# What do the three hardness levels mean



## BarryH (30 Mar 2020)

Just been checking the hardness of my water (Severn Trent in Derbyshire) and see there are three listings for hardness, Clark, French and German.

Anyone help with the reason we have three different parameters listed?


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## dw1305 (30 Mar 2020)

Hi all,
<"They are just different units">.

The English one is "Clark", but we usually use the German unit. Lenntech offer <"a convertor">.

One dH is "One degree German" and defined as 10 milligrams of calcium oxide (CaO) per litre of water. This is equivalent to 17.85 milligrams of calcium carbonate per litre of water, or 17.85 ppm.

These are from the very useful <"Krib Water hardness article">

It is the same amount of CaCO3 for both dKH or dGH.

cheers Darrel


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## BarryH (30 Mar 2020)

dw1305 said:


> Hi all,
> <"They are just different units">.
> 
> The English one is "Clark", but we usually use the German unit. Lenntech offer <"a convertor">.
> ...


Thanks Darrel. Severn Trent's reading for my area is 6.80 German which more or less agrees with the reading I get out of tap.


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## sparkyweasel (30 Mar 2020)

We use German degrees because the first readily-available, easy-to-use test kits aimed at aquarists were made in Germany. Water companies give that figure mainly for our benefit.
Plumbers and washing machine engineers use Clark (=English) degrees because they always have, it's for them that the water companies give that figure.
I don't know who uses the French degrees, - apart from the French I suppose.


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