Brix to Specific Grav confusion

[christoph200071]

Hello, would anyone mind helping me understand something please? I used this calculator: http://onebeer.net/refractometer.shtml to convert my refractometer reading (brix)to specific gravity. I did the test after a few days of fermentation to check conversion and found that if I calculate original brix to final brix (with above calculator), I get a potential ABV of 5.8%, but if I use the converted figures in specific grav (on other calculators) I get a potential ABV of 3.6%.

If you need numbers: My original brix was 12.6, and after 4 days, I’m at 6. Directly converted to potential ABV (using above calculator), this gives me a reading of around 5.8%, but if I convert both figures to specific grav (1.051 to 1.024) I get an approximate ABV of 3.6%

Can anyone explain why there’s a discrepancy?

Many thanks.

 

[ajdelange]

The approximate formula is 0.42*(OG - AE)/0.791 so in going from 12.6 to 6 you would have 0.42*6.6/0.791 = 3.5% ABV. But you don't have 6 Bx, you, have 6 read from a refractometer calibrated for sucrose, not beer. Alcohol and sucrose both pull refractive index higher (and thus cause a Bx reading to be higher). To figure out what the actual Bx of the beer is you must have a correction formula for the beer you are measuring or at least the class of beer (thus MEBAK has one formula for Schankbier and one for Vollbier). The website you are looking at tries to cover all beers under one formula and will, thus, be quite inaccurate unless you are lucky and there is no way to tell when that is.

It doesn't matter whether you do the ABV calculation in Bx or points. The final reading of the refractometer is going to be off. Refractometers are great for monitoring the decline in gravity during sparge but not good for much else in brewing at the home brew level. Commercial brewers use them to do alcohol concentration estimates but they have calibrations for the beers they brew week in and week out. The calibrations come from an accurate determination of the alcohol content of beers made by distillation or another method.

 

[christoph200071]

Thanks for helping me understand that ajdelange. It seems to me that the result you arrived at is reasonably similar to the coverted figures from brix to specific grav. But I'll do the math, and next time round use my hydrometer.

Thanks again, much appreciated!

 

[Yooper]

The refractometer is awesome for preboil gravity, and a post-boil OG. I love it for taking the brix of fruit (I'm a winemaker). But as ajdelange said, once alcohol gets into the mix, it skews the readings. There are some halfway decent converters to give you a half-way decent guestimate. But for the most part, refractometer pre-fermentation, hydrometer post-fermentation.