Device and method for detecting blood or blood constituents in the liquid system of a device for extracorporeal blood treatment

An apparatus for dialysis treatment comprising:

• a source of dialysis fluid for fresh dialysis fluid and a discharge for used dialysis fluid;
• a dialyzer which is divided by a semi-permeable membrane into a blood chamber and a dialysis-fluid chamber;
• a balancing system for balancing fresh and used dialysis fluid which comprises a first balancing chamber, a second balancing chamber, a third balancing chamber and a fourth balancing chamber;
• a conveying system for conveying fresh dialysis fluid into the balancing chambers and for taking used dialysis fluid away from the balancing chambers;
• a feeding system for feeding fresh and used dialysis fluid;
• and
• a control system configured to operate the feeding system,
• wherein the conveying system is configured such that fresh dialysis fluid can be fed from the source of dialysis fluid to each balancing chamber and fresh dialysis fluid can be fed from each balancing chamber to the dialysis-fluid chamber of the dialyzer, and used dialysis fluid can be fed from the dialysis-fluid chamber of the dialyzer to each balancing chamber and used dialysis fluid can be taken away from each balancing chamber, and
• wherein the control system is configured to cooperate with the feeding system in such a way that, in successive phases of operation, fresh dialysis fluid is fed to the dialysis-fluid chamber from one balancing chamber of one pair of balancing chambers and used dialysis fluid is fed from the dialysis-fluid chamber to the other balancing chamber of the pair of balancing chambers, while fresh dialysis fluid is fed from the source of dialysis fluid to one balancing chamber of another pair of balancing chambers and used dialysis fluid is taken away from the other balancing chamber of the other pair of balancing chambers.