The Twaddell Hydrometer Scale
A Scottish scale for liquids heavier than water.
Origin
Devised by William Twaddell, a Glasgow instrument maker, in the early nineteenth century for the Scottish chemical industry.
The catch
The whole scale exists to avoid decimals. Specific gravities of industrial liquids cluster between 1.000 and 2.000, and Twaddell simply multiplied the decimal part by 200, so a works chemist could call out whole numbers across a noisy factory floor.
Specific gravity = 1 + (degrees Twaddell / 200)Enter a reading in degrees Twaddell to convert to specific gravity. The scale runs from 0 (pure water) to 200 (twice the density of water).
Worked examples
Twaddell served the bleaching, dyeing and heavy chemical works that clustered around Glasgow in the nineteenth century, where alkali strength and acid concentration had to be checked constantly. It sits alongside the French Baumé scale, which covers both heavier and lighter liquids but is less convenient arithmetically. British works chemists generally used Twaddell for anything denser than water and Baumé for anything lighter.