A method, including injecting a current between an electrode of a catheter and tissue in proximity to the catheter, the catheter having a force sensor configured to measure a force between the catheter and the tissue. The method further includes measuring a succession of phase shifts of the current relative to a fixed reference and verifying that a cardinality of the measured phase shifts falling below a predetermined threshold increases over a predetermined time period. A zero-force point for the force sensor is calibrated according to the force measured by the force sensor during the predetermined time period.
