Abstract: In a blood dialysis system a proportioning pump accurately mixes precise quantities of water and concentrate. A double-acting piston assembly is disposed in a stepped cylinder which has proximity sensing switches at both ends thereof. These are operated as the piston assembly reaches the end of its stroke in each direction. The switches alternate a bistable solenoid valve system which directs pressurized water to opposite sides of a large central piston, thus causing the piston assembly to reciprocate. Small pistons, attached on opposite sides of the large piston, move in small pump chambers, with check valves. The small pistons pump concentrate in a precise ratio to the volume of water driving the large piston.

Abstract: A system continuously prepares dialysate by mixing water under pressure with concentrate to form dialysate under pressure. This dialysate is supplied on demand to a single patient station or to a plurality of patient stations. The pressure at which this dialysate is supplied to multiple patient stations is controlled so changes in the flow to one station will not change the pressure at which dialysate is supplied to other stations. A pressure regulator senses the pressure of the dialysate supplied to the patient stations and controls a regulator valve connected between the source of water under pressure and a proportioning pump which mixes this water with concentrate. The regulator includes a constant force spring assembly which provides a constant bias against the pressure in a sensing chamber over a range of linear movement of the pressure sensing element.