Improvements in a backwash system for diluting apparatus wherein a rinse solution is directed to and through a sample pick-up probe to a receptacle and therefrom to a waste depositary. Either the probe or the receptacle in turn is movable between a first uncoupled condition, along a predetermined or fixed path first to place the receptacle and the delivery end of the sample probe proximate, and second, to establish a fluid tight sealed coupling therebetween. The receptacle includes an entrance recess accommodating an apertured resilient insert which engages the delivery end of the probe to define the sealed coupling. A sensing switch is employed to assure that the rinse solution will not be dispensed as backwash unless the sealed coupling is known to have been established. The sensing switch can be presented in the path of the movable object, the switch being actuated when the movable object has completed its movement to establish the sealed coupling between the receptacle and the delivery end of the sample probe. The switch is scanned to ascertain its state and unless actuated, backwash is prevented.Either both, one or the other of said receptacle and probe are mounted for singular or cooperative combined translatory movement, simultaneously or sequentially.