MPPC array to measure nitrogen oxides in pollution gases
To measure the Nitrogen Oxides (NOx) emitted into the atmosphere by ducts or chimneys, the European Standard specifies as Standard Reference Method (SRM) the Chemiluminescence technique. This method consists on the reaction between the sample gas containing Nitrogen Monoxide (NO) and the Ozone produced by an Ozone generator, which produces a light emission (chemiluminescence) proportionally to the NO concentration.
Because of the Nitrogen Oxides are, generally, composed by about 90% of NO and 10% of Nitrogen Dioxide (NO2), the European Standard includes also the method to measure the concentration of NO2. In this case the sample gas should pass through a NO2 to NO converter which consist on a heated chamber which contains a catalyst material able to reach a conversion efficiency of at least 95%.
The NO chemiluminescence emission spectrum is peaked around 1200 nm with a lower cut-off around 600 nm. To avoid the detection of light produced by other reactions of other gases with the Ozone which emit, usually, into the visible wavelength range, an optical filter at 600 nm is used.
Nowadays because of the emission range of interest, almost all the instruments use a Photomultiplier Tube (PMT) to detect the produced light.
Due to the recent developing of the semiconductor technology specially regarding the light detection, in our application we are developing a system based on an MPPC array to replace the PMT. The reason of this choice is based on the possible advantages that an MPPC gives, such as the smaller size, the versatility of the possible configurations and the higher resistance to mechanical stresses. All these features are very useful to apply this technology into portable multi-gas analysers.