I spray lots into the top so that fog billows out and then get some more onto the inside neck of the bottle - it will form a little icy crust. Put the cap on and the bottle will inflate to fairly high pressure. At that point the butane has displaced (I'm guessing) maybe 3/4 of the oxygen/nitrogen mix. Place your hand on the bottle to warm it (evaporate that little crust) and the pressure will shoot up to several atmospheres (about 105psig = 8bar absolute). Now you release the pressure from the lid gently so that it comes down to just over 1 atm, and that will release 7/8 of the remaining oxygen. What remains is about 1/32 of what was in there before.