WATER-LEVEL DETECTION APPARATUS

PURPOSE: To obtain a water-level detection apparatus which can sharply reduce the influence of scale or the like and detect a water level in all water qualities by a method wherein the electric resistance between the face of an electrode and water is sensed when the face of the electrode arranged in the position of a prescribed water level has reached the water level. CONSTITUTION: Electrode E1 to E6 (of which E4 , E5 and E6 are used for a lowest water level, a highest water level and a warning water level, respectively) are arranged in prescribed water level positions in a water tank 30. When a water level inside the water tank 30 reaches the level or higher of the electrode E4 , the electric resistance between the electrode E4 and water is changed, the output pulse width of a monostable multivibrator 25 triggered by an oscillator 24 becomes large, and a relay Ry4 at a judgment output circuit 26 is turned on. When a water level reaches the electrode E5 , relays Ry4 , Ry5 are operated, contacts Ry4-1a , Ry5-1a are turned on, a control relay X1 is turned on, and a submersible pump 31 discharges water by a self-holding circuit. When the water level is lowered to the level of the electrode E4 , the relay Ry4 is turned off, the self-holding circuit is stopped, and the pump 31 is stopped. Since the water level is detected on the basis of the electric resistance between the electrode and the water, the influence of an insulator such as scale or the like is reduced sharply even when the insulator is stuck to the face of the electrode.