Water hardness - units and conversions
Water hardness is one of those parameters where the units cause the most confusion - the same result is given in several ways. It is worth knowing what hardness means and how to convert freely between units.
What hardness is
Hardness is the content of calcium (Ca²⁺) and magnesium (Mg²⁺) ions in water. It results mainly from the geology of the catchment (water from limestone areas is harder) - a good example of geochemical background. A distinction is made between total hardness, carbonate (temporary) and non-carbonate (permanent).
Units
- °dH - German degree (the most popular in Poland),
- mg CaCO₃/l (or ppm) - converted to calcium carbonate,
- mmol/l - molar hardness (the sum of Ca²⁺ and Mg²⁺),
- meq/l (mval/l) - in equivalent units.
Conversions
As a rough guide (rounded values):
- 1 mmol/l ≈ 100 mg CaCO₃/l ≈ 5.6 °dH ≈ 2 meq/l
- 1 °dH ≈ 17.8 mg CaCO₃/l ≈ 0.178 mmol/l ≈ 0.357 meq/l
- 1 meq/l ≈ 50 mg CaCO₃/l ≈ 2.8 °dH
Thanks to these conversions it is easy to bring results from different sources to one comparable scale.
How to read the result (as a guide)
In German degrees, water tends to be classified roughly as follows:
- soft - up to ~8 °dH,
- moderately hard - ~8-14 °dH,
- hard - ~14-21 °dH,
- very hard - above ~21 °dH.
(The boundaries are given differently in different sources - treat them as indicative.)
In practice
The most important thing is to stick to one unit in your monitoring and record it explicitly - otherwise mistakes in comparisons are easy (see exporting data to Excel). In LimnoLog you keep hardness as an indicator with a chosen unit, consistently throughout the series.
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