CLASSIFICATION OF WATER HARDNESS

The amount of hardness is sometimes expressed by descriptive words. The following table shows how these descriptions may be related to the numerical value of the hardness in common units.

Description	Hardness in mg/l as calcium	Hardness in mg/l as calcium carbonate
Soft Moderately soft Slightly hard Moderately hard Hard Very hard	0 – 20 20 – 40 40 – 60 60 – 80 80 – 120 Over 120	0 – 50 50 – 100 100 – 150 150 – 200 200 – 300 Over 300

(mg/l = milligrammes per litre = 1 part per million)

CONVERSION OF WATER HARDNESS UNITS
Water Hardness Unit  Difinition	International recommended mmol/litre	Physical measures mval/litre	America & states PPM	English oe	French of	German odH
	100mg CaCO3 per 1000 ml water	28 mg CaO or 50 mg CaCO3 per 1000 ml water	1 part CaCO3 per million = 1 mg CaCO3 per 1000 ml water	1 grainCaCO3 per gallon= 14,3 mg CaCO3 per 1000 ml water	10 mg/ CaCO3 per 1000 ml water	10 mg/CaO per 1000 ml water
1 mmol/litre 1 mval/litre 1 PPM 1 Oe 1 Of 1 OdH	1 0,5 0,01 0,1429 0,10 0,1786	2 1 0,02 0,285 0,20 0,357	100 50 1 14,29 10,00 17,86	7,0 3,5 0,070 1 0,700 1,250	10,00 5,00 0,10 1,429 1 1,786	5,6 2,8 0,056 0,7999 0,5599 1

WATER H₂O HARDNESS

TEMPORARY HARDNESS: Ca AND Mg BICARBONATES (precipitate minerals upon heating)

PERMANENT HARDNESS: Ca AND Mg SULFATES OR CHLORIDES (dissolve with sodium)

DEGREE OF HARDNESS EXPRESION. PPM Calcium (US norm), ^odH (European norm*)

SOFT: LESS THAN 1,6 mmol/l = 160 PPM = 9 ^odH

SLIGHTLY HARD: 1,6-3,2 mmol/l = 160-320 PPM = 9-18 ^odH

HARD: 3,2-4,6 mmol/l = 320-460 PPM = 18-26 ^odH

VERY HARD: ABOVE 4,6 mmol/l = ABOVE 460 PPM = ABOVE 26 ^odH

1 ^odH = 10 mg/CaO = 17,8 mg/l CaCO₃ = 0,18 mmol/l earth alkaline ions

(* 1 ^odH = 17,86 PPM American hardness = 1,25 ^o English hardness = 1,78 ^o French hardness)