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How Hard Water Effects a Swimming Pool

Water Hardness is nearly as important as pH and Total Alkalinity


After pH and Total Alkalinity,  water hardness is another very important aspect of swimming pool water quality.
It is normally measured in parts per million (PPM) but the French measure it in degrees f - but just multiply by 10 to convert degrees f to PPM

Water Hardness in France

How hard is the water at your French property? - Click on the link below to get some idea.

http://www.salines.com/index.php?cat=4&mot=16

But this does not give the full picture because the hardness can vary from town to town and even village to village. Your local water company should be able to give you the range of likely hardnesses for your property.

French levels 2 x that in the US


In the  Charente-Maritime and the Dordogne where the water is amongst the hardest in the world - the French classifications of soft to hard relate to roughly double the calcium content of the comparable US classes.
By now you are probably wondering why you should care how hard your swimming pool water is.

Limescale deposits


If you have a vinyl lined pool there is no need for the calcium hardness to be above 100 PPM. The higher it is and the hotter the water the more likely you are to get cloudy water and limescale deposits that can occur throughout the pump and filtration system and even on the vinyl lined pool walls.

This is particularly likely at calcium hardness levels are above 350 PPM and where the pH and TA are above their ideal levels - these are 7.2 and 120. High pH and a high TA lower the soluability of the calcium and other minerals, thus encouraging their deposition.


Please note that if you have deposits that are not pure white they may be the deposition of other minerals such as iron or copper.

Gunite or Tiled Pools and Water Hardness


If you have a gunnite or a tiled concrete pool the Calcium hardness should be maintained at about 200 PPM - otherwise the water will etch the walls until it reaches equilibrium with the calcium in the pool walls.
How do you get rid of scale deposits? - drop the pH value to below the optimum for a period - possibly as low as 6 until it has disappeared. You could also add some mineral treatment that will reduce the likehood of deposition of Calcium and other deposits.
How do you avoid their re-appearance? - rigorously control pH and TA and AVOID THE USE OF CALCIUM HYPOCHLORITE by using sodium hypochlorite.
Can you use a water softener? - not really - these cost an arm and a leg to run just for household water - the cost of softening pool water would be difficult to control and very expensive.
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