Figure 1: Schematic diagram of the SPE and components of the hardware of the amperometric sensor. The schematic structure of SPE consisted of following parts: (a) terminal post, (b) electric conductive rail, (c) insulation layer, (d) working electrode, (e) counter electrode and (f) polyvinyl chloride board. The hardware components of the amperometric sensor: (1) liquid crystal display and HT1621 driving chip, (2) MCP3201 A/D converter, (3) EM78P458 single-chip microcomputer, (4) operational amplifier, (5) function key, (6) data-interface, (7) SPE slot and (8) battery (output, 6 V).