PURPOSE:To obtain an open ditch flow meter, which is able to be easily installed and adjusted and yet is not affected by level of water of an open ditch, by allowing a supporting body, which is equipped with a water level measuring step and a flow velocity measuring step, to float on the surface of fluid in the open ditch. CONSTITUTION:On the surface 6 of a fluid 4 flowing on the bottom 7 of an open ditch, a supporting body 11 which is equipped with a water level detector 12 and a flow velocity detector 13 is provided, and signals sent out from the detectors 12 and 13 are converted by a signal converter 16 and transmitted to outside through a signal cable 17. A cable fixing section 19 of the supporting 11 is connected to a cable fixing section 21 on side wall of the open ditch in order to keep the flow measuring position of the supporting body 11 from being changed by the flow. By using this mechanism, a water level L and a flow velocity v are able to be sought by the water level detector 12 and the flow velocity detector 13, respectively, and further, if a width of the open ditch is set to B, an actual flow rate Q is sought from an equation Q=k.B.L.v using a flow velocity compensating constant k, and therefore, measurement of flow rate of the open ditch and installation and adjustment of the flow meter can be done with simplicity.
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