Apparatus and method for measuring the compressibility (K) of a fluid circulated by a positive displacement pump that comprises: a pressure sensor for sensing a pressure P of a fluid confined in a fluid module of the pump; a position sensor for sensing the position X, of a piston in the fluid module and a control unit communicating with the pressure sensor and the position sensor adapted to: log the pressure (P) and piston position (X) substantially synchronously; using (X), calculate the volume (V) of the confined fluid in the fluid module; and using (P) and (V), calculate the compressibility (K).