Refractometer calculator variations

duder · August 3, 2012, 5:39pm

So, what are all of you using for an online Refratometer Calculator?

NB has one, Onebeer.net has one, ProMash, Sean T.'s …

They all seem to provide a wide range of results. Who to believe? Ball park it?

The difference on one of my last brews was Sean T.'s (OG) = 1.074 & NB (OG) = 1.078. Brix 18.8 entered on both.

Kind of a big difference! Especially when trying to figure efficiency.

I have been using Sean T.'s, for the most part, and I’m also wondering what everyone seems to be using as far as Old Cubic/New Cubic/New Linear.

Thanks for your thoughts. :?

rustyhoover · August 3, 2012, 6:44pm

I might be wrong, but for OG calculations, I think the only thing that Sean’s does that others may not is to use a wort correction factor. Just for sanity-check, you can set the wort correction factor to 1.00 and it will probably give you the same answer as other calculators.

This is only for sanity-checking, though, because for calculations that need to be accurate, wort correction is a must.

Now, on FG and any other calcs after fermentation has started, the differences are much greater and more complex…

shadetree · August 3, 2012, 6:53pm

The wort correction factor, usually 1.04, is used to adjust the reading for wort versus the sugar-water solution that the refractometer is designed to measure, so don’t mess with that. Sean’s calculator is based on real-world results with post-pitch beer and he made a new model that is pretty darn accurate over a range of readings - I trust it more than any of the calculators using the “standard” formula.

ynotbrusum · August 4, 2012, 12:45am

Sean’s spreadsheet is highly reliable for beers that are not ultra huge or which are finishing ultra low, according to his statements. Highly accurate for most beers!

nyakavt · August 6, 2012, 4:21pm

If you’re talking about pre-fermentation OG reading, the calculators say different things because they are using a different polynomial to curve-fit the brix to SG tables. The ‘official’ polynomial is the ASBC polynomial, what promash uses, which AJDelange posted on the BN forums a couple years ago:

°P = -616.868 + 1111.14SG - 630.272SGSG + 135.997SGSGSG

Putting this into the excel solver says 1.07765 for 18.8°P. The refractometer correction factor is applied to this, so divide the right side of the decimal by your correction factor (1.02-1.04) to get the actual wort S.G., which should be most comparable to the hydrometer.