Taking hydrometer samples from primary

[sgillespie]

If I have a primary fermenter that has some significant head space, should I be concerned about taking the lid off and introducing oxygen when taking hydrometer readings?

 

[COLObrewer]

No worries, co2 is heavier than air, just don't go whipping it up or anything and you'll be fine. Also, it's fruit fly season, so be carefull you don't trap one of those little nasties in there when you close it up again.