Abstract: A calibration system and method for calibrating a medical sensor (16) that monitors chemical blood gases. A calibrator (12) is used in connection with a tray (14) in which the medical sensor is stored in a sterile environment, both before and during the calibration process. The medical sensor is immersed in a liquid (30) in a tonometry chamber (28) defined in the tray and covered by a membrane (120/122) that is permeable to gas, but impermeable to bacteria. During the calibration process, the tray is inserted into the calibrator, bringing a heated platen (26) into contact with the tonometry chamber, so that the liquid can be heated to a calibration temperature substantially equal to the temperature at which the medical sensor will subsequently be used. A first calibration gas is then bubbled through the liquid until a saturated condition is achieved.