In this post by Jack Dugan of Jenco Instruments, we provide background information on conductivity, resistivity, TDS and salinity.

Background

Conductivity, resistivity, totally dissolved solids (TDS) and salinity are all measurements of the alternating electrical current (AC) of liquid.  These parameters are measured by analytical instruments commonly called electrical conductivity meters, EC meters, or just conductivity meters.  Conductivity meters measure the alternating electrical current conducted between two or four electrical conducting plates of a conductivity electrode and convert the resulting conductivity reading into resistivity, TDS and salinity.  Some conductivity meters display only a single variable while others are combination meters and may display in several formats.

Conductivity

Conductivity is a gross, indirect measurement of the concentration of ions present in a liquid solution.  The unit of measure most commonly used for conductivity is the Siemen. Micro-Siemens, uS/cm, (10-6 ) is Micro-Siemens per cubic centimeter.  Milli-Siemens, mS/cm, (10-3 ) is Milli-Siemens per cubic centimeter and for highly conductive solutions there is the Siemen.  Conductivity is measured without regard to temperature.  Compensated Conductivity is measured at 25° Celsius or some other selected reference temperature.  Conductivity = 1/Resistivity.

Resistivity

Resistivity’s unit is the Ohm. Resistivity is the inverse of conductivity.  Resistivity = 1/Conductivity.The normal unit of Resistivity is the mega ohm, abbreviated M? = 1,000,000 ?.1uS = 1u ohm and 1mS =1m ohm.  Resistivity is an older unit of measure generally superseded by the Siemen but is often used for measuring deionized water especially for compliance with the regulations of governmental agencies.

TDS

TDS or totally dissolved solids is a measurement of the total dry mass of dissolved solids in water. TDS is measured by grams per liter, g/L and is based on compensated conductivity where a temperature coefficient is selected and a TDS calculated coefficient is selected based on the types of salts in the solution.

Salinity

Salinity is a measurement of dissolved salt and is measured in ppt, parts per thousand or ppm, parts per million.  Salinity is based on the conductivity and the temperature.  In general ppm times a factor of two equals the conductivity for common salts like NaCL.  Example: 1000 ppm = 2000 uS/cm.