Refractometer

A refractometer measures how light bends passing through a small sample, which correlates with dissolved sugar content, and displays that as a Brix or gravity value on an internal scale. Because it only needs a few drops, brewers like it for checking wort during the boil or mash without wasting a large sample or waiting for a hydrometer to settle.

You use it by placing a drop of liquid on the prism, closing the cover plate, and looking through the eyepiece toward a light source to read where the shaded line crosses the scale. It works well for pre-fermentation readings of wort, but once alcohol is present the reading needs a correction, because alcohol bends light differently than sugar and a raw refractometer reading after fermentation will read falsely high.

When shopping, look for models with automatic temperature compensation so ambient temperature does not skew results, and a dual scale showing both Brix and specific gravity. The most common mistake is trusting an uncorrected reading for final gravity, which makes a beer look far sweeter than it is; use a proper alcohol correction formula or calculator, or fall back on a hydrometer once fermentation is underway.