The measuring principle of thermal conductivity is used to measure gas concentrations up to 100 vol %. Thermal conductivity is suitable for the measurement of combustible and toxic gases. It is however not suitable for monitoring in the range of LEL. The principle depends on the effect that different gases have different conductivity for heat. In a thermal conductivity transmitter there are 2 sensors which are both catalytic in-active. The sensor elements are build into a Whatstone bridge. The detecting sensor ( D sensor ) is exposed to the measuring gas. The compensation sensor (K sensor) is situated in an atmosphere closed chamber that is filled with clean air. The measuring gas will cause the S sensor to cool down the temperature and change the electrical resistance of the wire. The change of the electrical resistance is proportional to the gas concentration. To insure that the change of the temperature is caused by the gas and not by other influences like the ambient temperature, the K sensor, compensating sensor is necessary.