Everyone knows water conducts electricity. What makes the water a good conductor is
not the water itself, but the positive and negative ions dissolved in it... ie: Sodium,
Calcium, Chloride, Carbonate, etc. The more ions there are dissolved in water, the more
conductive it becomes. Ultrapure water is a not a good electrical conductor as evidenced
by the 18 megohm-cm resistivity (.055µS/cm conductivity). Conductivity and Resistivity
are simply inverses of each other. Resistivity measurement is used exclusively for high
purity water applications, while conductivity measurement is employed for the entire
spectrum of water quality monitoring.

Temperature significantly effects the conductivity/resistivity of water requiring
instrumentation to be temperature compensated to 25°C.     

Water begins to become very electrically conductive with just a few ppm of dissolved
solids/ions. Water supplies vary substantially in the types and amounts of dissolved
solids present. The overall conductivity of the water is the cumulative effect of the
blend of ions. Many water sources have similar conductivity to Total Dissolved Solids (TDS)
correlations allowing for conductivity to be used as an accurate, real-time indicator. The
relationship of conductivity and TDS is non-linear being both ionic concentration and
temperature dependent. For typical “fresh” water supplies this conversion curve varies from
approximately .6 at the lower conductivities, to as much as .8 at the high end. Conductivity
instruments can be calibrated to display conductivity (microSiemens and milliSiemens)
or an estimated TDS (ppm).