Abstract: An impeller is rotatably mounted in a housing on the lower end of a sub which is lowered by means of a wire line into a bore hole. The fluid flow in either direction within the bore hole engages and rotates helical vanes of the impeller, which in turn rotates a digital sensing device. A magnetic coupling links the impeller to the sensing device which is sealably secured within the housing. Rotation of the sensing device in one direction produces a digital output signal which is distinguishable from the output signal produced when the sensing device is rotated in the opposite direction. The digital signal is produced by the rotation of a butterfly valve between signal emitter elements and signal receiving elements coupled to a distinguishing logic circuit. The direction of flow of fluids along the bore hole is thus easily detected in a reliable, highly sensitive manner.