The cell has a body (1) with a slit (10) extending across it and a sensor (2) supported at a given distance from the base (11) of the slit via a pressure ring (4). The interior of the pressure ring acts as a measuring chamber with an inlet and outlet. The sensor is pressed against the pressure ring via an eccentric lever (5) supported between the sides (12) of the slit. Preferably the pressure lever has a pressure surface (9) and a perpendicular outer surface (8) at a lesser distance from the rotation axis of the pressure lever than the pressure surface.
