The unit of Water Hardness measurements is usually ppm (milligrams/liter) Calcium Carbonate (CaC03) and a normal working range is 0.1 – 10 ppm. The most common method of softening is by ion exchange using resin beds (traditionally referred to as Sodium Zeolite). The Sodium ion attached to the resin is exchanged for the Calcium and Magnesium ions. Another method is “precipitation” (lime), but this is gradually being replaced by ion exchange. The same equipment will work for both methods; but please check if the application is a hot lime process.
Magnesium is more soluble than Calcium and so it is more readily replaced by Sodium. This means the first indication that the softener train is failing is the increase of Calcium in the effluent. The measurement / control packages are used to detect this “breakthrough” of Calcium, indicating the need for the bed to be regenerated and a new bed to be switched in.
Sodium Zeolite does not reduce the alkalinity of the softened water; in some installations a Hydrogen Zeolite softener is run in parallel to the Sodium and the resulting effluents blended to give the desired pH (this provides an opportunity to provide pH analysis). The softener regeneration requires acids or bases (depending on the exchange method) requiring Conductivity measurement, as % acid or % base.