I recently replaced my glass carboy with a 20 liter Spiedel plastic fermenter. One problem, the unit is opaque so I cannot read the hydrometer. After one week fermenting at 50 degrees i opened the fermenter and dropped in the hydrometer (sprayed with star san) to get a read. The container was open a half minute, but now I am concerned if that brief time allowed too much oxygen to enter. I want to check the hydrometer again but don’t want to risk spoiling my lager.
I’d be more worried about introducing possible contamination by putting your hydrometer right in the fermenter. If there’s any active fermentation at all, then the yeast should pick up what little oxygen may be introduced. I’ve never run into any issues by opening the fermenter to draw a hydrometer sample.
To each his own (should be the Homebrewers’ mantra). But I think there is a benefit to drawing off a sample that you may be missing; with a sample, you get to taste your beer. I find it instructive to experience how my beer tastes at every reasonable opportunity.