[hiphophead]

I know they say take readings durning your fermentaion and if the stay the same for 3 days or so then its complete and ready to rack.  but how am i supposed to take reading 3 days in a row.  alll i have is my filled carboy.  is it safe to uncork the recork like that.  or is there something i can do like have a smaller fermentaion bottle going too?   

[a10t2]

Once the krausen drops take a reading, then take another three days later. If they aren't identical, take another three days after that, etc.

The small sample idea (fast ferment test) is a good one to ensure you don't have a stuck fermentation. IMHO, though, if you're pitching an adequate quantity of yeast it isn't always necessary.

[hiphophead]

so it would be ok to pop the air lock off to take a reading then place it back on.  that wouldnt taint it.

[tygo]

Nope, just use good sanitation.  Sanitize whatever you use to get the sample (I use a turkey baster) and whatever else will come in contact with the beer.

[corkybstewart]

But keep in mind that if you're expecting an FG of 1.012 and you get 1.020 for 3 days the beer may not be done.  You may have to warm it or take other measures.  Hydrometer readings are not the absolute indicator of a beer that is completely fermented.  I've had beers stall at a particular gravity for a week until I warmed it, shook it or added more yeast. 

[majorvices]

It is totally safe to pop the stopper off the carboy or crack the bucket lid from time to time. When using carboys, just be sure to sanitize the opening with a few sprays of Star San on a paper towel.

That said, you really shouldn't need to take a lot of gravity readings. I normally take 2 readings, one when I am running off into the carboys and one before I run off into the kegs.

If you have strong fermentation techniques, meaning you 1) pitch enough healthy yeast (for liquid yeast you must pitch starters - see http://www.mrmalty.com/calc/calc.html for more info.), 2) aerate well (if you shake the carboy make sure you shake it off and on for about 20-30 minutes), 3) and control fermentation temps (never pitch warmer than 70 degrees, never let the fermentation get over 68-70 for most ales, never let the temp crash much more than 2 degrees in 24 hours, be aware that fermentation generates as much as 4-6+ degrees of heat over ambient at high krausen) - you usually should have no problem hotting your FG within 1 - 2 weeks. Temperature and amount of healthy yeast are extremely important, if you pitch too warm and the temp spikes and then crashes you could stall fermentation (not to mention off flavors from warm fermentation temp).

Also, in some cases some yeasts tend to flocculate and drop out early, especially English ale yeast. Often times this happens with about 4-6 points left to go. In this case you usually simply need to rouse the yeast with a sanitized racking cane and maybe bump the temp up a couple degrees, and usually it will finish.

[bluesman]

I typically have a target gravity in mind that I would like to achieve. If I hit it, I move to the next step. Each yeast behaves a little differently in that they attenuate to different levels.

For example US05 typically attenuates between 75-80%. So if your making an APA with an OG=1.050...and conditions are ideal for fermentation, you should finish anywhere from 1.010 - 1.013 (hydrometer reading).

Once you acheive that level of attenuation you know you're ready to rack the beer into a keg or secondary.