passed with a further UV lens to a high-sensitivity UV detector, which converts the radiation into a measuring voltage. The radiation absorption in the cuvette is then a measure for the gas concentration (c) according to Lambert Beer’s law:
With increasing pressure (p) and sinking temperature (T), the proportion of radiation absorption in the cuvette increases.
Spectral distribution of absorption bands of nitric oxide, nitrogen dioxide and sulfur dioxide
UV LED technology
By the use of the AlGaN-LED technology spectral ranges from 360nm to 230nm can be covered completely. In order to increase the lifetime of the sensitive UV-LEDs a special control electronics (smartPOWER) was developed with which the expected life-span could be extended by the factor > 10. Under these operating conditions, the lifetimes of the sensitive UV radiation sources have risen to > 2 years for the first time. This is an important prerequisite for the usage in continuously operating gas measuring systems. Furtherrmore, the UV LED can be modulated with almost any desired frequencies so that rapid changes in the gas concentration can also be captured. Response times of t90% <100 milliseconds can therefore be achieved easily.
Light area of the microchip
The physical structure consists of a photometer with two reception detectors. The reference detector continuously monitors the radiation intensity of the UV LED and this serves to compensate aging effects and temperature influences. The measuring detector detects the selective radiation absorption in a 100-mm-long measuring cuvette, which is provided with a special coating on the inside in order to suppress disturbing hang-up effects. The control of the UV LED and the evaluation of the detector signals are carried out 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. As a result of this measure, the usage of optical filters for the spectral restriction to the particular type of gas becomes obsolete.
Physical composition of photometers