An apparatus connected to operate a balloon system that enables in-vivo calibration of a blood pressure sensor associated with a balloon of the balloon system, the sensor and balloon being associated such that the sensor is in-vivo when the balloon is in-vivo, and wherein the blood pressure sensor is a fiber optic pressure sensor, the apparatus comprising: means for inflating the balloon to a target displacement volume so that a gas pressure in the balloon system is indicative of a patient's blood pressure; means for monitoring the patient's blood pressure by observing the gas pressure in the balloon while simultaneously monitoring the patient's blood pressure through the sensor; and means for using blood pressure readings obtained by monitoring the gas pressure as reference blood pressure measurements and determining a mathematical relationship between blood pressure measurements obtained through the sensor and the reference blood pressure measurements such that future blood pressure measurements obtained through the sensor can be modified according to the mathematical relationship to generate calibrated blood pressure measurements, characterised: - in that the means for inflating the balloon to the target displacement volume employs Boyle's law to determine the target displacement volume; - in that the means for monitoring includes means for recording a pressure signal generated by a gas pressure sensor and a pressure signal generated by the blood pressure sensor, and the means for using blood pressure readings includes means for filtering the pressure signal generated by the blood pressure sensor to generate a filtered signal, wherein the means for filtering includes a filter that simulates the effect of the balloon system configuration upon the gas pressure as reflected in the pressure signal generated by the gas pressure sensor; and - by means for correlating the filtered signal and the pressure signal generated by the gas pressure sensor to determine a relative time delay between the filtered signal and the pressure signal generated by the gas pressure sensor, and wherein the means for using blood pressure readings includes means for generating a filtered and time-shifted signal by time-shifting the filtered signal to compensate for the relative time delay. means for inflating the balloon to a target displacement volume so that a gas pressure in the balloon system is indicative of a patient's blood pressure; means for monitoring the patient's blood pressure by observing the gas pressure in the balloon while simultaneously monitoring the patient's blood pressure through the sensor; and means for using blood pressure readings obtained by monitoring the gas pressure as reference blood pressure measurements and determining a mathematical relationship between blood pressure measurements obtained through the sensor and the reference blood pressure measurements such that future blood pressure measurements obtained through the sensor can be modified according to the mathematical relationship to generate calibrated blood pressure measurements, characterised: - in that the means for inflating the balloon to the target displacement volume employs Boyle's law to determine the target displacement volume; - in that the means for monitoring includes means for recording a pressure signal generated by a gas pressure sensor and a pressure signal generated by the blood pressure sensor, and the means for using blood pressure readings includes means for filtering the pressure signal generated by the blood pressure sensor to generate a filtered signal, wherein the means for filtering includes a filter that simulates the effect of the balloon system configuration upon the gas pressure as reflected in the pressure signal generated by the gas pressure sensor; and - by means for correlating the filtered signal and the pressure signal generated by the gas pressure sensor to determine a relative time delay between the filtered signal and the pressure signal generated by the gas pressure sensor, and wherein the means for using blood pressure readings includes means for generating a filtered and time-shifted signal by time-shifting the filtered signal to compensate for the relative time delay. - in that the means for inflating the balloon to the target displacement volume employs Boyle's law to determine the target displacement volume; - in that the means for monitoring includes means for recording a pressure signal generated by a gas pressure sensor and a pressure signal generated by the blood pressure sensor, and the means for using blood pressure readings includes means for filtering the pressure signal generated by the blood pressure sensor to generate a filtered signal, wherein the means for filtering includes a filter that simulates the effect of the balloon system configuration upon the gas pressure as reflected in the pressure signal generated by the gas pressure sensor; and - by means for correlating the filtered signal and the pressure signal generated by the gas pressure sensor to determine a relative time delay between the filtered signal and the pressure signal generated by the gas pressure sensor, and wherein the means for using blood pressure readings includes means for generating a filtered and time-shifted signal by time-shifting the filtered signal to compensate for the relative time delay.