The invention relates to the balancing of fluid streams in a dialysis system. In particular the invention relates to a cassette for conveying a first and a second fluid stream in a dialysis system, wherein the first and the second fluid streams can be medical fluid streams such as for example dialysate streams or blood streams, wherein the cassette has a sensor as a device for balancing the first and the second fluid stream, and wherein the sensor has a first channel for the first fluid stream and a second channel for the second fluid stream. The invention further relates to a dialysis system, which is configured to accommodate at least one cassette which is configured as described above. Furthermore, the present invention relates to an arrangement by which two channels for the first and the second fluid streams are formed. In addition, the invention relates to a method for construction of the two channels or the arrangement.