That is a great question. The “simple” answer is that no, hydrometry and refractometry are not interchangeable and that you shouldn’t try to use a refractometer during active fermentation. Refractometry relies on measuring how a ray of light will bend (refract) through liquids of different densities to be read out on a scale. Hydrometry relies directly on density of a solution itself and accounts for a density change as the sugar disappears and alcohol (alcohol is less dense than water) is generated. When I was a winemaking student at UC-Davis we were taught that refractometers are best used for unfermented juice as an initial Brix reading and that once fermentation started, hydrometers were the best tool to monitor fermentations. This is because alcohol and carbon dioxide bubbles will both interfere with how the ray of light passes through the refractometer and are difficult to control for. However, as you bring up, there should be a way to calculate the effect of the alcohol to arrive at a “corrected” sugar level. Some people have attempted to do so. There is an
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