The Twaddell scale is a hydrometer scale for reporting the measured specific gravity of a liquid relative to water. On this scale, a specific gravity of 1.000 is reported as 0, and 2.000 reports as 200. Concentrated sulfuric acid with a specific gravity of 1.8 has a Twadell scale measurement of 160 (as per the linear relationship between readings and sp. gravity). The Twaddell scale is only used for liquids with specific gravity greater than water. The scale was used in the British dye and bleach manufacturing industries.
The scale is named after the scientific instrument manufacturer W. Twaddell of Glasgow, who first developed hydrometers on this scale at the start of the 19th century.
- C.R.A. Wright, On the valuation of solutions for technical purposes by means of the hydrometer in Practical magazine, Volume 1 Simpkin, Marshall & Co., 1873 page 239