Sidearm fitting for inserting 12 and 18mm diameter probes used to secure O2, pH and temperature sensors in larger ProCulture® 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 Viton® o-ring and a polypropylene-sealing cap. The sensors are held in place by Noryl® nuts with Viton o-rings. The fittings are completely autoclavable.