The trouble is: evaporative effects. Regardless of what many idiots will tell you about Automatic Temperature Correction (ATC) and the like, the fact is, a refractometer does not read accurately with a steaming hot sample. Even when you leave a hot sample to cool off a little bit, it is cooling by evaporation, which increases the sugar concentration, causing further inaccuracy and unpredictability, since sugar does not evaporate while the water vapor comes off. You could TRY to get accurate readings from hot samples by keeping cups cold in your refrigerator and drawing a tiny sample into a cold cup so that it is instantly cooled close to room temperature. But if this isn't done consistently, measurement might still be a crapshoot. I mean..... I'm a scientist, and I've tried to do things like this to make it work, but... real-life results have been surprisingly inconsistent to the point where I no longer bother trying to measure samples that started hot because.... it ain't easy to do it right.
