Conductivity Theory and Measurement


Courtesy of IC Controls Ltd.

Electrical conductivity is a measure of the ability of a solution to cany* a current. Current flow in liquids differs from mat in metal conductors in that electrons cannot flow freely, but must be carried by ions. Ions are formed when a solid such as salt is dissolved in a liquid to form electrical components having opposite electrical charges. For example, sodium chloride separates to form Na' and CI* ions. All ions present in the solutions contribute to the current flowing through the sensor and therefore, contribute to the conductivity measurement. Electrical conductivity can therefore be used as a measure of the concentration of ionizable solutes present in the sample.

Conductivity Units
Electrical resistivity uses the unit of ohm meter or ilm. Electrical conductivity is the reciprocal of electrical resistivity. Rather than use the units Q 'm', in 1971 the unit 'Siemens' (symbolized by the capital letter S) was adopted by the General Conference on Weights and Measures as an SI derived unit. The unit for electrical conductivity becomes Siemens per meter. The Siemens unit is named after Werner von Siemens, the 19* century German inventor and entrepreneur in the area of electrical engineering.
North American practice continues to see me use of unit mho cm to measure conductivity. where the unit 'mho' is a reciprocal ohm The word 'mho' is the word 'ohm' spelled backwards. Because of the history of conductivity measurements in micromho cm and uullmiho cm. it is common to see these measurements translated to uiicrosiemens'cm and niilhsieniens/cni because there is a one-to-one correspondence between these units.

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