Blood treatment device with improved bag weight monitoring

A blood treatment device includes an extracorporeal blood circuit, dialyzer and dialysis fluid circuit. The blood circuit and dialysis fluid circuit are separated from each other by a membrane in the dialyzer. A weighing device measures and monitors the weight of a bag containing a fluid. A fluid pump pumps fluid out of and into the bag. A control unit interrupts fluid supply by stopping the pump when a weight variation of the bag occurs and temporarily maintains the interruption at least until the malfunction disappears, and automatically restarts the fluid supply if the bag or the weight of the bag stabilizes within a predetermined time and the weight variation does not exceed a predetermined value. The control unit generates an alarm if the bag or weight of the bag does not stabilize within a predetermined time and/or if the weight variation exceeds a predetermined value.