A fluid processing system may include a flow control cassette comprising at least one interface sensor chamber in fluid communication with at least one of a plurality of separate channels, the at least one interface sensor chamber defined at least in part by a wall, and at least one capacitive sensor disposed on the wall of the at least one interface sensor chamber. The fluid processing system may include, in the alternative or in addition, at least one syringe comprising a wall defining a barrel having a first end and a second end, the barrel having a bore with or without a piston or plunger disposed therein, and at least one capacitive sensor disposed on an outer surface of the wall of the syringe.