The invention relates to methods of preparation of highly sensitive potentiometric sensors with an electroconductive polymer film as a sensing element. The first developer transport path and the second developer transport path include a first transporter and a second transporter, respectively, to transport the developer in a longitudinal direction. The BGA package is connected to the mounting pads on the printed circuit board due to melting of the solder balls, thereby mounted on the printed circuit board. The bottom end of the short-circuiting conductive plate is soldered to the ground plane and the bottom end of the power-supply conductive plate is connected to a power-supply circuit. An EDM electrode is shaped to fill completely or a portion of a cavity in the casting, the electrode is then connected to an electric discharge machine, and the internal surfaces of the internal core cavity are then machined to form a roughened surface.