An apparatus to design a via pad of a via is described. Of outer shape of the grooves, the outer shape of the grooves communicating with a space in the hermetic container at least when the piston is in a bottom dead center is a shape not forming parallel line to an axial center of the piston when the grooves are developed in a plane. This sealing ring and the groove in which it is located can advantageously be formed with simple and cost-effective lathe cutting operations. A field sensor is arranged within the sensing cavity to measure the field.