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CAN YOU DETERMINE WATER HARDNESS FROM CONDUCTIVITY OR TOTAL DISSOLVED SOLIDS MEASUREMENTS?
     General water hardness is related to the dissolved minerals in the water.  General hardness is a misleading term that is often confused with carbonate hardness or temporary hardness, which is actually related to alkalinity and relates to the "buffering capacity" of the water (its ability to resist pH changes).  This means that if the carbonate hardness is high then the pH will be extremely stable or alternatively if the carbonate hardness is low the pH of the water will be able to fluctuate easily.  The term general hardness should be replaced with a simpler term: Hardness.
     Water hardness is the measurement of the amount of ions which have lost two electrons (divalent cations) dissolved in the tested water and is therefore, related to total dissolved solids.  The more divalent cations dissolved in the water the "harder" the water.  Generally the most common divalent cations are calcium and magnesium, however other divalent cations may contribute including iron, strontium, aluminum, and manganese.  Typically the other divalent cations contribute little to no appreciable additions to the water hardness measurement.  A stream or river's hardness reflects the geology of the catchment's area and sometimes provides a measure of the influence of human activity in a watershed.  For example, sites that have active or abandoned mines nearby often have higher concentrations of iron ions in the water resulting in a very high hardness degree.
     Water hardness can be expressed in many different units including French degrees, German degrees, Clark degree, grains per gallon, mg/L CaCO3 (calcium carbonate), and ppm (parts per million).  General conversions are below:
1 ppm = 1 mg/L CaCO3
1 ppm = 0.058 grains/US gallon
1 ppm = 0.07 Clark degrees
1 ppm = 0.10 French degrees
1 ppm = 0.056 German degrees
1 French degree = 1 hydrotimetric degree
1 Clark degree = 1 grain / Imperial gallon as calcium carbonate
1 French degree = 1 part / 100,000 calcium carbonate
1 German degree = 1 part / 100,000 calcium oxide
1 grain/US gallon = 17.1 ppm
1 grain/US gallon = 1.20 Clark degrees
1 grain/US gallon = 1.71 French degrees
1 grain/US gallon = 0.958 German degrees
     Total dissolved solids (TDS) refers to a measure of all inorganic solids dissolved in the water.  This means that it will measure ions that contribute to water hardness, tike calcium, but also those that do not, like sodium.  The TDS measurement is a better reflection of the total mineral content of the water rather than a water hardness measurement.  However, for estimation purposes, the water hardness can be roughly calculated by dividing the ppm (parts per million) measurement of the TDS by 10 giving a hardness value with an error of only 2-3 French degrees.  TDS measurements can also be derived from relative conductivity measurement.
     Conductivity is similar to TDS measurements.  Conductivity is a measure of the ability of a substance to conduct electric current.  Conductivity measurements offer a rapid and non-destructive way to measure ion content in the sample.  The conductivity measurement is made with an electronic sensor or meter in micro/milli-Siemens per centimeter or ppm.  Conductivity increases with increasing ion content, which means that in most cases it gives a good approximation of the TDS measurement using the conversion factor of 1 ppm = 2 uS/cm.  Conductivity is temperature sensitive and is typically standardized to 25°C.  While conductivity is a convenient way to get an approximation of the hardness of water it does have the drawback of combining all ions in the measurement, including those that do not contribute to the water's hardness.  This hardness approximation gives an error similar to the TDS measurement of 2-3 French degrees of hardness.
     Can you measure water hardness with a conductivity sensor or TDS sensor?  Yes, however it depends on the accuracy that you want to have in your measurement.  In general the following table describes the water hardness as measured by a TDS, conductivity, or hardness measurement.
TDS(ppm) Conductivity(uS/cm) °f 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
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