Abstract: In a pulser for transmitting the reciprocating motion of a piston to a fluid contained in a chamber such as a solvent extraction column, at least one pulsation diaphragm having the shape of a hollow torus is joined to the piston and to a rim of the duct in leak-tight manner, means being provided for applying a pressure which compensates for the static pressure of the fluid on the piston and the pulsation diaphragm. The difference between the pressure exerted on the internal wall of the diaphragm on the fluid side and the pressure exerted on the external diaphragm wall is minimized during operation but maintained at a positive value.