Abstract: A system is disclosed for measuring the oxygen concentration in a fluid contained in a fermentation vessel, comprising a fermentation vessel containing a nozzle aperture disposed in the wall thereof, an electrode holder slidably disposed within the nozzle aperture, the electrode holder being provided at one end thereof with an oxygen permeable membrane which provides a wall across the aperture in the vessel for contact with the fluid to be examined, a diaphragm oxygen electrode mounted in the electrode holder of the fermentation vessel so that the diaphragm of the electrode and the oxygen permeable membrane are in close contact with each other, whereby the oxygen concentration of the fluid contained in the fermentation vessel is measured. A method for measuring the oxygen concentration in a fluid contained in a fermentation vessel is also disclosed.