The invention concerns a device and a method for a peritoneal dialysis treatment in several cycles. A cycler (1) controlled by a processor (31) fills the abdominal cavity with dialysis fluid, and drains the abdominal cavity. A sensor (37) senses a variable related to the draining of the dialysis fluid. The draining is interrupted when the variable reaches a breakpoint at which the variable is radically changed, thereby leaving the residual volume.