Peritoneal dialysis systems and methods move liquid using a pump chamber (P1, P2) that is operated in response to pneumatic pressure variations applied by a pump actuator (PA1, PA2). The systems and methods periodically measure air pressures in the actuator (PA1, PA2) and an associated reference air chamber (VS1, VS2) and derive from these a measurement of liquid volume moved through the pump chamber (P1, P2). The systems and methods minimize derivation errors by compensating for temperature differences among the pump chamber (P1, P2); the pump actuator (PA1, PA2); and the reference chamber (VS1, VS2).