unit 300 keep spaced apart from each other so that the sensor unit 300 can sense any liquid additive shortage in a non contact manner thus even if scale builds up on the buoyancy body 200 sensing ability and accuracy will not be affected this advantageously enhances the reliability furthermore as illustrated in fig5 the buoy portion 210 may include a magnetic body 211 and an air chamber 212 the magnetic body 211 is installed within the buoy portion 210 and enclosed the magnetic body 211 may be disposed within the buoy portion 210 in a position facing the sensor unit 300 and may generate a magnetic field as a result if the buoy portion 210 moves toward the sensor unit 300 and is located within a predetermined range the sensor unit 300 senses the magnetic field of the magnetic body 211 without directly contacting the buoy portion 210 as illustrated in fig5 the air chamber 212 may be formed within the buoy portion 210 and filled with air to increase the buoyancy effect as illustrated in fig6 the display unit 500 may include at least one of an alarm 510 and a flickering monitor 520 which can receive a control signal from the control unit.