An apparatus for determining a magnitude of a compressive load applied to a piston including a compliant film disposed between first and second elements is provided. The apparatus includes a first part movable with the first element in a movement direction along which the magnitude of the compressive load is to be determined, a second part movable with the second element in the movement direction and a sensor to measure a distance between the first and second parts in the movement direction, the measured distance being related to a deformation of the compliant film as the compressive load is applied.