If you're on a municipal water system, call your water district and request a copy of their most recent analysis. These instruments are one of the most important tools for a -grower to have and use regularly. All of these instruments work in essentially the same way. They measure the electrical conductivity of the water. It is the dissolved salts in most water that allows it to conduct electricity.
Pure water is a poor conductor since there are none of the conductive salts found in impure water.
It is not uncommon to find high levels of salts in well water or municipal water supplies. Calcium and Magnesium carbonates are among the most common ingredients in tap water and in well water. Since calcium and magnesium are important plant nutrients. However, even a good thing can become a problem if the levels are too high. Generally, a calcium content of more than 200 PPM, or 75 PPM for magnesium, are on the verge of excessive for most hydroponic applications.