Pipet signalling system

Claim 1

For use in pipetting apparatus of the type comprising a generally tubular body having an open forward end for receiving and expelling fluid, a plunger supported for axial movement in the body to draw fluid into and dispense fluid from te body with axial movement away from and toward the open end during successive fluid pickup and fluid dispensing strokes of plunger movement, a signalling system for signalling occurrence of the fluid dispensing stroke comprising: signalling means for responding to plunger positioning in a forward axial position for signalling the occurrence of such plunger positioning; a disabling element movable between first and second locations in the body, the disabling element in the first location cooperating to inhibit response of the signalling means to said plunger positionings, the disabling element in the second location cooperating to allow response of the signalling means to said plunger positioning; means for retaining the disabling element in the first location thereof; and means responsive to plunger movement away from the forward end during a first plunger pickup stroke for moving the disabling means to the second location thereof whereby the signalling means is conditioned for detecting and signalling said plunger positioning during the succeeding second plunger dispensing stroke thereby signalling occurrence of the fluid dispensing stroke.