Abstract: In measuring the turbidity of a liquid, especially for determining water clarity in a water purification process in which after the addition of flocculant the water is centrifuged, light is emitted as a diffuse beam from a light source and the intensity of light reflected from gas bubbles in the liquid is detected by a photosensor. The light source and photosensor are incorporated in a probe having transparent cylindrical wall. The probe is sealed within a housing confining a liquid receiving chamber. A plurality of seals are provided forming a space containing a cleaning solution. An actuator is coupled to the probe for reciprocating the probe relative to the housing and seals for cleaning deposits from the wall.