Abstract: An arrangement for measuring the content of crystals in a mixture of liquid and crystals comprises a pipe line (2) and a pump (5), by which a constant flow of the mixture is pumped through the pipe line (2). In this, a unit (6), preferably an electrical resistance wire, is arranged to transmit a constant amount of heat to the flow. There are also temperature sensors (7, 8) before and after the unit (6), arranged to transmit impulses representing the temperature difference to a unit (11), which with the aid of information about the velocity of the flow, the added amount of heat, the specific heat of the liquid and the melting heat of the crystals, calculates the content of crystals in the mixture. The content may be shown on a screen or a corresponding signal may be transmitted to a controlling unit, which influences the formation of crystals in the liquid.