A sensor well (100, 200) for use with a bioreactor vessel, the sensor well comprising: a sheath (116, 228) having proximal end (118, 230) and a distal end (120, 238), wherein the sheath is enclosed at the distal end; a sensing element (126, 236) comprising a fluorescing material disposed on the distal end of the sheath (116, 228); a signal transmitter (127, 240) comprising an optical waveguide disposed within at least a portion of the sheath (116, 228), one end of the signal transmitter (127, 240) located at the distal end of the sheath (116, 228), the signal transmitter (127, 240) positioned to transmit light from a sensor controller to illuminate the sensing element (126, 236), the optical waveguide configured to receive illumination signals from the sensing element (126, 236) and provide signals from the sensing element (126, 236) to the sensor controller; a connector (110, 208) comprising an aperture, the connector configured to attach the sensor well to a portion of a bioreactor vessel; and a collapsible bellows (114, 222) coupled on one end to the connector and on the other end to the proximal end of the sheath (116, 228), the bellows (114, 222) housing the sheath and the sensing element on the end of the sheath when the sheath is in an undeployed position not extending through the aperture in the connector (110, 208), wherein the bellows (114, 222), the connector (110, 208), and the sheath (116, 228) are connected to form at least a portion of a hermetically sealable and sterilizable enclosure (102, 216) in which the sensing element is enclosed when the sheath is in the undeployed position, a removable sealing layer (112, 226) covering the distal end of the sterilizable enclosure (102, 216) configured to hermetically seal the sterilizable enclosure (102, 216), a calibration gas inlet with a first submicron filter and a calibration gas outlet with a second submicron filter configured to allow passage of a gas through the enclosure to calibrate the fluorescing material enclosed in the enclosure, wherein the sheath (116, 228) and the bellows (114, 222) are configured so that the sheath (116, 228) can be deployed through the aperture in the connector (110, 208) and into a bioreactor vessel that is attached to the connector thereby exposing the sensing element (126, 236) on the distal end of the sheath (116, 228) to media contained in the bioreactor vessel while maintaining a sterile environment in the bioreactor vessel, and the sensor well is provided pre-sterilized and sealed within a sterile package. a sheath (116, 228) having proximal end (118, 230) and a distal end (120, 238), wherein the sheath is enclosed at the distal end; a sensing element (126, 236) comprising a fluorescing material disposed on the distal end of the sheath (116, 228); a signal transmitter (127, 240) comprising an optical waveguide disposed within at least a portion of the sheath (116, 228), one end of the signal transmitter (127, 240) located at the distal end of the sheath (116, 228), the signal transmitter (127, 240) positioned to transmit light from a sensor controller to illuminate the sensing element (126, 236), the optical waveguide configured to receive illumination signals from the sensing element (126, 236) and provide signals from the sensing element (126, 236) to the sensor controller; a connector (110, 208) comprising an aperture, the connector configured to attach the sensor well to a portion of a bioreactor vessel; and a collapsible bellows (114, 222) coupled on one end to the connector and on the other end to the proximal end of the sheath (116, 228), the bellows (114, 222) housing the sheath and the sensing element on the end of the sheath when the sheath is in an undeployed position not extending through the aperture in the connector (110, 208), wherein the bellows (114, 222), the connector (110, 208), and the sheath (116, 228) are connected to form at least a portion of a hermetically sealable and sterilizable enclosure (102, 216) in which the sensing element is enclosed when the sheath is in the undeployed position, a removable sealing layer (112, 226) covering the distal end of the sterilizable enclosure (102, 216) configured to hermetically seal the sterilizable enclosure (102, 216), a calibration gas inlet with a first submicron filter and a calibration gas outlet with a second submicron filter configured to allow passage of a gas through the enclosure to calibrate the fluorescing material enclosed in the enclosure, wherein the sheath (116, 228) and the bellows (114, 222) are configured so that the sheath (116, 228) can be deployed through the aperture in the connector (110, 208) and into a bioreactor vessel that is attached to the connector thereby exposing the sensing element (126, 236) on the distal end of the sheath (116, 228) to media contained in the bioreactor vessel while maintaining a sterile environment in the bioreactor vessel, and the sensor well is provided pre-sterilized and sealed within a sterile package.