The following advice should NOT be used until the user educates him/herself on the proper and safe use of compressed gasses. I shall not be personally responsible. OK?
I would never have air in headspace: I'm an amateur, don't leave anything to chance, won't risk a batch=time, money, work. I use nitrogen, left over from chemistry, shall switch to CO2 ASAP. Approached fellow at AirGas, got encouraging answers. You should talk to local gas dealers. AirGas is even getting into "beer gas" for us HBers. Can also use gas pressure to push fluid out of a vessel.
CO2 is cheap, and going in with others is cheaper still.
I always wanted to try: Dry Ice(careful....-80 degrees=frostbite. you are warned!!!) will slowly warm, giving off CO2. And, CO2 is heavier than air, and COLD CO2 is heavier still. Use this fact. Careful: CO2 at 10% concentration will render you unconscious, and, you will die. Buuuuutttttt......CO2 as Dry Ice has compressor oil on it, but a clever fellow should be able to construct a filter to remove this oil. Why? 'cause DI is dirt cheap. Hmmmm. Also, CO2 has formula weight of 44, and this expressed as grams is 44g., the weight of what chemists call one mole of CO2, and with 454 grams per pound, that is ten moles of CO2 per pound. Soooooooooooooo wot? Well, one mole of a gas at room temp. occupies 22.4 liters. Now, you can compute how much gas is needed to flush how much space. Test it by holding a lit match by opening. Go online to find conversion factors for, well, stuff. Good luck!