Originally Posted by boscobeans
Put the lid on and wait another week or two. Opening it like that can allow the CO2 layer to be "breezed" out and allow oxygen to take its place.
Agreed, however this problem can be averted if you have a CO2 tank and a regulator (although this may be an issue of u bottle rather than keg). Since CO2 is heavier than air, if you "pour" a little co2 into the fermentor after you open it and take a reading, it will displace the air/oxygen and fill it with a layer of pure CO2 which will hover over the krausen. Just make sure the pressure is low enough otherwise the jet of co2 will blow the krausen everywhere (think blowing the foam off the top of a poured beer).
I use this technique when kegging, i just fill the keg w co2 prior to kegging, that way when i pour the brew into the keg i dont have to worry about absorbing o2 and oxydizing the beer. The beer lasts longer in the keg and tastes better as a result!