The measuring and warning devices for explosive gases and vapors, uses the measuring principle catalytic combustion as a matter of priority. The measurement according to the measuring principle catalytic combustion is effected with the help of the "Whatstone bridge". One side of the bridge has two sensors, which are placed in the measuring chamber. These sensors are also called pellements. They are made from thin platinum wire which are rolled to a spiral, similar to the wire in a light bulb. The other side of the bridge is found in the electronics of the transmitter. The potentiometer in this side of the bridge makes calibration at the site possible (single person calibration).

One sensor, the detecting sensor (D-sensor), is catalytic active. The catalytic combustion takes place at the D sensor. The catalytic combustion of the gas and the oxygen in the atmosphere causes a raise of temperature of the platinum spiral. By raising the temperature, the resistance of the wire changes.

The change in resistance upsets the Wheatstone bridge.The second sensor is the compensating sensor(K sensor). The K sensor is catalytic in-active and no reaction will take place on the K sensor. The K sensor is used to compensate for temperature, pressure and humidity.

On the D sensor all combustible gases are burned and send to a signal. The signal height depends on the energy that is set free when the gas is burned. When the sensor is calibrated to one gas, the amplification of the bridge is adjusted so that the concentration of the gas corresponds to the display of the monitor.

Safety calibration The advantage of catalytic combustion lies in it's unique ability to detect all combustible gases and vapors. At a safety calibration the transmitter is calibrated on a gas, to which the catalytic combustion reacts with the least cross sensitivity. A punctal alarm more reliable, and independent from the attacking gas.

Catalytic poisons At catalytic combustion it should be noted that some materials can damage the catalyst. These are in particular heavy metal combinations ( Lead, quicksilver etc.), sulphur and halogen compounds like silicone. In these circumstances special poison resistant sensors or infrared instruments are used. In addition, the sensor shall be tested regularly with test gases for its cross sensitivity.