Does an ABV reading lose accuracy if the beer has been sitting out?

I dont know the real answer but with that being said I do not think time out would effect the abv.

 

You'll probably want to put this in the Homebrewing forum. I'm actually looking to start brewing this year.

 

In theory the alcohol would evaporate first from the beer if it is left exposed to the air, but the rate of evaporation would depend on the surface space that is exposed as well as the humidity of the air in the room. I doubt that very much would be lost, but it's a tough call based on the factors I mention above. I'd say that you could take a gravity reading and see if it sounds realistic. You might also post this question in the Homebrewing forum where you may may get a more definitive answer.

 

Welcome! Don't know the answer but post here and someone will know for sure: http://www.beeradvocate.com/community/forums/homebrewing.8/

 

How are you checking abv? If you mean taking a final gravity (FG) reading to plug into a formula or online tool with your original gravity reading to find your abv, that should have been done before bottling (carbonation). You will need a de-gassed sample to get an accurate FG reading, as CO2 will throw off the density of the sample. I think just shaking your sample vigorously a few times would do the trick, but honestly, I'm not sure. I think others have done this and can weigh in.

 

As @Mothergoose03 stated, the alcohol could have started evaporating which would effect the FG reading. Leaving it open for a week now I would also be concerned that bacteria or wild yeast may have gotten into that leftover open bottle and started fermenting which would lower the gravity reading even more. Just crack another bottle, test the gravity, then drink it all. If you are serving this at your wedding you need to know that the beer isn't crap, it is carbonated fully, that you don't have uncarbonated bottles and over carbonated bottles from uneven distribution of priming sugar, etc, etc (original post and questions lead me to believe that you are probably not an experienced brewer).

 

I'm not one who uses brewing calculators, but if you took an OG reading and you know exactly how much beer was produced and what the recipe was, possibly a calculator will give you the expected final gravity and the ABV. Was this an extract recipe or an all-grain brew session? Possibly someone who uses these programs can chime in with what other info is needed.

 

How do you know the beer was done fermenting before you bottled?

How many bottles did you leave open? Are you planning on serving the bottles that were left open?

 

Normally, determining ABV is done by taking a hydrometer reading before the yeast is pitched and taking another hydrometer reading after fermentation is done.

Some of the booze may have evaporated from the open bottle. Probably not enough to matter. Take a hydrometer reading of the open beer. If you know the pre-fermentation gravity, you can use this calculator to determine ABV.

http://www.brewersfriend.com/abv-calculator/

 

OP, I misread your original question. You can't use a beer that has been opened for a week. Do as @jbakajust1 suggests and open another one if you want to get some sort of a final reading. But since it is carbonated, you will need to de-gas it like @carteravebrew suggests.

 

To a 1st approximation ... the last two digits of the OG x 0.10 = ABV.

Ex.
OG = 1.070
Last two digits: 70
Last two digits x 0.1 = 7 (IOW 7% ABV)