Abstract: A sensor capsule to facilitate positioning of an analyte sensitive element such as a CO2 sensor at the end of a fiber optic probe, which discloses a dye layer defined by a dye well, fluorescent dye solution and apertures formed in a silicone sensor membrane. A TEFLON® covering membrane is positioned over the silicone membrane to retain the dye layer within the well, and perforated metal disc is positioned beneath the silicone membrane to provide support and resist swelling of the dye layer. The perforated metal disc, silicone membrane and TEFLON® membrane define a sandwich sensor structure which is positioned within an outer housing. An insert member is positioned over the sandwich sensor structure and is snap fit into outer housing to provide a sensor capsule which may be removeably positioned/replaced in association with a probe tip used for forming the fiber optic CO2 SENSOR.