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Brewed a big Belgian beer last week. And using my refractometer (poorly, as it turns out), I wasn't able to get a solid reading of the OG.

Now a week later I've taken a sample of the beer and want to retroactively determine the OG using the current gravity, as determined by a hydrometer. Will this work?

My method was this:

  1. Get a hydrometer reading. In this case, 1.024.
  2. Take a refractometer reading. 11.1 brix.
  3. Using the MoreBeer refractometer correction worksheet enter the current refractometer reading on the first line.
  4. Enter various numbers for the "Starting Refractometer Reading" until the FG column for the 11.1 brix reading results in a gravity of 1.024.

Doing that gave me an OG of 1.079. This is much lower than I'd expect. According to the recipe and readings I was doing during brewing, my OG should be up in the 1.090s. That makes me think my method for retroactively determining OG is wrong.

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up vote 1 down vote accepted

Your method for determining OG isn't technically wrong, but I'm not sure how much error exists in the formulas in the spreadsheets. I just tested your method, plugging in post fermentation gravity, and then adjusting OG (OB) until the final gravity reading matched what I measured with the hydrometer.

For the specific recipe I used, it was one point away in it's calculation against the actual OG measured with a hydrometer. So I would say that for my recipe, the method worked, and was accurate to less than the standard error of the measurements (assuming 95% confidence interval). Is it possible that your efficiency was just lower than you'd expected, and that the reverse calculation was accurate for your recipe as well?

If anyone else has a post-fermentation gravity from both a hydrometer and an uncorrected refractometer, can you please try this and see if the OB in the spreadsheet matches your measured OG? I'll also open another bottle and test a second recipe tomorrow.

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