A hydrometer scale usually used for liquids denser than water, 19ᵗʰ – 20ᵗʰ century, mostly used in England, for example in the leather industry to check tanning solutions, and for sulfuric acid and milk. Abbr, °Tw.

To convert a Twaddell number to a scale in which the specific gravity of water = 1, multiply by 0.005, then add 1. For example, 20°Tw is equivalent to a specific gravity of 1.100. To convert specific gravity to a Twaddell number:

sources

In the Twadell hydrometer, which was used in this country [the United Kingdom] long before the introduction of Fleischer's densimeter, and which is still in general use for technical work, the degrees are half of the above value. Each degree corresponds to 0005 sp. gr., prefixed by unity. Thus, 7° Tw. = 1.035; 20° Tw.= 1.100; 100° Tw.= 1.500 sp. gr., etc. No table is therefore required for the conversion of degrees Twaddell into ordinary specific gravities, and the value of a degree Twaddell is quite definite. It is remarkable that this rational and practical hydrometer, the scale of which is usually distributed over six spindles, should be almost universally used in this country, which is so very conservative and unpractical with regard to weights and measures, whilst the continental nations, which have all adopted the metric system of weights and measures (with the exception of Russia), have not yet adopted a rational hydrometer.

George Lunge, editor of the German edition. Technical Methods of Chemical Analysis.
Charles Alexander Keane, editor of the English translation.
New York: D. Van Nostrand, 1908.