Cue Thomas Dolby.... I check gravities in two ways, depending on the fermentor in use. Checking gravity with a "thief" doesn't scare me much - the fermenting/fermented beer has a CO2 blanket atop it, the thief doesn't create too much turbulence or displace too much of the gas so I don't worry there. Some of my fermentors are ported - I take the sample from the bottom. Yes, a little air gets sucked in but that's not enough to worry about. Any time I rack, I flush the new container with CO2 so any splashing won't cause oxidation. And that's made quite a positive contribution to my beer quality as well!
