Using your hydrometer at 81 will cause it to read .001 or .002 below the actual value depending on your calibration temp. Warmer solutions are slightly less dense.

Did you test the hydrometer in plain water to see if it is accurate? Hydrometers can also have a significant amount of error in some cases and you can compensate for it if you know it.

You might look in the manual of the device to see what kind of formula they are using for the Brix to SG conversion, but it is apparently not accurate at higher gravities. There is a old rule of thumb that gravity points = 4 X Brix, which actually works ok for gravity that you may find in beer brewing, so that if you have a batch with 10 Brix, the SG would be around 1.040. Such a simplified formula become more and more inaccurate as the gravity becomes higher. A calculator like Vinocalc

can give you better results. You can also program a formula into a spreadsheet to use.

Vinocalc uses the following:

brix = 143.254 * sg3 - 648.670 * sg2 + 1125.805 * sg - 620.389

sg = 0.00000005785037196 * brix3 + 0.00001261831344 * brix2 + 0.003873042366 * brix + 0.9999994636

One more thing to remember about refractometers is that they will give you inaccurate readings once fermentation starts as the refractive index of alcohol is higher than that of a sugar/water solution. With a finished batch, you may get a hydrometer reading of 1.000 while the refractometer will still be reading 10 Brix. Vinocalc and other calculators have some formulas to make a correction for this, though they could be refined a bit more. You’ll want to use those formulas or you’ll be thinking a batch is stuck when it is not. At the end of fermentation I always like to get a gravity reading rather than trusting the refractometer number.

I hope that helps.

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