Apparatus for sensing the concentration of toner in a developer mix

In the developer assembly of a copying machine including a developer mix sump for retaining a quantity of developer mix and developer means for transporting developer mix from said sump into contacting relation with the surface of a member carrying an electrostatic image to be developed, toner concentration detection means including in combination:

• surface means comprising a material being triboelectrically dissimilar from said developer mix, said surface being isolated electrically from the sump and developer means and coupled through impedance to system ground potential and being positioned in the path of said developer mix so that said mix flows thereover in contact therewith, and current sensing means coupled for sensing the current produced triboelectrically by the interaction of said developer mix and said surface, and flowing through the impedance, the value of said current being proportional to the concentration of toner in said mix.

In an electrostatic copier apparatus of the type that employs recirculating toner to which toner material must be periodically added during use to maintain a desired level of concentration, which toner circulates in a path from a reservoir to the image to be developed and back to the reservoir, the improvement comprising:

• a surface of a material which is triboelectrically dissimilar from that of the toner which surface is provided in the circulation path away from the reservoir, such that the toner makes contact with said surface and flows over said surface to generate a triboelectrically d.c. voltage on said surface which voltage is related to the concentration of the toner, said surface being electrically isolated from the remainder of the apparatus;
• and
• means for measuring the d.c. voltage being generated at said surface in response to the flow of toner thereover and for automatically controlling the addition of toner material in response to the d.c. voltage so generated.