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Old 01-24-2013, 12:05 AM   #1
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Default Estimate OG after the fact?

So I have a batch fermenting that I forgot to grab an OG reading for, and I couldn't be bothered to open it back up and take the reading initially. However, when I pulled a sample today, I thought of something. I have both a refractometer and hydrometer, so I took a reading with both. The refractometer read about 7.1, and the hydrometer read 1.018. With the calculation to compensate for the presence of alcohol, and knowing what the actual gravity reading is, would it not make sense that I can then basically 'triangulate' to find the right OG to fit the equation? I estimated that my original Brix was about 11.5 or 11.6, meaning somewhere around 1.047 for my OG.

I used the morebeer refractometer spreadsheet tool to figure this out by just changing numbers until everything lined up.

Anyone have any reason to refute this estimate?

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Old 01-24-2013, 12:12 AM   #2
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Well, I've actually often wondered about that myself- it definitely makes sense. The refractometer conversion charts aren't perfect, so I'd expect that working backwards to get to OG from refraction and FG would be less accurate than going the other way round. Add to that the fact that brix measures sucrose in solution, and beer is mainly maltose and some other sugars (almost no sucrose), in a mixture that will be very different from beer to beer, and I'd say you'd be in the ballpark, but you could also be off by a LOT. What's the idea, apart from curiosity?

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Old 01-24-2013, 12:20 AM   #3
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I've been working on the math for that for a couple of days and then I saw Northern Brewer has it as part of their calculator. :-/

If the gravity is off by 0.001 then it really throws things off. Abv is off by about 0.5% and OG by 0.010 in a few test cases I ran.

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Old 01-24-2013, 12:26 AM   #4
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The main reason was just to figure out the alcohol content, really. I wouldn't plan on using it as a replacement for a normal reading. I am curious though, although brix measures sucrose, it does give a good useable ballpark of the other sugars, right? If it could indeed be off by a lot, wouldn't that make it kind of a moot point to take readings?

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Old 01-24-2013, 09:55 AM   #5
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Yes. Brix is percent sucrose by weight in solution. Most sugars have about the same density and index of refraction. Initial gravity, when measuring the sugar content by density (with a hydrometer) or by refraction index (with a refractometer) both of them are reasonable. Where it becomes a problem is when alcohol is involved. Neither measument will give you the percent sugar. Alcohols have a density about 75% of water and a index of refraction about 10% higher than water.

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Old 01-24-2013, 05:06 PM   #6
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Quote:
Originally Posted by Brewsit View Post
So I have a batch fermenting that I forgot to grab an OG reading for, and I couldn't be bothered to open it back up and take the reading initially. However, when I pulled a sample today, I thought of something. I have both a refractometer and hydrometer, so I took a reading with both. The refractometer read about 7.1, and the hydrometer read 1.018. With the calculation to compensate for the presence of alcohol, and knowing what the actual gravity reading is, would it not make sense that I can then basically 'triangulate' to find the right OG to fit the equation? I estimated that my original Brix was about 11.5 or 11.6, meaning somewhere around 1.047 for my OG.

I used the morebeer refractometer spreadsheet tool to figure this out by just changing numbers until everything lined up.

Anyone have any reason to refute this estimate?
Your estimate looks ok.

ProMash has a tool for this (inserted your #'s and pasted a screenshot below). I use a refractometer to get a good idea of the FG and also check when bottling with a hydrometer to get a better # (the differences have been +/-2 gravity points on average). I've also used the ProMash tool and it predicts the OG within a few points well but not as good as a true measurement.

I'd trust a calibrated refractometer (or temperature/calibration compenated hydrometer) for OG and a temperature/calibration compensated hydrometer reading for FG.

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