So I have about 13 to 14 gallons of beer fermenting right now and am getting ready to force carbonate it in a Sanke keg. However, I have a couple questions regarding the actual kegging process. First I plan on taking the ball lock and spear assembly out of the keg and carefully filling the keg trying to avoid getting excess oxygen into the beer. Then I am going to put the ball lock and spear back on. My question though is that I will have about 2 gallons of head space that is just air. So do I need to offgas that air to prevent the oxygen from getting in the beer? If so how would i do that in a sanke keg?