Help calculating Brix after fermentation start

I'm getting odd readings with my refractometer, so I'm doing something wrong, but I don't know what.

I want to see if my beer is done.

OG was .56, using a brix/OG calculator I found this to equal 13.8 Brix.

Then, I use this calculator: http://onebeer.net/refractometer.shtml which calculates OG/FG & ABV in Brix.

I fill in 13.8 in the OG box, and 5.6 in FG box (which is the reading from my refractometer). The result is 1.001. Which is not correct.

With my hydro the FG is .12.

I have calibrated the refractometer with sterile water and wort.

 

A refractometer won't work correctly after fermentation. The alcohol in it skews the results. You need to still use a hydrometer. It will work for you to compare results on multiple days to see if its done as all results will be equally skewed. But for FG you still need a hydrometer

 

Argh. I thought there was some kind of formula taking into account the levels of alcohol. But now I see that it cant tell how much alcohol theres in the brew. Doh!

But, if I check OG and FG with a hydro. And taking SG in readings in between with the fracto, can I after I've taken the FG reading do some math which tells me what the SGs were when I took the readings with the fracto?

 

Sean Terrill has a calculator and downloadable spreadsheet on his site which I've found to be pretty accurate for calculating FG compared to the actual hydrometer reading. He did his own calcuations to come up with it (he has an article in this month's Zymurgy on refractometers - the one with Vinnie on the cover). The spreadsheet is nice for keeping a log of batches. I also now rely on this for estimating FG on small batches where I don't want to use a whole sample. It's close enough for me.

 

Hmm.. i didn't get it.. does this measure the SG? All I can see is original and final values.

 

Yes you plug in the original and final values and hit calculate and it shows the OG and FG. I notice that the calcuations on the site are slightly different than the older spreadsheet I have been using. The wort correction factor has been updated to 1.040 while my spreadsheet uses 1.020. Not exactly sure how you use this or if you are supposed to adjust it for your own refractometer. Probably explains on the site but I haven't waded through it.

At any rate, I have compared FG on 14 beers with a hydrometer against the calculations on the older spreadsheet I have. 10 were within 1.001 of the hydrometer reading (many of those pretty much right on), 3 were within .002. The only one way off like 6 pts was a saison that went to 1.001. So pretty reliable for me except for that one extreme.

 

But no SG? Between OG and FG?

 

I don't understand what you are asking, OG and FG are both specific gravities.

Do you mean will the calculation work for partially fermented wort? On his site where he is describing the new calculations he talks about how they do not seem to work so well for very underattenuated worts, like 50% attenuation. So I guess it may be useful early in fermentation, but I wouldn't think most people are wanting that info anyway. I would email Sean directly if you have quesitons.

 

Thank you for the link to the calculator. I use a refractometer exclusively.