Did you know that it's possible to obtain the value of the hardness of water, even in French Degrees using conductivity or TDS meters with good approximation. The main cause of hard water is the presence of dissolved calcium or magnesium ions.

The most common measurement unit of hardness is the French degree (^of), defined as:

1^of = 10ppm CaCO₃

Dividing the ppm measurement of the TDS by 10 gives the hardness value of water with and error of only 2-3^of.
Also the relationship between TDS and Conductivity is that 1ppm = 2uS/cm conductivity, thus

1^of = 20uS/cm

Dividing the microSiemens conductivity measurement by 20 gives the French degree hardness of water with a 2-3^of error.

Table of Water Hardness

TDS(ppm)	EC(uS/cm)	of	HARDNESS
0-70	0-140	0-7	Very Soft
70-150	140-300	7-15	Soft
150-250	300-500	15-25	Slightly Hard
250-320	500-640	25-32	Moderately Hard
320-420	640-840	32-42	Hard
above 420	above 840	above 42	Very Hard