Resistivity Measurement
The resistivity of ionic liquids varies tremendously by the concentration – while distilled water is almost an insulator, salt water is a very efficient electrical conductor.

There are an increasing number of industrial processes and applications that use low conductivity or high-resistivity water and require Resistivity measurement. Generally resistivity measurements are only used in low conductivity or pure water applications. Industries that use Resistivity measurement extensively are pharmaceutical, cosmetic, semiconductor manufacturing and power generation.

Resistivity is the reciprocal of conductivity and uses the ohm as a unit of measurement. This also requires a length unit added and becomes ohm’s/cm. An ohm is too small a unit for practical measurements so MΩ (1 million ohms) and kΩ (1 thousand ohms) are used. From this 1,000kΩ = 1MΩ.

Solution Resistivity at 25˚C
Ultra-pure water 18.2MΩ.cm
De-ionised water 10 MΩ.cm
Distilled water 1.0 MΩ.cm
Boiler feed (condensate) 100 kΩ.cm

 

For Resistivity measurement we use a nominal cell constant of K=0.01 or 0.1 and the range of LTH controllers for Resistivity measurement allow the user to configure the cell constant to ensure the most accurate resistivity measurement.

You will notice that when Resistivity measurement of a solution is quoted it is usually referred to a temperature. This is because the Resistivity or conductivity of the solution varies not just with the amount of dissolved salts but also with temperature. The temperature coefficient of a solution is normally between 1-3% per °C but it should be noted that the Resistivity or conductivity of pure water – above 10MΩ/cm has a very large and non-linear temperature dependence.

In order to ensure the Resistivity measurement is accurate it is essential that the instrument used has the correct temperature compensation algorithms for pure water measurement.

LTH Electronics offer a range of on-line instruments and sensors for Resistivity measurement and the AquaCal 2000 pure water measurement kit for the measurement and validation of pure water systems.

Call us on +44 (0)1582 593693 or email sales@lth.co.uk to find out how we can help you.