Sidearm fitting for inserting 18mm diameter probes used to secure pH, O2 or temperature sensors in larger Spinner Flasks with vertical sidearms during cell culture to monitor important cell culture parameters. Together with gassing and media handling fittings, they allow Spinner Flasks to operate as bioreactors when they are attached to appropriate cell culture controllers at costs considerably below those of traditional Bioreactor vessels with expensive stainless steel headplates. This provides a cost-effective way of enhancing cell culture productivity. The fittings are comprised of a PET insert with a fluorocarbon O-ring and a polypropylene-sealing cap. The sensors are held in place by modified polyphenylene oxide thermoplastic nuts with fluorocarbon O-ring. The fittings are completely autoclavable. Sidearm fitting for inserting 18mm diameter probes used to secure pH, O2 or temperature sensors in larger Spinner Flasks with vertical sidearms during cell culture to monitor important cell culture parameters. Together with gassing and media handling fittings, they allow Spinner Flasks to operate as bioreactors when they are attached to appropriate cell culture controllers at costs considerably below those of traditional Bioreactor vessels with expensive stainless steel headplates. This provides a cost-effective way of enhancing cell culture productivity. The fittings are comprised of a PET insert with a fluorocarbon O-ring and a polypropylene-sealing cap. The sensors are held in place by modified polyphenylene oxide thermoplastic nuts with fluorocarbon O-ring. The fittings are completely autoclavable.