Abstract: A device for measuring the viscosity of a fluid is shown which can be used to continuously measure the viscosity of the ink used in an ink jet printer. The pressure between two restrictors which respond differently to a change in viscosity is related to the total pressure drop across both restrictors to provide an indication of the change in viscosity of the fluid being dispensed by the printer. By utilizing this information, the viscosity of the ink can be modified to maintain the viscosity within the desired range.

Abstract: The invention relates to a method for monitoring the viscosity of ink in a continuous jet ink jet printer, which method comprises causing at least part of the ink to flow through a restricted orifice into a vessel and measuring the time taken to fill the vessel between a lower level limit and an upper level limit and allowing ink to escape from the vessel to reduce the level to below the lower level limit, characterized in that ink is automatically removed from the vessel by a syphon when the ink has reached the upper level limit.

Abstract: Fluid is applied in droplet form to a substrate by feeding the fluid to a nozzle so that the fluid issues from the nozzle as a single substantially coherent jet following a single jet flight path, causing the jet to break up into a series of substantially uniformly sized droplets, and applying a sufficiently large electrical charge to the fluid by means of a charge electrode so as to form mutually repellant droplets having flight paths which diverge from one another. The single jet path is directed into a catching device by which the fluid is caught and prevented from being applied to the substrate. The jet of fluid is broken up into a stream of substantially uniformly spaced droplets and the divergent stream of droplets is directed away from the catching device and allowed to reach the substrate so as to deposit fluid on the substrate.