A method, including inserting a probe into a cavity in a subject's body and receiving, from a force sensor in the probe, first readings indicative of a first change in measured contact forces between the probe and the cavity by less than a predetermined limit over at least a predetermined interval of time. The method continues by receiving second readings from the force sensor when the second readings have changed by more than a predetermined force threshold or location coordinates of the probe have changed by at least a predetermined location change threshold. The method continues by receiving third readings indicative of a second change in the measured contact forces between the probe and the cavity by less than the predetermined limit over at least the predetermined interval of time. The method concludes by calibrating a zero-force point for the force sensor according to the third readings.