What exactly is conductivity?

Conductivity is a measure of the ability of a solution to conduct electric current. It is expressed as microsiemens (micro-Siemens per centimeter or µS/cm) or, in higher conductivity levels, millisiemens. It is also the reciprocal of resistivity.

Some industries, such as water treatment, have adopted a measurement known as TDS (Total Dissolved Solids). The TDS value is approximated to conductivity using a multiple factor and is expressed in parts per million (ppm).