Don’t trust your refractometer blindly

I just learned that it is always a good idea to check the correction factor that applies to your refractometer. A commonly accepted correction factor for converting a refractometer’s Brix reading to a hydrometer Plato reading is 1.04. I was always under the impression that the Brix reading has to be multiplied with this value, which works for me, but Sean Terril pointed out to me that the commonly accepted formula divides the Brix value by this correction factor to covert to a Plato reading. (Refractometer Calculator)

Since my approach was working for me I had to check what’s going on here. I thus tested water, a 20 Plato sugar solution and a 20 Plato wort with both the hydrometer and my refractometer. All solutions and the refractometer had the same temperature.

hydrometer in water This is the hydrometer in water. I have a table that I use to correct the hydrometer reading in Plato for temperature and the slight offset that the hydrometer has
water on refractometer water measured with the refractometer
hydrometer in 20.4 Plato sugar water The sugar water (~40g table sugar, 160g water) read after correction 20.4 Plato
hydrometer in 20.4 Plato wort The wort read after correction 20.4 Plato as well
20.4 Plato sugar water on refractometer In the refractometer the sugar water read 19.4 Brix. Since refractometers are calibrated for sugar water, it should have read 20.4 Brix
20.4 Plato wort on refractometer In the refractometer the wort reads 19.5-19.6 Brix (remember that water read slightly under the zero-line). To get the hydrometer reading I have to multiply this reading with 1.04

The reason why my correction factor is different is that the scale on my refractometer is off. I.e. there is already a conversion factor for sugar water that is not 1. But since I checked my refractometer against my hydrometer at various wort gravities and found that Plato is Brix * 1.04 I can still use this refractometer in brewing.

BTW, the ATC of this refractometer is also broken. For every brewing session I have to re-calibrate it with water. Since this is done very quickly, it doesn’t bother me too much.


If you buy cheap gear, check its calibration.

8 thoughts on “Don’t trust your refractometer blindly

  1. I had the exact same thing on my refractometer which confused me for a while until I tracked enough readings to be sure of it. Regarding the ATC, though, it’s not about keeping the calibration on the zero point. If your ATC wasn’t working or you used the refactometer outside it’s range you would just end up with bogus readings regardless of the zero point.

    • Dave,

      If this type of error in the refractometer readings is more common than I thought, I wonder to what extent it is the reason that the refractometer correction formulas for measurements on beer are not working for some brewers.

      As for the ATC. From what I have found out, the ATC mechanism shifts the scale up and down based on the temperature of the unit via a bi-metal strip. Hence simply calibrating the zero point should do the same thing. I think that’s what you would have to do on non ATC models.

  2. I see the same problem on my new refractometer with ATC – at 20C 14.6Brix while the hydrometer shows ~15.2P; 13.0Brix and 13.5P at hydrometr.
    Refractometer has a manual calibration and is calibrated to exact 0Brix at 20C on distilled water; there’s no need to calibrate it every time.
    ATC is not broken, it seems to correct the measurement to wrong direction :) But the temperature of one drop of the wort could change very quickly, so it’s more reliable to let this drop to get cold.

    Probably it’s a common problem with cheap chinese refractometers.

    • Jason,

      I took a look at the write-up of your experiment and those are interesting results. Given the low concentration of hop oils and resins, I’m skeptical that they have the effect that you have shown. It could be that more evaporation occurred when you added the hops. You can easily compensate fort that by letting all the experiments cool and weigh them. Then add just enough water to compensate for evaporation. The scales I use might not be accurate enough for this, but I may give this a try.

      But thanks for bringing this up.

  3. Did you compare the final gravity by your refractometer and the hydrometer?
    As for me, the refractometer reeding after the fermentation is completed seems to be lower then correct value, but the difference is less then [reading*0.08] (as works for OG).

    And it’s unclear how this FG reeding can be corrected.

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