Review on Brix Refractometer with ATC - Dual Scale for Wine Making, Beer Brewing, and Homebrew Kit: Specific Gravity & Brix Hydrometer by Victoria Brown

Using the formula from the brewery I found that the calculated specific gravity was within 3 points or so of the hydrometer reading. So it's not entirely accurate, but it's much more convenient - a sample is a few drops, not a full bottle. It is very easy to follow the progress of the brew and to indicate the end of fermentation. I would still use a hydrometer before and after fermentation to calculate ABV and sometimes in between to check it. For some reason the dividing line of the cherry mead in the eyepiece is not as sharp as with the calibration water, but it still works.