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Old 12-29-2012, 12:22 AM   #21
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Originally Posted by passedpawn View Post
No, sorry Goldie, but you're wrong some of that. I will agree that a refractometer is great for doing measurements during the boil (that's the only time I use one). Also, when I said "cal" your hydrometer, I meant to check it for accuracy. You're right, if it isn't accurate, it's a tosser.

Refactometers are calibrated for a sucrose solution. Beer is a combination of different types of sugars, including sucrose, maltiose, dextrose, and maybe others. The rub is that different beers contain different ratios of these sugars. So, you could calibrate your refractometer to one type of beer, say a pilsner, and it would work great for that, but then make a porter and it's way off again.

Further, the correction formulas in software that attempt to correct for FG, when alcohol is present, are not accurate as well. Sean Terrill has spent a ton of time working this out. You can form your own opinion by listening to him on the following podcast, and checking out his invaluable studies on this subject. I'll post the links below, as well as his online calculator. In his words, "[The current correction algorithm] is also noticeably less precise than even the relatively poor “triple scale” hydrometers used by most home brewers".

When you see the calculator, you'll also notice there is a "Wort Correction Factor" that you need to enter. This must be empirically determined for each different recipe. If you brew the same exact beer often, you could figure this out and expect great FG accuracy from the refractometer. If you brew different beers, you'll never have this number.

Take a look at the stuff below and you be the judge. His blog is full of great discussion with research.

Sean Terrill Discussion on Refractometer FG

Basic Brewing Episode with Sean Terrill

Sean Terrill Refractometer Calculator
This may be what I am seeing, once you hit a certain point the readings no longer correlate with wort and there is a separation. It was most obvious in my latest brew of Westy 12 with a OG of 1.094.

I did a step series today using simple table sugar at certain ratios and they matched perfectly.

Now, will understanding this ruin my brew day now... no. This isn't rocket science, but it is important to understand the various measurement of uncertainty in our testing systems. Great that we rediscover the wheel ever now and again.
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