The amount of mineral and salt impurities in the water is called total dissolved solids (TDS). TDS shows how many units of impurities there are for one million units of water. One way to measure impurities in water is to measure the electric conductivity of water. Absolutely pure water is a poor conductor of electricity. Water shows significant conductivity when dissolved salts are present. Over most ranges, the amount of conductivity is directly proportional to the amount of salts dissolved in the water, or Salinity.
Four basic styles of Conductivity / TDS / Salinity meters are available for varying applications: Pens, Portable Meters, Data Loggers and Benchtop meters.