A Brix Refractometer that includes a specific gravity scale. You won’t have to do conversions between Brix and gravity when measuring the Original Gravity of musts or wort. Measuring final gravity accurately will still take some calculations, as the presence of alcohol does affect the reading.
Best of all, it only requires a few drops of liquid for an accurate reading. No need to lose a whole hydrometer test jar of beer or wine in the process.