disc has a center hole 10 about which the disc may rotate at a distance from the center hole 10 are a plurality of orifices 11 of different apertures ranging from 3 mm to 8 3 mm the orifices are placed equidistant from the center hole 10 fig2 c shows a pipe insert 12 positioned relative to the disc 9 the pipe insert represents the flow channel of the inhibitor the disc 9 may be rotated to place a selected orifice 11 centrally in the flow channel the angular distances between the orifices 11 see fig2 b are chosen so that when the disc 9 is rotated to position another orifice in the flow channel the orifice will be situated at a predetermined position within the flow channel fig3 a b shows a dosage valve having a valve house 13 containing a disc 9 according to fig2 a c an inflow line 14 is connected to the valve house 13 at one side and an outflow line 15 is connected to the house 13 at an opposite side an actuator 16 is connected to the housing 13 and is operatively coupled to the disc 9 to rotate this fig4 shows schematically a longitudinal section through an orifice 11 upstream of the orifice 11 is an inlet pipe 17.