An electrochemical gas sensor

An electrochemical gas sensor comprises, a working electrode 20; a counter electrode 60 having an outer periphery; a reservoir for electrolyte 90, wick means 80 providing a path for the electrolyte from the reservoir 90, around a first part or parts of the outer periphery of the counter electrode 60, to provide electrolytic continuity between the counter electrode 60 and the working electrode 20; and a gas diffusion path for gas to diffuse to or from a second part or parts of the outer periphery of the counter electrode 60. The sensor may also comprise a reference electrode 50. The sensor is suitable for sensing for example carbon monoxide, sulphur dioxide, hydrogen sulphide and nitric oxide. The electrodes may comprise for example a platinum black / PTFE catalytic layer covering one face of a hydrophobic microporous PTFE support. The electrolyte may be concentrated sulphuric acid. The wick means may be an unbound glass fibre.