Abstract: A calibration vial is described for storing and calibrating under sterile conditions a blood gas sensor that comprises an optical fiber whose sensing end carries a sensing element, such as a chromophore or fluorophore. The vial contains a calibrating liquid and, in its preferred embodiment, is structured to keep the end of the filter wet and permit fast calibration of the sensor without spillage of the calibrating liquid. The vial has a tubular inner calibration chamber in which the end of the fiber resides and which has a sterile filter-plugged gas inlet in its bottom and an outer concentric calibrating liquid reservoir chamber. The bottoms of the two chambers are interconnected by one or more liquid reflux ports that are positioned such that the gas coming into the bottom of the calibration chamber aspirates liquid from the reservoir chamber into the calibration chamber. This permits rapid mixing and circulation of the liquid upwardly through the calibration chamber.