Conductivity

Electrical Conductivity is the ability of a solution to transfer (conduct) electric current.

It is the reciprocal of electrical resistivity (ohms). Therefore conductivity is used to measure the concentration of dissolved solids which have been ionized in a polar solution such as water.

The unit of measurement commonly used is one millionth of a Siemen per centimeter (micro-Siemens per centimeter or µS/cm).

When measuring more concentrated solutions, the units are expressed as milli-Siemens/cm (mS/cm) i.e.- 10-3 S-cm (thousandths of a Siemen).

For ease of expression, 1000 µS/cm are equal to 1 mS/cm. Often times conductivity is simply expressed as either micro or milli Siemens.

However this unit of measurement is sometimes (incorrectly) referred to as micro-mho's rather than micro-Siemens. The expression "mho" was simply the word ohm spelled backwards. Conductivity measurement unit of expression into whole numbers.

TDS is really a gravimetric measurement, because in solution the solids are predominately present in ionic form, they can be approximated with conductivity.

The TDS scale uses 2 µS/cm = 1 ppm (part per million as CaCO3). It is also expressed as 1 mg/l TDS.

Temperature plays a major role in conductivity. Because ionic activity increases with increasing temperature, conductivity measurements are referenced to 25ºC. The coefficient used to correct for changes in temperature, β is expressed as a percentage per degree Celsius. For most applications, beta has a value of two. In order to establish the true value of beta a solution is measured at the elevated temperature (without temperature compensation). Then the solution is cooled and re-measured. β can then be exactly calculated for that particular solution.