Abstract: A sensing head for use in turbidity measurements has a body with a sensing surface, for example a photocell enclosure, which is immersed in liquid in use of the head. Periodic cleaning of the sensing surface is effected by powered movement, for example under control of an electric motor, of the body relative to a housing in which a wiping element such as an O-ring is seated. As applied to a photoelectric head a further body enclosing a light source may be mounted for movement with the photocell enclosure to be wiped clean by a separate wiping element in the housing.

Abstract: A turbidity measuring instrument has a chamber through which liquid to be examined flows, the chamber having a constant head inlet and a weir overflow outlet so that a static liquid level is maintained in the chamber, light being directed vertically into the liquid from above and back-scattered light from particles suspended in the liquid being measured photoelectrically by a detector head located in a sealed housing with a light source above the liquid level.