Abstract: To prevent signal degradation of a sensing element, which may be a potentiometric or a polarographic sensor, having a reference electrode exposed to ambient air, the reference electrode (22) is placed within a chamber (21) which has included therein granules (39) of a material inert with respect to oxygen and having a surface hardness at least as hard as the reference electrode (22) so that, upon subjection of the sensor, when coupled to the exhaust system of an internal combustion engine, to vibrations, shocks and jolts, the granules will move in the chamber, rub against the electrode, and hence keep the electrode surface activated and free from contamination which may have penetrated with ambient air.