The Twaddell scale is a hydrometer scale used for measuring the specific gravity of liquids relative to water. On this scale, a specific gravity of 1.000 is reported as 0, and a specific gravity of 2.000 is reported as 200. For example, concentrated sulfuric acid with a specific gravity of 1.8 has a Twaddell scale measurement of 160, reflecting the linear relationship between readings and specific gravity. The Twaddell scale is used exclusively for liquids with specific gravity greater than that of water.