Embodiments provide a calibration tool for measuring a distance between a transducer and a liquid surface of an open channel, including: a pole; a flat board attached to the pole, configured to reflect a signal from the transducer, wherein the flat board is substantially perpendicular to the pole; a level device provided on the flat board, configured to ensure that the flat board is substantially parallel to a bottom of the open channel during a measurement; and a measuring scale provided on the pole.