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-   -   Estimate OG after the fact? (http://www.homebrewtalk.com/f14/estimate-og-after-fact-384055/)

DSmith 01-24-2013 05:06 PM

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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.

Attachment 96338

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