Apparatus for detecting oil mist or other light-affecting contamination in a fluid comprises a measuring chamber across which a light source directs a beam of light on to a photocell. Solenoid valves each control the connection of a respective inlet pipe (connected, for example, to a particular part of an internal combustion engine being monitored for oil mist) to the measuring chamber. The solenoid valves are operated in sequence so as to admit respective samples of the atmosphere into the measuring chamber where the degree of contamination is detected by the changing photocell output. The samples are drawn into the measuring chamber by suction from a fan unit. The fan unit is also connected by a by-pass chamber and by-pass pipes in the individual valve units to the inlet pipes on the upstream sides of the valves. In this way, the inlet pipes are kept charged with atmosphere from the areas being monitored so that each sample is very rapidly sucked into the measuring chamber when a valve is opened.
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