Abstract: Pressure converter for mounting above the drill bit at the lower end of a drill pipe for generating a higher fluid pressure in a portion of a drilling fluid flow, including a piston which is moveable in a cylinder. The piston has a large piston area which when moved in a first direction is affected by the pressure of the drilling fluid in the drill pipe, and an opposite small piston area which when moved in the first direction provides an increased pressure in a smaller portion of the drilling fluid flow. A valve arrangement controls the movement of the piston in the cylinder. Conduits communicate with drilling fluid flow passages inside the drill pipe and the annulus outside the drill pipe respectively. The space in front of the small piston area is adapted to receive a particular working fluid and there is provided a moveable membrane so as to separate the working fluid from the drilling fluid.