A refractometer is a handheld optical tool used to test engine coolant and battery fluid. It works by measuring how light bends (refracts) as it passes through a liquid sample. To use it, you place a few drops of fluid on the glass prism, close the cover plate, and look through the eyepiece while pointing the tool toward a light source. A scale inside shows you the exact freeze protection point of glycol mixtures or the specific gravity of battery acid.
Standard hydrometers use floating plastic needles, but they are easily throw off by air bubbles or temperature differences. The refractometer is far more accurate and requires only a tiny fluid sample. Most models feature automatic temperature compensation, meaning they adjust for the liquid's temperature automatically. This ensures you get a correct reading whether the coolant is warm from the engine or cold from storage.