Abstract: A device for keeping liquids separate in a pipe (16), especially for use in a petroleum well, where the device comprises a diaphragm (14), which in its operative position and through use of pretensioned slats (12) is stretched across the cross section of the pipe (16), and where each of the two end portions of the slats (12) is connected to a boss (8) and (10), respectively, in a manner such that each the slats forms an outward bow relative to the common axis of the bosses (8, 10), when in the neutral position.