How can I control the oxygen in an erlenmeyer flask in a bioprocess?
Controlling oxygen in a flask can be achieved by bubbling a purge gas of some description through it. This will entrain and remove oxygen from the solution as well as creating an oxygen-depleted headspace.

If you have a regulated nitrogen supply, a rubber stopper with a hole in it, an open-ended pasteur pipette and a length of hose, you can bubble nitrogen into the solution to purge oxygen. This requires the vessel to be open somewhere else to equalise pressure, which is achievable with a multi-neck flask but not with a standard Erlenmeyer flask. You can get around this with a stopper with two holes. Continual positive pressure should keep the external atmosphere out.



Depending on how vigorously you want to shake your flask, this may or may not be reasonable. If you are using a stirrer bar you will obviously want to give the pipette sufficient clearance.

As far as controlling addition of oxygen, this would seem to be a bit trickier as you are probably not going to be providing continual positive pressure. You might be able to get around this, depending on the specifics of what you are doing, by pumping in oxygen in the same way and using a balloon or a gas syringe or something to deal with the overpressure.