Twaddell scale

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.^[1] 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.

This scale was historically employed in the British dye and bleach manufacturing industries. While the Baumé scale was widely adopted across England, the Twaddell scale remained in use in both England and Scotland.^[2]

The scale is named after William Twaddell, a scientific instrument manufacturer from Glasgow, who first developed hydrometers using this scale at the beginning of the 19th century.

Converting between Twaddell scale and specific gravity

let a = any degree of Twaddell's Hydrometer, x = specific gravity in relation to water taken at 1.000

${\displaystyle x=0.005a+1}$

${\displaystyle a={\frac {x-1}{0.005}}}$^[3]