Abstract: For measuring an interface between two fluids, a device (10, 75, 90) having n electrical circuit comprising transmitting means, a detector, a first antenna (14, 18, 78, 82, 92) and a second electrically grounded antenna (15, 18, 79, 83, 93), the transmitting means being arranged to transmit electrical impulses between the first and second antennas through one of the two fluids so as to be absorbed thereby, and the detector being arranged to detect a change in either an impedance or a rate of absorption of energy which occurs at the interface. The device includes a housing (11, 91) for placing in the fluids so as to contact the interface along a periphery of the housing towards a first end thereof, the first antenna (14, 18, 78, 82, 92) projects from the housing at said periphery, and the electrical circuit includes a coupling means (28) for coupling thereto a remote monitoring means whereby the interface may be measured remotely.