Liquid level sensing device

Claim 1

Apparatus for developing diazo-type copy sheets by the application of an electrically conductive developing fluid to said copy sheets, comprising in combination, a developer section including a main reservoir containing a quantity of said conductive developer fluid and developer means mounted at said reservoir and operable to apply to the copy sheets said fluid, said developer means including a liquid applicator roller having a surface adapted to carry a predetermined quantity of developer fluid, mounted for rotation within said main reservoir, drive means for rotating said roller, means for supplying developer fluid from said main reservoir to the surface of said applicator roller, and wiper means mounted in wiping engagement with said roller surface for removing excess developer fluid therefrom prior to the application of said fluid to said copy sheets by said roller, a reserve reservoir also containing a quantity of said conductive developer fluid, connected to said main reservoir, from which fluid is dispensed to maintain the supply of developer fluid in said main reservoir at an operating level, pump means for dispensing developer fluid from said reserve reservoir to said main reservoir, and liquid level sensing means comprising first sensing circuit means including a first sensing electrode positioned within said reserve reservoir at a predetermined level and normally immersed in said conductive fluid for monitoring the level of developer fluid present in said reserve reservoir, said first sensing circuit means including a source of electrical power connected to said electrode providing a current flow between said electrode and said conductive fluid, circuit means operable to a first state while current flows between said electrode and fluid and to a second state when the flow of current between said electrode and fluid ceases due to the depletion of developer fluid in said reservoir below said predetermined level, and indicating means for indicating the depletion of developer fluid from said reservoir below said predetermined level upon the operatIon of said circuit means from said first to said second state, and second sensing circuit means including a second sensing electrode positioned in said main reservoir at a predetermined level, and normally being immersed in said developer fluid for monitoring the level of developer fluid present in said reservoir, said second sensing electrode connected to said source of power with the latter providing a current flow between said electrode and said conductive fluid in said main reservoir, and further including circuit means operable to a first state while current flows between said electrode and fluid and to a second state when the flow of current between said electrode and fluid ceases due to the depletion of said fluid in said main reservoir below said predetermined level, and means connected to said applicator roller drive means, responsive to the change of state of said circuit means from said first to said second state to prevent further operation of said developer means, including the prevention of the further operation of said applicator roller drive means, thereby to avoid damage to said roller surface.