the quantity of urea solution that is to be injected and transmits a corresponding electrical signal to the metering valve 15 via an electrical connecting line 23 the urea is hydrolyzed and mixed as a result of being injected into the exhaust pipe 4 the catalytic reduction of the nox in the exhaust gas to form n 2 and h 2 o takes place in the catalytic converter units the metering valve 15 for introducing the urea solution into the exhaust pipe 4 substantially corresponds to a standard low pressure fuel injection valve which for example is secured releasably in a valve holding device that is fixedly connected to a wall of the exhaust pipe 4 0050 fig2 a shows a typical ceramic pressure sensor diaphragm 181 which is circular in cross section belonging to the pressure sensor 18 from the side which is remote from the pressurized medium i e reducing agent fig2 b shows this ceramic pressure sensor diaphragm 181 in section for reasons of chemical stability sensor elements made from al 2 o 3 ceramic are particularly suitable for pressure measurements carried out in aqueous urea solution these sensor elements comprise a ceramic shaped part with integrated sensor diaphragm 181 a resistor network 182 is applied to the sensor membrane 181 using thick film technology the network includes the sensor and calibration resistors the measurement principle is based on the effect of.