Electrical conductivity is the measure of electrical current a material (or solution) can carry. Conductivity is one of the most common analytical parameters in the process industry and has been for numerous years.
Conductivity measurements are the sum of all dissolved ionic activity within a sample solution. It is important to note this as the measurement process is unable to distinguish between specific ions. Throughout all industry, conductivity measurements monitor the purity of water, concentrations of processes, breakthroughs of solutions being cooled and even discharge of wastewater into municipal streams and rivers. The measurement process is reliable and repeatable and inexpensive to make. Process temperature also effects conductivity. Electrodes include integrated temperature detectors for continuous measurement and compensation of the measured value.
In the following bullet points, we will have a look at the three most common methods of measuring conductivity and highlight the benefits of each measurement principle.
Two-Electrode Conductivity Measurement Principle
An important factor when determining the conductivity however is the geometric makeup of the two electrodes. The geometric makeup is the cell constant. It is the distance between the two electrodes as well as the surface area of each. The cell constant is the most important factor to determine the measurement range of your sensor. The smaller the cell factor, the tighter the measurement range (example: 0.03/cm for high purity applications 0-1,000 µS/cm), therefore larger the cell constant, the wider the measurement range (example: 1.0/cm for standard application 10 µS/cm – 20 mS/cm).
Typical measuring ranges: 0 µS/cm to 20 mS/cm (20,000 µS/cm)
Four-Electrode Conductivity Measurement Principle
A four-electrode conductivity cell introduces two more electrodes onto the sensor. An AC voltage on the outermost pair of electrodes generates a current in the sample media. The two inner-most electrodes are “current-less” and measure the potential difference in the sample which depends on the sample’s conductivity. The transmitter then considers the measured potential difference and the given current. It then provides a conductivity measurement which is unaffected by “polarization”. Four-electrode conductivity sensors are great for applications with a wide measurement range.
Typical measuring ranges: 1 µS/cm to 600 mS/cm
Inductive/Toroidal Conductivity Measurement Principle
An inductive or toroidal conductivity sensor is sometimes referred to as “non-contacting”. This is because the technology does not involve electrodes which are in direct contact with the process. Toroidal sensors use two tightly-wound metal toroids. A corrosion resistant plastic body encapsulated these toroids. The sensor uses an integrated temperature detector inside the plastic body to compensate measured values.
The first toroid acts as the “drive” coil and an alternating voltage is applied to it. This induces a voltage in the liquid surrounding the coil which causes an ionic current to flow proportional to the conductance of the liquid. The second coil is the “receive” coil. It transfers the ionic current into an electric current and your transmitter measures this current. Toroidal conductivity sensors offer a wide measurement range are great for use in dirty applications due to their immunity to buildup. Because of their wide measurement range, we often use used these types of sensors in concentration applications.
Typical measuring ranges: 2 µS/cm to 2,000 mS/cm
Summary
Whether it is controlling corrosion and scaling in your process or monitoring the purity of your water, conductivity sensors play a pivotal role in process applications throughout many different industries. In short, understanding the key differences among the three technologies used to measure conductivity will help you to determine proper sensor selection for optimal performance within your application!
