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Old 08-09-2010, 01:26 PM   #11
ajdelange
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I edited the previous post putting OG and ABV data in there so it would all be in the same place.

As to the goodness of the fit: I haven't got that much data here and, while it is true that they span a fair range of OG, I don't really put too much faith in a model based on so little data. My impressions of refractometry at this point is that it works great except when it doesn't i.e. it's generally pretty good but occasionally will have you off by more than 1°P.

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Old 08-10-2010, 04:12 AM   #12
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Thanks again for the update. It does seem like your data don't correlate very well with mine. I'm back to being frustrated now. It may just be that refractometers aren't all that accurate when it comes to FG. Then again, since your OG data are based on the ABV readings, maybe the innacuracies come at at that point. Like I said, there should be a simple solution based on two IVs. If nothing else, it seems like I have a good correlation for my own beers. The last three have all been <1 point off the curve.

BTW, would you mind me mentioning your results "publicly"? James Spencer has asked me to discuss refractometers on his podcast.

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Old 08-10-2010, 12:13 PM   #13
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If you delve into the subject a bit you'll find that refractometers have their uses in the brewery but they are pretty restricted. Refractometry is not, in general, considered applicable to binary mixtures except in those cases where a measurement of some other type is available. For example, in the sugar industry, they sometimes use polarization measurements. Here you are trying to use the OG and that works up to a point. More Beer has a spreadsheet that calculates AG from OG and RI. When I try that against my 7 beer data set I get within a point or 2 for most of the beers but am off by over 4 points for one of them. I think this is pretty good as considering the potential differences in sugar spectrum for a given OG and variability in yeast attenuation it doesn't seem that OG is going to be that great an indicator. Not to mention that there are lots more than 2 optically active substances in beer.

On OG: Is what you measure with the hydrometer or especially a refractometer really the OG? If you measure before pitching the starter did you adjust for the starter (including the fact that it is fermented)? If after pitching how do you treat the alcohol in the starter? How much water wast lost to evaporation during fermentation? Should OG be adjusted for that? Was the wort absolutely uniformly mixed? This can be a factor if you sparge the hops in the kettle (don't want to leave any extract on those hops) and/or push the last of the wort out of the chiller into the fermenter. I do both so the hydrometer reading I take in the fermenter are at best approximate. That's why I back calculate as do many commercial brewers. As an example, the last beer in the list of data was the first Alt out of a new brewpub around here. The brewer said the OG was 16°P but it back calculated to 14.75. That's a pretty typical spread. Note that this doesn't mean that 14.75 is spot on. It's based on Balling's formula. If more or less than 0.11 grams of yeast are produced per gram of alcohol the OG calculation will be off.

The place where a refractometer can be used in a brewery is in the determination of alcohol but to use it effectively you need to calibrate against the beers you are brewing and that's where my interest lies. If I can demonstrate a tight cal on that Alt, for example, the brewer should be able to estimate his ABV to within a tenth of a percent or so and thus back calculate his OG. There would be a separate curve for the Kölsch and another for the APA etc. The catch is, of course, that one needs to get the calibration (the ASBC method uses ABW and AG) data and that's a lot of work (unless you have an Alcolizer setup into which you put the beer and out of which all these numbers pop).

Yes, you are certainly welcome to use the data any way you like. I just wish there were more of it.

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Last edited by ajdelange; 08-10-2010 at 12:15 PM. Reason: Added refractometer ot OG reading bit
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Old 08-10-2010, 05:32 PM   #14
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I've found that the Morebeer one is nuts-on for my readings whenever I take a final with both hydrometer and refractometer. I'll give yours a shot for comparision.

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Old 08-10-2010, 06:03 PM   #15
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Great. It would be nice to have someone check the math. Note that I have edited the post with the data because the conversion to Brix polynomial was in error (I fat fingered in 30°C instead of 20°C and, as it that weren't enough, 0 for vacuum when I should have put in 1 for air). Or, if you use your own formula be aware that the nD's are relative to air.

Over the 7 beers I found the spreadsheet gave average error of 2.75 points too big. RMS error was 3.24 points and the max error was 6 points (1.5°P).

The refractometer data were taken with a Reichardt hand held digital refractometer, corrected manually to 20 °C (the ATC tables in refractometers are not suitable for beer). The FG measurements were taken with an Anton Paar DMA 5000 M. So the data should be good but, as you see, I can fat finger most anything when it comes to the keyboard!

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