purgeing kegs: If you add co2 to an empty (air filled) keg and then release the presure and then repeat you will not get all the o2 out. yes o2 is lighter than co2 but it is still a gas. alchahol is lighter than water but if you pour water into a glass of rubbing alchahol until it overflows an amount equal to the volume of alchahol you do not end up with pure water. fluids mix. They may not stay mixed but they do mix...
Fluids DO mix. However, with gases you can take advantage of the pressure difference from your resident gas (air - oxygen) and your purging gas (CO2).
If you're purging with CO2, you're using 5-20 psig gas to purge 0 psig (atmospheric) gas. A few purges is with pressurized CO2 is PLENTY - as long as the purges come up to pressure and you're not just blowing CO2 through an open valve.
When we have to push ALL the oxygen out of our batch reactors (at work), we use nitrogen at 30 psig. We use 3-30psig purges to get the O2 levels < 0.5%. Obviously, N2 and O2 are miscible - its the pressure difference that counts.