Drip free, volume-adjustable, automatic liquid dispenser

A liquid dispenser of the type comprising a tank providing a supply of liquid to be dispensed, a pump comprising a cylinder communicating with the tank and having a cylinder head connected, via a normally closed check valve, to an outlet passageway, a working piston having a piston rod with a piston head mounted for reciprocation in the cylinder thereby defining with the cylinder head a liquid pressure chamber of variable size whereby liquid is drawn into the pressure chamber from the supply during an intake stroke and a prespecified amount of liquid is expelled from the pressure chamber therefrom through the check valve to the outlet passageway during an exhaust stroke, the improvements residing in that

• ducts are formed in the piston head for permitting flow of liquid from the supply therethrough into the pressure chamber, the piston rod includes a valve member for closing the ducts and check valve engaging means at a leading end thereof, and in that the piston head is mounted on the piston rod for limited lost motion therewith in the direction of reciprocation during initial portions of both the intake and exhaust strokes whereby,
• for an initial portion of the exhaust stroke, as a result of the lost motion, the piston rod and valve member can move relatively towards the piston head and, subsequently, when such lost motion has been taken up, engage and push the piston head towards the cylinder head with the valve member in sealing engagement with the ducts, closing the pressure chamber to prevent flow of liquid therethrough and causing an increasing pressure of liquid therein to open the check valve and expel a prespecified volume of liquid, determined by the total displacement of the exhaust stroke less the displacement of the lost motion, through the check valve into the outlet passageway, the check valve engaging means being carried into engagement with the check valve towards the end of the exhaust stroke and maintaining the check valve open at a first stage of the intake stroke, during which, return movement of the piston rod causes liquid remaining in the outlet passageway to be sucked back towards the pressure chamber and the valve member to be removed from the ducts and so that, after take up of the lost motion, the piston rod can engage and move the piston head, permitting liquid to be drawn from the supply through the ducts into the pressure chamber.