The pNa is an easy-to-use and inexpensive instrument for determining water hardness/softness. The hardness of water is due to the presence of magnesium and calcium. These make washing difficult, waste soap and create unpleasant scum and scale deposits. With a zeolite system, the calcium and magnesium ions are substituted on a one-to-one basis with sodium ions from a conditioning resin. Once all the sodium ions are exhausted, the resin has to be regenerated. This is currently determined by estimating the volume of water that goes through the softener and guessing when to change the resin!