A surface-contact sensor includes a housing defining an opening, an ion-permeable membrane at the opening, a counterelectrode within the housing, and a reference electrode within the housing that is spaced apart from the current-collector. A current-collector pad includes a grommet support having a base and a lumen defining an opening. An ion-permeable membrane spans at least a portion of the opening of the grommet. The ion-permeable membrane is held in place by a gasket surrounding the lumen. A current collector, at least in one embodiment, is proximate to a portion of the ion-permeable membrane that spans the opening.