Abstract: Gas sensors 1 are mounted on respective explosion proof housings 2 that contain microprocessor circuitry 11 for operating signals from the sensors. Signals from the circuitry 11 are transmitted along a digital highway 4 to a central control unit. The sensors may be located at remote locations in an industrial process plant or for example along the roof of a tunnel. A non-contact hand held unit 12 is provided for individually setting operating parameters of the circuitry 11, to enable factors such as zero set, span and linearity to be set for the sensors individually. A two way wireless communication link is provided between the unit 12 and a transducer 15 on the housing 1, so that the current value of the circuit operating parameters can be displayed on a display 18 on the hand held unit.