Abstract: A turbidity meter has a sensor unit supported in a fluid under test with a light source and at least two light sensors supported so that one light sensor is in line with the source to receive transmitted light and the remaining sensor or sensors are arranged to receive light scattered by the fluid. Both the source and the sensors have flow forming chambers connected to a source of pressurized fluid so that a thin layer of this fluid is caused to flow over lenses of the source and sensors to prevent deposition of material from the fluid under test. The signals from the sensors are digitized, and the intensity of the source is digitally controlled to maintain at least one of sensor signals within a suitable range, thus enabling operation over a wide range of turbidities, and automatic selection of turbidimetric and nephelometric modes of operation as appropriate.