According to an aspect there is provided an apparatus comprising at least one electrode and a movable sensor membrane, wherein the apparatus comprises means for: measuring a voltage or a current; determining an amount of external pressure based on the measured voltage or current; based on the determined amount of external pressure, providing electrostatic feedback force such that the movable sensor membrane is undeflected; and determining a correlation between the electrostatic force feedback and the external pressure.

A pressure testing device for calculating a pressure in a flexible line comprises a housing unit, a force sensor mounted on the housing unit and a clamp assembly having a clamp mounted on the housing unit. The clamp is operable to compress the flexible line against the force sensor by a predetermined degree of deformation of the flexible line. The device includes a displacement sensor adapted to measure a displacement of the clamp. The device also includes a controller having a processor in communication with the force sensor and the displacement sensor, and a memory unit containing stored data. At the predetermined degree of deformation of the flexible line, the processor compares a first signal from the force sensor and a second signal from the displacement senor with the stored data to estimate the pressure within the flexible line.