I have left a hydrometer in a fermenter many times in the past. Just while making mead. That way I could just peek in and see how things are going. Not as much krausen with mead so no problems there. Just remember that the reading proly won't be 100% accurate with the CO2 in solution its gonna read higher then it really is. But your gonna get a ball park figure.
I started doing this with wine recently where the wine is close to the top and you can just grab it out. I'd be concerned with putting it in a beer fermenter where you couldn't easily remove it, on account of a hydrometer's fragility and all.
I've left my hydrometer in a test jar from my OG test and let it ferment out to check the readings FG readings. I never cross examined them to see if they match, but that wouldn't be hard to do...
i thought about this a while back. They sell digital float hydrometers (or a submersable hydro) for testing water. Some eco tool or something. I always thought it would be cool to see the fermentation rate real time.
Why not just get a wine thief or something similar to pull a sample out? I never considered leaving the hydrometer in there - because I would never get it out until I racked.
Mylo
"Life is too short to bottle homebrew." - Me
"HEINEKEN? Fuck that shit! Pabst Blue Ribbon!!!" - Dennis Hopper, in Blue Velvet