Conductivity measurement is a critical aspect of various industrial and laboratory processes, especially when monitoring water quality, chemical analysis and bioethanol production. Understanding the fundamentals of conductivity, the types of sensors used and the best practices for accurate measurements can significantly enhance the reliability of the results.

What is conductivity?

Electrical conductivity is defined as the ability of a material to carry an electric current. In the context of liquids, it refers to the capacity of a solution to conduct electricity due to the presence of dissolved ions. The measurement of conductivity provides essential insights into the concentration of these ions, which can include salts, acids, bases and other conductive materials.

Several factors influence a solution's conductivity, including the type and concentration of ions present, temperature and the solution's physical properties. Typically measured in MicroSiemens per centimeter (µS/cm), conductivity is a non-specific parameter, meaning it does not differentiate between the types of ions present but rather gives a sum effect of all dissolved ionic species.

Sensors for conductivity measurement

Selecting the right conductivity sensor is essential for achieving accurate and reliable measurements. Given the diversity of conductivity measurement applications, it is not surprising that there is no single measurement technology that is ideal for every situation. The following three technologies are commonly used:

• 2-pole conductivity cell

• 4-pole conductivity cell

• Inductive conductivity measuring cell

Inductive measuring cells are mainly used for inline measurement for process control in industrial plants. This article focuses on the 2- and 4-pole conductivity cells.

2-pole conductivity cells

Classical 2-pole conductivity cells consist of two plates. Normally, the plates are surrounded by an outer tube that protects them from mechanical damage and reduces the errors caused by field effects. There are also other designs of 2-pole cells. Another widely used construction type is a pin that is encircled by the second electrode. These sensors are manufactured from robust materials such as stainless steel or titanium and are much less susceptible to mechanical damage. The easy setup of the 2-pole cell allows miniaturized measuring cells to be constructed. These micro sensors also enable measurement of small amounts of the sample.

Schematics of a 2-pole conductivity cell. Source: Mettler Toledo

4-pole conductivity cells

The 4-pole cell has different layouts, but the functional principle is always the same:

• The outer poles are the current poles to which an AC is applied. They are driven in the same manner as the 2-pole sensor. The inner measuring poles are placed within the electric field of the current poles, and the voltage is measured using a high impedance amplifier.

Schematics of a 4-pole conductivity cell. Source: Mettler Toledo

The circuit can accurately measure the current flowing through the outer poles and the solution. If the voltage across the inner poles and the current are known, the resistance and the conductance can be calculated. To obtain the conductivity, the conductance must be multiplied by the cell constant of the inner poles.

The strength of a 4-pole conductivity cell is measuring conductivity over a wide measuring range from 10 µS/cm up to 1,000 mS/cm with excellent linearity. The main applications of this sensor type are measurements over expanded midrange conductivity as found in seawater, wastewater or diluted acids or bases.

Mettler Toledo's InLab conductivity sensors. Source: Mettler Toledo

How to select the right sensor?

Selecting the right sensor is often the tricky part. The following three criteria can help users choose the right sensor for their conductivity measurements.

Chemical stability

There must be no chemical reaction between the sensor material and the sample.

Construction type

2-pole sensor: Best for low conductivity measurements

4-pole sensor: Best for mid to high conductivity measurements

Cell constant

Use a sensor with a low cell constant (0.01 to 0.1 cm-1) for low conductivity measurements and a sensor with a higher cell constant (0.5 to 1.0 cm-1) for mid to high conductivity measurements.

Conductivity measurements are vital in various applications. Understanding the definition, selecting the right tools and adhering to the best practice significantly improves the accuracy and reliability of measurement results.

For all the necessary information about Mettler Toledo’s conductivity sensors and how well they are suited to customer applications, visit www.electrodes.net.

To contact the author of this article, email pHmatters@mt.com