CEM - Abstract

Abstract Information :

The gas analysis of NO, NO2 and SO2 is prescribed in many application areas for the monitoring of legal limit values. The measuring ranges result from the requirements of the different national and international guidelines. These measuring ranges have tended to be gradually reduced over the last decades. This trend seems to continue for CEM in the future. Furthermore, these high measurement requirements are also required for mobile applications. Especially for mobile applications, electrochemical gas sensors have been preferred so far because they are very small and light, have a low power consumption and cause low costs. However, this technology is increasingly being replaced worldwide by physical measurement methods. This process is accelerated by legal changes. In Asia/China in particular, a physical method based on UV absorption is prescribed for the analysis of NO, NO2 and SO2.

A compact gas measurement module (ULTRA.sens®) has been developed for this application area to meet this requirement. Further fields of application are the monitoring of marine diesel engines (MARPOL) and automotive exhaust gas analysis.

This module contains 2 optical measuring benches. In one of these benches, NO2 and SO2 are measured simultaneously using the NDUV principle. The radiation sources used are UVLEDs (285nm and 400nm), which have a very long service life and low power consumption. The NO measurement is performed in the spectral range around 226.5nm (ϒ 0/0 Bande). Here a gas discharge lamp (EDL) filled with air is used. Excited NO is formed in the discharge zone, which emits exactly the radiation required to measure NO during the transition to the ground state. This is also referred to as a molecule resonance absorption (UVRAS). This results in a highly selective NO measurement which is not influenced by other accompanying gases. The module is very compact and has no moving parts that could be subject to wear. Due to these design details, the module is also ideally suited for mobile use. The measurement stability is achieved by appropriate reference measurements. In addition, the module is thermostatted to a working temperature of 50°C and is then located in an insulated box.

The module is supplied with 24 VDC and requires approx. 25 Watt in the heating phase, which is reduced to approx. 5 Watt in the stationary state. Compared to NDIR technology, more accurate measurements with a lower detection limit can be achieved. Cross sensitivities to water vapour and high CO2 concentrations do not exist with this method. The CLD method is also not suitable for this application, since only NO can be directly detected and NO2 must first be converted into NO via a converter (delay due to chemical conversion).

Typical measuring ranges that can be realized with the module are 0-100ppm SO2, 0-100ppm NO2 and 0-300ppm NO. A wide dynamic range can be achieved by appropriate linearization functions, so that even high concentrations (up to 3000ppm) can be measured with the module. Especially for applications with fast concentration changes (e.g. automotive) the module can be set to a time constant of t90% > 4 seconds.