element and an outer layer overlaying the first layer and containing a catalytic compound capable of trapping gases which poison the precious metal catalyst the catalytic compound being supported on a porous oxide application_number US 77188201 A description as shown in fig1 a gas sensing element 10 is connected in one arm of a wheatstone bridge circuit the other three arms being constituted by a compensating element 11 a variable resistor 12 and a fixed resistor 13 having a value such that the bridge can be balanced by adjustment of variable resistor 12 across the two diagonals of the bridge are respectively connected a voltmeter 14 and a voltage source 15 the output voltage of the source 15 is chosen so as to heat the gas sensing element 10 and the compensating element 11 to a desired operating temperature the variable resistor 12 is adjusted so that the voltmeter 14 indicates a zero reading when the gas sensing element 10 and the compensating element 11 are exposed to normal atmosphere the gas sensing element 10 and the compensating element 11 are calibrated to indicate a known combustible gas concentration and then exposed to the atmosphere that is required to monitor at the operating temperature any combustible gas present.