A fluid processing system comprising: a disposable fluid circuit comprising: a separation chamber; and a flow control cassette comprising a housing containing a plurality of separate channels (230, 232, 234, 236, 238, 240, 242, 244, 246, 248, 250, 252, 254, 256, 258, 260) connected to a plurality of selectable junctions, at least one interface sensor chamber in fluid communication with at least one of the plurality of separate channels, the at least one interface sensor chamber defined at least in part by a wall (390, 392, 394, 396, 398), and at least one capacitive sensor (380, 382, 384, 386, 388) disposed on the wall of the at least one interface sensor chamber; and reusable hardware configured to accept the disposable fluid circuit and comprising: a separator connected to the separation chamber; a control cassette interface having at least one actuator for each of the selectable junctions and a coupling connected to the at least one capacitive sensor (380, 382, 384, 386, 388); and at least one controller coupled to the separator, the at least one actuator and the capacitive sensor (380, 382, 384, 386, 388) via the coupling, the controller configured to selectively operate the separator and the at least one actuator to provide a procedure according to a protocol. a disposable fluid circuit comprising: a separation chamber; and a flow control cassette comprising a housing containing a plurality of separate channels (230, 232, 234, 236, 238, 240, 242, 244, 246, 248, 250, 252, 254, 256, 258, 260) connected to a plurality of selectable junctions, at least one interface sensor chamber in fluid communication with at least one of the plurality of separate channels, the at least one interface sensor chamber defined at least in part by a wall (390, 392, 394, 396, 398), and at least one capacitive sensor (380, 382, 384, 386, 388) disposed on the wall of the at least one interface sensor chamber; and a separation chamber; and a flow control cassette comprising a housing containing a plurality of separate channels (230, 232, 234, 236, 238, 240, 242, 244, 246, 248, 250, 252, 254, 256, 258, 260) connected to a plurality of selectable junctions, at least one interface sensor chamber in fluid communication with at least one of the plurality of separate channels, the at least one interface sensor chamber defined at least in part by a wall (390, 392, 394, 396, 398), and at least one capacitive sensor (380, 382, 384, 386, 388) disposed on the wall of the at least one interface sensor chamber; and reusable hardware configured to accept the disposable fluid circuit and comprising: a separator connected to the separation chamber; a control cassette interface having at least one actuator for each of the selectable junctions and a coupling connected to the at least one capacitive sensor (380, 382, 384, 386, 388); and at least one controller coupled to the separator, the at least one actuator and the capacitive sensor (380, 382, 384, 386, 388) via the coupling, the controller configured to selectively operate the separator and the at least one actuator to provide a procedure according to a protocol. a separator connected to the separation chamber; a control cassette interface having at least one actuator for each of the selectable junctions and a coupling connected to the at least one capacitive sensor (380, 382, 384, 386, 388); and at least one controller coupled to the separator, the at least one actuator and the capacitive sensor (380, 382, 384, 386, 388) via the coupling, the controller configured to selectively operate the separator and the at least one actuator to provide a procedure according to a protocol.