A small volume prover (SVP) apparatus and method for providing variable volume calibration. The apparatus includes a precision bore cylinder and a piston configured with a valve arrangement in order to permit fluid to pass through an annular passage when the piston is traveling from one position to an opposite position. A position sensor and a pair of optical slot detectors can be integrated in association with the piston for continuously sensing an accurate position and stroke of the piston. The position sensor verifies the optical slot detectors at every run and provides calibration of flexible volumes as the fluid travels via the annular passage. The position sensor also checks the correct return of the piston after the flow stroke and detects the stuck mechanics. Such small volume prover apparatus associated with the position sensor can be employed for precise and fast position sensing of the piston and variable volume calibration of a fluid flow sensor.