Abstract: A flow detector, for use adjacent a steam trap to detect an unacceptable level of leakage of steam through the trap, comprises a tubular housing substitutable for a standard pipe nipple. The housing comprises a boss which provides a condensate collecting well in which an electrode is exposed. Immediately opposite the well, on the opposite side of a flow passage provided by the housing, is a baffle in the form of a ramp arranged to divert steam towards the well. In normal conditions of use, the presence of condensate in the well is signalled by an electric current passing between the electrode and the housing. Should an excessive steam flow occur in the passage, steam diverted by the ramp displaces the condensate from the well around the electrode and stops the flow of current. A similar detector could be of use in a compressed air line.