Powder flow control valve

A powder flow control valve for dispensing a predetermined weight of powder in which a downwardly extending powder flow path is surrounded by a perforated boundary surface past which powder may flow under gravity, said perforated boundary surface having a first side which faces the powder flow path, the valve having first means for creating a pressure drop across the boundary surface with a higher pressure on the first side of the boundary surface to terminate flow of powder along the path and second means for creating a pressure rise across the boundary surface with a lower pressure on said first side of the surface to promote flow of powder past the surface;

• wherein the improvement comprises control means responsive to the quantity of powder delivered for selectively operating said first and second means in a sequence having a first period of time during which the first means is deactivated and the second means is activated to dispense powder with accelerated flow past the boundary surface, a second period of time during which the second means is deactivated and the first means is maintained deactivated to allow flow to continue past the boundary surface under gravity until a required quantity of powder has been dispensed by the valve as detected by the control means, and a third period of time during which the first means is activated by the control means to terminate flow of powder through the valve.