Method and device for measuring the flow differential between incoming liquid flow to a dialysis machine and outgoing liquid flow from a dialysis machine, as well as a method for calibration of the device. The dialysis machine comprises a heat exchanger (10) for transfer of heat energy from the outgoing (29) to the incoming (8) liquid flow, a connection (23,24) to a dialyser (2) for feeding the dialysis solution through the dialysate side of the dialyser, a device (20, 28) for attaining an ultrafiltration in the dialyser, and a plurality of temperature sensors (30,31;32,33) for detecting the temperature of the dialysis solution. The flow differential is calculated with help of the equation ΔQ = Q x (ΔT 1 - ΔT 2 )/ΔT 2 where ΔQ = the flow differential, Q = the liquid flow, ΔT 1 = temperature difference across one side of the heat exchanger and ΔT 2 = temperature difference across the other side of the heat exchanger. Calibration takes place with built in concentrate pumps (6,16,18).