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.[1] Concentrated sulfuric acid with a specific gravity of 1.8 has a Twaddell 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. While the Baumé scale was adopted throughout England, the Twaddell scale was used in England and Scotland.[2]

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.

## 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

${\displaystyle x=1+(a\times 0.005)}$

${\displaystyle a=(x-1)\div 0.005}$[3]

## References

1. ^ 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
2. ^ Griffin, John Joseph (1873). Scientific Handicraft: A Descriptive, Illustrated, and Priced Catalogue of Apparatus, Suitable for the Performance of Elementary Experiments in Physics. Glasglow: J. J. Griffin and Sons. p. 77.
3. ^ Griffin, John Joseph (1877). Chemical Handicraft: A Classified and Descriptive Catalogue of Chemical Apparatus, Suitable for the Performance of Class Experiments, for Every Process of Chemical Research and for Chemical Testing in the Arts. Accompanied by Copious Notes, Explanatory of the Construction and Use of the Apparatus. Glasglow: J. J. Griffin and Sons. p. 51.