Technical Conductivity and Resistivity
Conductivity measures the ability of a solution to conduct an electric current between two electrodes. In solution, the current flows by ion transport. Therefore, with an increasing amount of ions present in the liquid, the liquid will have a higher conductivity. If the number of ions in the liquid is very small, the solution will be "resistive" to current flow. AC current is used to prevent complete ion migration to the two electrodes.
Conductance = 1/Resistance Conductivity: mho = Siemen Normal unit of measurement is: 1 micromho (╡mho) = 1 microSiemen (╡S), 1 millimho (mmho) = 1 milliSiemens (mS) = 1,000 ╡S Resistivity: ohm Normal unit of measurement is: megohm = 1,000,000 ohm Example - Conductivity to Resistivity Conversion: Conductivity = 20 ╡S = 20 x 10-6 S = 2 x 10-5 S = 2 x 10-5 mho Resistivity = 1/conductivity 2 x 10-5 S = 1/2 x 10-5 ohm = 0.5 x 10-5 ohm = 5 x 10-4 ohmConductivity and Resistivity (NaCl and CaCO3 Solutions at 25°C)
Probe Constants
Probe constant defines the volume between the electrodes. Solutions with an extremely high conductivity require a sensor with a probe constant greater than 1.0. Solutions with extremely low conductivity require a sensor with a probe constant less than 1.0. The greater the distance between the electrodes, the smaller the current signal.
