A unit expressing the hardness of water, primarily used in Great Britain, equal to 1 grain (64.8 milligrams) of calcium carbonate per imperial gallon, equivalent to about 1 part per 70,000 parts of water. Symbol, °Clark. These are sometimes called English degrees (°E), to distinguish them from the German (deutsche häte, °dH) and French degrees (°f).
The unit was named for Hosiah Clark (-1898).
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Last revised: 10 February 2009.