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Hardness of water

“Water hardness" is the amount of calcium and magnesium ions. This hardness, or hydrometric title (H.T.), is expressed in “French degrees” (°fH):

1 °fH = 4 mg/L (Ca2+) = 2.4 mg/L (Mg2+)
Water hardness scale
Information pictogram
Picture of a box of test strips with different values on the right. From 0 to 6°F: very soft water, from 7 to 13°F: soft water, from 14 to 27°F: medium hard water, from 28 to 39°F: hard water, over 40°F: very hard water

Water hardness can be determined by using test strips.
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