ISPE Glossary of Pharmaceutical and Biotechnology Terminology

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The reciprocal of conductivity (R=1/C). A measure of specific resistance to the flow of electricity. In water, provides an easy mean of continuously measuring the purity of very low Total Dissolved Solids (TDS), or ionic concentration. The fewer the dissolved ions in water, the higher its resistivity. Resistivity is normally expressed in Megohm-cm (Mohm-cm) and is equivalent to one million ohms of resistance measured between two electrodes one centimeter apart. The theoretical maximum ionic purity of water is 18.3 Megohm-cm at 25°C.


The measure of the resistance to the flow of electrical current through high purity water. This is measured in millions of ohms-cm or Megohm-cm (Mohm-cm). Resistivity is the reciprocal of Conductivity (R=1/C, 1 Mohm-cm = 1 µS/cm). This provides an easy means of continuously measuring the purity of very low TDS water or ionic concentration.

Publication Source: ISPE Baseline® Guide, Vol. 4: Water and Steam Systems (Second Edition)

Publication Date: 2011

See also: Absolute Purity Water

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