be taken to assure that the probe extends well below the central axis of the conduit because the greatest concentration of sand in a flow stream will occur in the lower portion of the conduit due to the action of natural gravitational forces to counteract the erosive effect of the particulate matter on the probe in the conduit the probe proper 10 may be coated with a thin layer of wear resistant material such as titanium carbide to extend its operational life extending from the opposite end of the probe proper 10 are the threaded portions 16 and 12 threaded portion 12 adjacent the probe proper 10 is designed to permit the probe to be screwed into a pipe fitting in the conduit not shown perpendicular to the flow stream adjacent the threaded portion 12 of the probe 1 is a hexagonal pipe fitting 14 adapted to receive a wrench to screw probe proper 10 into the conduit adjacent to the hexagonal fitting 14 is threaded portion 16 which may be used to anchor a bolting or clamping device for the purpose of maintaining the acoustical transducer 19 in constant mechanical contact with the probe adjacent the threaded portion 16 the probe tapers in steps so that the backside 18 is roughly the same circumference as the attached transducer 19 the backside 18 of the probe is flat adapted to make contact with a flat surface of the acoustical transducer 19 acoustical transducer 19 consists of a piezoelectric crystal 9 having output lead 13 connected to an external electrical system for processing the output signal and a ground lead 11 the piezoelectric crystal has its output signal externally tuned to the primary resonant frequency of its thickness mode.