Electrical conductivity is an indicator of water quality.  Conductivity data can determine concentration of solutions, detect contaminants and determine the purity of water. YSI conductivity sensor measures conductivity by AC voltage applied to nickel electrodes. These electrodes are placed in a water sample (or other liquid), where the current flows through the electrodes and the sample. Current level has a direct relationship with the conductivity of the solution.

Conductivity is the ability of a material to conduct electrical current. The principle by which instruments measure conductivity is simple - plates/wires are placed in the sample, a potential is applied across them (normally a sine wave voltage), and the current is measured. Conductivity, the inverse of resistivity is determined from the voltage and current values according to Ohm's law.

Since the charge on ions in solution facilitates the conductance of electrical current, the conductivity of a solution is proportional to its ion concentration. In some situations, however, conductivity may not correlate directly to concentration. Ionic concentrations can alter the linear relationship between conductivity and concentration in some highly concentrated solutions.

The basic unit of conductivity is the siemens (S), sometimes referred to as mho. Since cell geometry affects conductivity values, standardized measurements are expressed in specific conductivity units (S/cm) to compensate for variations in electrode dimensions.

Conductivity measurements, along with temperature, also allow for salinity values to be calculated through algorithms.