cut out portion 12 is disposed on the upper plate 10 at a position above the reservoir 100 the reservoir 100 is arranged so that a reservoir cover 102 can be exposed out through the cut out portion 12 an introduction opening 104 is formed at the reservoir cover 102 and the introduction opening 104 may be opened or closed by a lid 120 a cover 130 configured to open or close the cut out portion 12 is rotatably coupled to one side of the cut out portion 12 a central portion of the bottom surface of the reservoir 100 is upwardly protruding to form a calibration display portion 106 as both side walls 108 of the calibration display portion 106 are connected to the bottom surface of the reservoir 100 the calibration display portion 106 and the reservoir 100 are integrally formed with each other here the calibration display portion 106 is arranged so that its surface can form an acute angle with respect to the surface of the reservoir cover 102 corresponding to an upper surface of the reservoir 100 also the surface of the calibration display portion 106 forms an acute angle with respect to the surface of liquid stored in the reservoir 100 the angle may be determined according to a height and.