a gas sensing unit designed to be protected against explosions and serving for the detection of combustible gases in an atmosphere , containing a gas sensor functioning according to the principle of catalytic oxidation or a semiconductor gas sensor . the gas sensing unit contains , within a housing , at least two mutually separated chambers or spaces . the one chambr is a compression - proof space and contains an electronic evaluation circuit for the gas sensor , the other chamber , which is a space structured to be protected against explosions , contains the gas sensor and a balancing element and is separated from the atmosphere by a sintered metal bushing or sleeve . a third explosion - proof chamber or space can be provided in the gas sensing unit for the purpose of connecting the lines or conductors . the gas sensor together with the balancing adapter forms an assembly or unit which can be pluggably arranged in the cover of the chamber containing the electronic system or hardware . with such type gas sensing unit it is possible to easily exchange the gas sensor at its erection site , without the need for having to open the compression - proof chamber containing the electronic components . the testing and balancing of the gas sensor can be accomplished at the manufacturing plant .