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.
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.