AIR MONITORING ULTRA.sens®


Photometric gas analysis is based on Lambert-Beer’s law and is implemented in the commercial ULTRA.sens®


photometer as follows. The use of LEDs for gas analysis was first described in


1983 by Wiegleb [5]. However, powerful LEDs that extend down into the spectral range < 260 nm have only been available for a few years. A UV LED is used as the radiation source, whose central wavelength is optimally tuned to the center of the O3


absorption spectrum (Hartley band) (Fig. 2).


The radiation from the UV LED is split into a measurement beam and a reference beam by a beam splitter (BS). The measuring beam is directed through the cuvette containing the gas mixture to be analysed. Both beam paths are detected by separate photodiodes and converted into electrical signals, which are then used to calculate the gas concentration c (Fig. 3). The maximum cuvette length L is 250 mm (AK250) and is suitable for very low O3 range. The smallest measuring range is 0-1000 ppb O3


concentrations in the ppm and ppb


resistant versions in 20 mm and 50 mm are also available for high O3 vol.% range.


. Shorter cuvettes (100 mm) and corrosion- concentrations, up to the


The ULTRA.sens also has the option of coupling the radiation from a second UV LED into the photometer. The second UV LED can be used, for example, to measure in a different O3


band (Huggins or Chappuis) in order to detect high concentrations in the vol. % range in the same photometer if necessary. Furthermore, simultaneous NO2


at the minimum of O3 absorption measurement is also possible absorption at 380 nm. This suppresses cross-sensitivity to ozone. This


measurement method is used successfully, for example, in the control of low-pressure plasmas. Furthermore, the ULTRA.sens can be combined with an AutoZeroFunction (AZF) to eliminate zero drift (Fig. 4).


ozone concentrations, plasma discharges are used, which allow O3 range. The O3


concentrations up to the vol.%


concentration is determined by using a method in accordance with ISO 13964 [4]. The absorption coefficient α is specified in this standard with a value of 1.44·10-5


m2 /µg at 253.7


nm (Hg line). This value is based on the work of Daumont et al. (1992). The calibration of the ULTRA.sens uses this database [2] to calculate the integral absorption across the entire emission range of the UV LED. To do this, the emission curve must be determined using a monochromator for the UV LED and calculated using the data from [2]. This results in a very high accuracy of the measured values. During practical use, no regular adjustments of the endpoint are necessary. It is recommended to have the manufacturer perform a check every 1-2 years.


AZF Auto-Zero-Function


Particularly in small trace measurement ranges (0-1000 ppb), the changes in absorption are so small that they can be influenced by temperature effects, humidity [3] and contamination. This leads to a drift in the zero line and thus also to undesirable measurement errors. To eliminate these measurement errors, an automatic switchover to O3


-free air can be performed at a specified time


interval (e.g. once per hour). During this phase, the last measured value before switching is stored in the software so that the zero-gas flow does not appear in the output signals (frozen). Once the current and, if applicable, deviating zero value has been determined, it is set to zero in the software. The measurement then continues as usual. This process typically takes less than 1 minute. Wi.Tec has developed a special module (Figs. 5 and 6) for this automatic zero-point adjustment, which communicates with the ULTRA.sens®


evaluation software. Both, time interval and the purge time can be set over a wide range via the firmware.


Fig. 2 : Absorption spectrum (red) of ozone in the range from 200 nm to 300 nm (Hartley band) compared to the spectral position of the UV LED emission without additional optical filtering


Fig. 5: The AZF is a compact module that integrates all necessary pneumatic and electrical components as well as the corresponding connections on a circuit board.


Fig. 3 : ULTRA.sens®


and vol.%) or for simultaneous measurement of O3 and NO2


setup with two UV LEDs for simultaneous gas analysis in different measuring ranges (ppm in a photometric setup


Fig.6: Integration of the AZF in a measurement setup with the ULTRA.sens® ambient air, which is passed through an O3 scrubber, thereby filtering out all O3


Corrosion-resistant design


For use in chemical processes or applications with high ozone concentrations (e.g. >1000ppm), corrosion-resistant analysis cuvettes made of special stainless steels, Monel® recommended. The cuvette windows are sealed using an O-ring (Viton®


or Hastelloy® , Kalrez® are or similar) and a


flange connection (Fig. 7). This creates a gas-tight connection and ensures that all components that come into contact with the gas are resistant to the corrosive ozone gas.


. The O3 -free air is generated from the components and converting O3 to O2 .


Fig.4 : Illustration of the ULTRA.sens with a 250 mm analysis cuvette (AK250). The AutoZeroFunction (AZF) with the corresponding valve and pump is located below the photometer. The O3


has a fine dust filter upstream. Calibration the ULTRA.sens


Since there is no test gas for ozone due to its decay properties, the ozone must be generated in real time using an ozone generator. UV radiation sources with a spectrum of λ<200 nm are used as ozone generators, which are particularly suitable for trace ranges (ppm and ppb). For higher


Fig. 7: ULTRA.sens® with a corrosion-resistant 20 mm analysis cuvette (AK 20)


scrubber can also be seen, which also


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