Estimating ABV using Brix refractometer

[danielcook]

Jim,
This falls in line with my empirical readings from my refractometer, which was freaking me out because I was assuming a WCF of just 1.04 and I was convinced my beer had failed to attenuate properly.

The attached screenshot shows the relevant calculation from http://seanterrill.com/2012/01/06/refractometer-calculator/. Prior to using this calculator I had been baffled by my refractometer reading of 4.2P (1.0165 if used on unfermented wort), because it was in stark contrast to my hydrometer's FG reading of about 1.005. These measurements left me with competing theories that either (a) my beer had stopped fermenting about 4 points early, or (b) my beer had attenuated way below its intended FG. Obviously neither outcome seemed easy to explain!

But now in hindsight it's apparent that the 1.005 SG reading was due to the pleasing presence of ethanol, which has about 79% the density of water. I haven't yet computed a wort conversion factor specific to my equipment, or to this style of beer, but using the default factor of 1.04 took away a lot of my concern.

Live and learn!

 

[JimGossett]

Jim,

RE: a 1.25 WCF of finished beers, does that mean a refractometer reading of a de-gassed sample of finished beer, whose FG = 1.013, would be about 4.125? (1.013 = 3.3P, and 3.3P x 1.25 = 4.125P)

This falls in line with my empirical readings from my new refractometer, which was freaking me out because I was assuming a WCF of just 1.04 and I was convinced my beer had failed to attenuate properly.

danielcook, I'm not sure I have completely understood your question. Is your FG of 1.013 a hydrometer reading or the reading on a refractometer that has a SG scale on it? There are both refractometers and hydrometers that have dual SG and Plato (or Brix) scales on them, and the interconversion of scales built into the devices only applies to unfermented wort – not to beer.

To keep things less confusing, I'm using "gravity" to refer hydrometer readings, and I'm using Plato or Brix to refer to refractometer readings.

There is also ethanol (EtOH) in finished beer, and it contributes significantly to both hydrometer and refractometer final readings (in negative and positive manners, respectively). EtOH lowers SG but raises refractometer reading. The relationship between final refractometer reading and FG (hydrometer-based) depends on OG (or original refractometer reading) because the higher the OG, the more EtOH in the final refractometer reading. WCFs are merely attempts to correct for non-sucrose-like constituents in worts. WCFs don't deal with EtOH.

In initial wort (before there is any ethanol, EtOH, present), the WCF is, in part, used to account for non-carbohydrate components in the wort that would contribute to a refractometer reading, but wouldn't really be part of the original, "real extract" (RE). The refractometer scale is based on a pure sucrose solution, and wort is not sucrose (and, in fact, wort is not all carbohydrate, either). These non-carbohydrate components are a greater fraction of the final apparent extract after the fermentable carbs are gone. That's why the WCF goes up. However, there is EtOH also present in the final beer, and it also contributes significantly to a refractometer reading. The WCF of 1.25 that I measured was on final beers whose EtOH had been removed. It should not, alone, be applied to a final refractometer reading to estimate final RE or FG.

If you visit at my calculator

http://www.ithacoin.com/brewing/ABV_calc_frontpage.htm

you can experiment with different initial and final refractometer readings and see the effect on initial and final RE (first two pale blue cells) and final SG (hydrometer-based) – the last entry in the output column.

 

[JimGossett]

The attached screenshot shows the relevant calculation from http://seanterrill.com/2012/01/06/refractometer-calculator/. Prior to using this calculator I had been baffled by my refractometer reading of 4.2P (1.0165 if used on unfermented wort), because it was in stark contrast to my hydrometer's FG reading of about 1.005. These measurements left me with competing theories that either (a) my beer had stopped fermenting about 4 points early, or (b) my beer had attenuated way below its intended FG. Obviously neither outcome seemed easy to explain!

But now in hindsight it's apparent that the 1.005 SG reading was due to the pleasing presence of ethanol, which has about 79% the density of water. I haven't yet computed a wort conversion factor specific to my equipment, or to this style of beer, but using the default factor of 1.04 took away a lot of my concern.

Live and learn!

Great!

As an aside: If I recall correctly, Sean Terrill's calculator to estimate FG from initial and final refractometer readings is based on using the same WCF (something like 1.04) to final refractometer reading, as well as to intial refractometer reading. Terrill put considerable effort into developing his calculator to improve estimates of FG over another available calculator – that of Bonham (Bonham, L. K., "The Use of Handheld Refractometers by Homebrewers," Zymurgy, 43-45, January/February (2001)). However, Bonham does not call for using a WCF when inputting the refractometer reading of the finished beer. As I've previously pointed out, there is no basis for using the same WCF for both wort and finished beer. Therefore, Bonham used no WCF for the finished beer. This, I believe, is why Terrill perceived inaccuracies in using Bonham's calculator: Terrill was inputting "corrected" final refractometer readings for finished beer when no correction was called for, if using Bohham's method. This doesn't make Terrill's method "wrong." I'm merely explaining why I think Terrill found "inaccuracies" using Bonham's method: Terrill, in effect, did not use Bonham's method as Bonham intended.

 

[ajdelange]

Wonder whatever happened to Lou. He would have loved this forum. He was a major contributor to HBD for years.

 

[chrisgg]

I've been working on a web-based calculator to estimate %ABV from initial and final Brix. Yes, there are polynomials that allow OG and FG to be estimated from initial and final Brix, and then one can use one of the many equations to estimate %ABV from OG and FG. But that seems circuitous. You're converting one indirect measure of carbohydrate content to another. Plus, I never see anyone evaluate the resulting accuracy. That was my motivation. Check it out here, if you're interested:

ABV Calculator Frontpage

== Jim

Thanks Jim for your ABV from Brix calculator. I tried it on my current brew, a deliberately low gravity British bitter. The OG and FG were 1.030 and 1.010 on a hydrometer, the FG reading after most of the fermentation was complete. (From my experience, the FG will probably end up at ~1.003 due to further fermentation, after transferring to a keg for about a week before bottling; not due to any added sugar. I add 1.5g/500ml priming sugar when bottling). On a refractometer, the corresponding readings were 8.1% and 4.3% Brix. The ABV estimate from an SG calculator (1.030-1.010) x 131.25, would be about 2.6% before priming. Your formula for Brix gives 3.2% before priming. However, Brix to SG calculators suggest 8.1% Brix is equivalent to 1.032 SG, though my hydrometer disagrees. If I enter 1.032 in the SG formula it would give 2.9%, a bit closer to your figure. I still find the formula useful to keep a check on Brix once the beer is bottled. It's no use wasting 100ml of bottled beer to estimate FG on a hydrometer. I have often had beer bombs that have continued to ferment once in bottle (in addition to priming) despite having apparently finished fermentation before bottling with a stable FG. They mnay foam and gush on opening. I always use swing top bottles so I can relieve the increase of pressure if necessary. It is a nuisance. This unwanted secondary fermentation could be due to a significant increase of room temperature during a summer heatwave. In the UK very few houses have cool air conditioning, mine included. It would just not be needed apart from a few weeks a year. Or the yeast might flocculate too early and then start again when disturbed by the syphoning process. Or the addition of a different yeast for priming could upset the balance. I've stopped doing that now. Whatever the reason, it's very useful to use your formula to give an estimate of whether the alcoholic strength of the beer is increasing while in the bottle.