Conductivity measurement is a critical process that quantifies a solution's ability to conduct electrical current, directly correlating to the concentration of the fluid's ionic species.
This metric is essential across various industries, including water treatment, semiconductor, power generation, pharmaceuticals, chemical, and food and beverage production, as it plays a pivotal role in water purity assessment, industrial process control, and chemical analysis.
These measurement readings transcend routine checks, forming a vital part of comprehensive strategies for quality control, regulatory compliance, and process optimization, where even slight inaccuracies can result in significant consequences, from product spoilage to regulatory penalties. Therefore, selecting the appropriate conductivity sensor, tailored to the specific application's conductivity range, environmental demands, and material compatibility, is paramount to ensure both precision and adherence to stringent quality standards.
As we examine the various types of conductivity sensors - from two-electrode to inductive sensors - we will explore how the right choices can empower industries to uphold these critical standards and enhance their effectiveness.
A conductivity sensor measures a solution's ability to conduct electrical current, which is directly proportional to the concentration of ions in the solution. When a voltage is applied across the sensor's electrodes, ions in the solution carry the electric current.
The conductivity is calculated based on the measured current and voltage, allowing the determination of resistance and, subsequently, conductivity.
Given the diversity of applications, no single measurement technology is ideal for every situation. The following three technologies are commonly used for both industrial process and laboratory conductivity measurements:
• Two-Electrode Conductivity Sensors
• Four-Electrode Conductivity Sensors
• Inductive Conductivity Sensors (Non-Contact)
Classical two-electrode conductivity sensors consist of two parallel electrodes supported by structural insulators that prevent shorting and maintain a stable cell constant by ensuring a proper distance between the electrodes. Early designs often used platinum electrodes, which were susceptible to bending, resulting in constant changes in the cell with minor mishandling. Newer designs now incorporate stainless steel or titanium electrodes.
A widely adopted concentric design, introduced by Thornton in 1964, features a center electrode encircled by an outer electrode. These concentric sensors are made from robust materials such as titanium or stainless steel, which provide resistance to changes in cell constant, mechanical damage, and temperature influences.
An AC voltage is applied across the fluid between the two electrodes, and the resistance of the fluid is determined.
The resistance combined with a calculation using the cell constant gives us values for conductivity and resistivity.
The two-electrode conductivity sensor is particularly effective for measuring low to medium conductivity (high resistivity) with high accuracy, making it suitable for a wide range of applications.
Furthermore, these sensors are valued for their simplicity and ease of use, making them a practical choice for routine measurements
As we mentioned, two-electrode sensors excel at measuring low to medium conductivity with high accuracy. This makes them ideal for laboratory or in-process applications such as:
The UPW UniCond 2-electrode resistivity sensor was developed collaboratively with semiconductor industry partners to provide best-in-class accuracy, stability, and environmental isolation in ultrapure water applications.
Four-electrode conductivity sensors include an additional pair of electrodes to enhance measurement accuracy.
Four-electrode conductivity sensors work by applying an AC current to the outer electrodes and measuring the resulting voltage between the inner electrodes.
This enables a precise calculation of the solution's resistance, which then allows for the determination of its conductivity by factoring in the cell constant.
A notable advantage of the four-electrode sensor is that negligible current flows through the inner electrodes during measurement, reducing polarization effects that could otherwise skew results.
This design also reduces measurement errors due to electrode fouling. The combination of these two factors makes four electrode sensors particularly well suited for high conductivity measurements.
The four-electrode sensor is suited for medium to high conductivity measurements, making it ideal for:
Analyzing seawater, cleaning solutions, and acids/bases in laboratory experiments.
Research applications where accurate conductivity measurement is critical for ionic exchange and formulation studies.
In industrial applications, four-electrode sensors excel in:
Monitoring process waters and effluents in real-time, such as for total dissolved solids measurement in cooling water and leak detection.
Providing continuous feedback in systems requiring ion exchange regeneration, where conductivity levels indicate process efficiency (widely used in the pharmaceutical, food and beverage, and semiconductor manufacturing industries for water treatment).
Food and beverage applications, such as cheese production.
CIP batch tanks and other industrial cleaning applications.
In this guide, you will find the fundamental concepts of conductivity measurement along with some good measurement practices.
Our experts guide you through the following topics:
Inductive sensors operate on the principle of electromagnetic induction and provide a non-contact method for measuring conductivity.
Constructed like transformer coils, inductive sensors consist of parallel coils closely spaced and embedded within a polymer body resembling a donut. One coil is energized with AC current, this current induces an additional current in the surrounding fluid via the magnetic field being applied by the energized coil.
The combined magnetic fields of the two currents induce a current in the receiver coil. Since the current in the energized coil is known, its effects can be subtracted, and the resulting current calculation is directly proportional to the ions in the surrounding fluid.
Inductive sensors excel in applications where electrode fouling is a concern, as they have no electrodes contacting the solution. This design also allows for measurement in aggressive media.
Furthermore, they are suitable for medium to very high conductivity measurements.
While typically more common in industrial settings, inductive sensors can also be employed in laboratories for:
Measuring conductivity in aggressive chemicals where traditional sensors might fail due to fouling or corrosion.
Inductive sensors are particularly advantageous for:
Continuous measurement in harsh environments such as chemical processing, where electrode fouling is a concern.
Monitoring high conductivity levels in processes like wastewater treatment and industrial cleaning operations.
Food and beverage applications, such as media separation, product monitoring.
CIP batch tanks and other industrial cleaning applications.
Chemical control in slurry dispense systems and concentration control of developer, etching, and cleaning solutions in microelectronics applications.
METTLER TOLEDO provides a comprehensive range of conductivity sensors designed to address our customers' diverse needs. To help you navigate this range and choose the perfect sensor for your application, we also offer a dedicated sensor selection service. By exploring our product offerings or contacting our experts, you can find sensors that align with your specific measurement requirements.
Our focus on technology and performance aims to support accurate and reliable conductivity measurements, ensuring you have the tools necessary for effective monitoring and analysis.
