A flow channel conduit system, comprising: a longitudinal flow channel (10; 40); a sensing area (28; 30) in the longitudinal flow channel; a first inlet port (14; 32), communicating through the longitudinal flow channel with a first end of the sensing area; a first outlet port (18; 36), communicating through the longitudinal flow channel with the first end of the sensing area; a second inlet port (16; 34), communicating through the longitudinal flow channel with a second end of the sensing area; a second outlet port (20; 38), communicating through the longitudinal flow channel with the second end of the sensing area; and a waste selector valve connected to the first and second outlet ports such that only one of the first and second outlet ports can be open at one time, the waste selector valve having a first actuation position in which the first outlet port is closed and the second outlet port is open and a second actuation position in which the first outlet port is open and the second outlet port is closed, thereby said waste selector valve controls longitudinal flow through the flow channel so as to enable simultaneous input flow through the first and second inlet ports but sequential output flow through the first and second outlet ports; wherein the first and second inlet ports (14, 16; 32, 34) are positioned between the first and second outlet ports (18, 20; 36, 38), wherein the first sensing region (28; 30) is positioned between the first and second inlet ports (14, 16; 32, 34), and wherein the sensing area (28; 30) is in longitudinal alignment with the first and second inlet ports (14, 16; 32, 34) and the first and second outlet ports (18, 20; 36, 38). a longitudinal flow channel (10; 40); a sensing area (28; 30) in the longitudinal flow channel; a first inlet port (14; 32), communicating through the longitudinal flow channel with a first end of the sensing area; a first outlet port (18; 36), communicating through the longitudinal flow channel with the first end of the sensing area; a second inlet port (16; 34), communicating through the longitudinal flow channel with a second end of the sensing area; a second outlet port (20; 38), communicating through the longitudinal flow channel with the second end of the sensing area; and a waste selector valve connected to the first and second outlet ports such that only one of the first and second outlet ports can be open at one time, the waste selector valve having a first actuation position in which the first outlet port is closed and the second outlet port is open and a second actuation position in which the first outlet port is open and the second outlet port is closed, thereby said waste selector valve controls longitudinal flow through the flow channel so as to enable simultaneous input flow through the first and second inlet ports but sequential output flow through the first and second outlet ports; wherein the first and second inlet ports (14, 16; 32, 34) are positioned between the first and second outlet ports (18, 20; 36, 38), wherein the first sensing region (28; 30) is positioned between the first and second inlet ports (14, 16; 32, 34), and wherein the sensing area (28; 30) is in longitudinal alignment with the first and second inlet ports (14, 16; 32, 34) and the first and second outlet ports (18, 20; 36, 38).