Originally Posted by Math0
You are saying that using a refractometer to get FG is bad? So what's the point of buying one if I cannot use it all the time? I calibrate it every time with water before and I assume a margin of error of about 0.002.
It's useful for things like checking to see if the beer is done fermenting, or to see if your sparge run-off gravity has gone below a particular level (at which point you might start extracting tannins), and of course for OG. You can use a refractometer alcohol correction formula (Google "refractometer alcohol correction".), to estimate your true FG, but it's more accurate to use a hydrometer. The presence of alcohol raises the refractive index of the solution, counteracting (somewhat) the drop you see from the yeast eating sugars in solution.