A method, including injecting a current between anelectrode of a catheter and tissue in proximity to thecatheter, the catheter having a force sensor configuredto measure a force between the catheter and the tissue.The method further includes measuring a succession ofphase shifts of the current relative to a fixed referenceand verifying that a cardinality of the measured phaseshifts falling below a predetermined threshold increasesover a predetermined time period. A zero-force point forthe force sensor is calibrated according to the forcemeasured by the force sensor during the predeterminedtime period.
