Detection apparatus

A system for detecting liquid level changes in a motor vehicle cooling circuit containing an aqueous solution susceptible to formation of electrically-conducting froth, said system including a radiator having a core and a header tank, the header tank incorporating an electrical conductivity probe unit comprising a chamber containing an electrical probe electrically insulated from the body of the header tank and connected to an electrical detection circuit, the chamber being in restricted liquid communication with the interior of the header tank through a froth barrier in the form of a foraminate obturating member so that said detection circuit is unaffected by froth which may be present in said header tank.