The pNa is an easy to use and inexpensive instrument for determining water hardness or softness. The hardness of water is due to the presence of magnesium and calcium. These make washing difficult, wastes soap, and creates 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. A common but inaccurate way to determine when to change resin is to estimate the volume of water that goes through the softener and guessing when to change the resin! Even though this may work in some cases, it fails in most since the sodium content of feed water is never constant. As a result, either the resin is regenerated too early and wastes resources, or too late, risking damage due to scaling. With the pNa, you can measure the sodium content of feed and exit water in seconds. When the resin is exhausted of sodium, there will be no exchange and the pNa will read the same value at the two ends. Only then should the resin be changed. Order Information: HI 98202 ( pNA) is supplied with protective cap, calibration screwdriver, batteries and instructions.