A fluid flow system for peritoneal dialysis, comprising: a pump with an inlet and an outlet, the pump being configured to pump fluid from the inlet to the outlet; first flow paths (914, 915, 916, 919, 920A) with control valves (935, 934, 933, 932, 931) selectably connectable to the pump inlet, the first flow paths (914, 915, 916, 919, 920A) being connected respectively to respective sources (910, 912, 919) of at least one concentrate and water, and to a dialysis fluid batch container; second flow paths (917, 920B, 945) with control valves (928, 929,930) selectably connectable to the pump outlet, the second flow paths (917, 920B, 945) being connected respectively to the dialysis fluid batch container, a patient access line, and a drain line; the drain line having first and second conductivity sensors (984, 985) connected in series in said drain line; and a controller (610, 907) connected to the pump, said control valves (935, 934, 933, 932, 931; 928, 929, 930), and said first and second conductivity sensors (984, 985), configured to operate the pump, connect the first and second flow paths (914, 915, 916, 919, 920A; 917, 920B, 945) selectably, and measure conductivity of fluid in said drain line; wherein the controller (610, 907) is configured to operate the pump and the control valves (935, 934, 933, 932, 931; 928, 929, 930) to create a fluid interface between first and second fluids and convey the interface to the drain line and further to store time data representing a time interval between a time the interface crosses the first conductivity sensor and the time the interface crosses the second conductivity sensor. a pump with an inlet and an outlet, the pump being configured to pump fluid from the inlet to the outlet; first flow paths (914, 915, 916, 919, 920A) with control valves (935, 934, 933, 932, 931) selectably connectable to the pump inlet, the first flow paths (914, 915, 916, 919, 920A) being connected respectively to respective sources (910, 912, 919) of at least one concentrate and water, and to a dialysis fluid batch container; second flow paths (917, 920B, 945) with control valves (928, 929,930) selectably connectable to the pump outlet, the second flow paths (917, 920B, 945) being connected respectively to the dialysis fluid batch container, a patient access line, and a drain line; the drain line having first and second conductivity sensors (984, 985) connected in series in said drain line; and a controller (610, 907) connected to the pump, said control valves (935, 934, 933, 932, 931; 928, 929, 930), and said first and second conductivity sensors (984, 985), configured to operate the pump, connect the first and second flow paths (914, 915, 916, 919, 920A; 917, 920B, 945) selectably, and measure conductivity of fluid in said drain line; wherein the controller (610, 907) is configured to operate the pump and the control valves (935, 934, 933, 932, 931; 928, 929, 930) to create a fluid interface between first and second fluids and convey the interface to the drain line and further to store time data representing a time interval between a time the interface crosses the first conductivity sensor and the time the interface crosses the second conductivity sensor.