Abstract: Calibration apparatus and method for calibrating a movable pressure sensor include the use of a stop device to establish a reference position outside the sensing movement range of a first sensor. While in vitro, a reference pressure is established by applying pressure to the sensor to move it against the stop surface with a controllable pressure source. This reference pressure is stored. During use when the sensor is in an unknown position, such as when in vivo, the controllable pressure source applies pressure to move the sensor against the stop surface and the pressure required to do so is measured. The measured pressure is compared to the reference pressure to determine the unknown pressure and the system is then recalibrated. An optical system may be used as a second sensor to determine the position of the first sensor.