Conductivity measurements in the process industry are influenced by a variety of factors that can compromise accuracy and reliability. Electrical interference from nearby equipment, especially in analog systems, can cause signal noise or drift. Chemical composition of the process media also plays a key role. Different ions have different conductive properties, meaning a change in the type or strength of dissolved salts, acids, or bases can alter conductivity.

Additionally, without proper compensation, even small temperature fluctuations can cause misleading readings. All of these threats to accuracy are important to keep in mind, yet none of them will matter if your conductivity sensor has the improper cell constant.

Understanding Cell Constant

So, what is the cell constant? It is the distance between the two electrodes of the conductivity sensor, as well as the surface area of each. It is the most important factor to determine the measurement range of your sensor. To convert the raw conductance reading to actual conductivity, the sensor multiplies the measured conductance by the cell constant. Thus, the cell constant is essential for converting a sensor’s electrical measurement into meaningful data about the solution. The smaller the cell factor, the tighter the measurement range (example: 0.03/cm for high purity applications 0-1,000 µS/cm), therefore, the larger it is, the wider the measurement range (example: 1.0/cm for standard application 10 µS/cm – 20 mS/cm).

How the Wrong Value Derails Accuracy

A few things can occur when using a conductivity sensor with the improper cell constant. When the constant is too high for a process prone to lower, more precise measurements, the measured conductivity will likely appear lower than it actually is. Too low of a constant, and the measured conductivity will appear artificially high. Either case can negatively impact process control, from dosing errors to compromising product quality. This is especially concerning in high-purity or chemical-critical applications.

When it comes to cell constants, not all conductivity sensors are created equal. However, two sensors with identical cell constants should yield the same reading in the same fluid. It allows interchangeability and predictable performance when replacing or maintaining sensors. The cell constant is the link between a conductivity sensor’s physical design and its ability to deliver accurate measurements. Choose the right cell constant, and the integrity of your data should be maintained. Get it wrong, and even the best sensor won’t save you.

Have questions or need help selecting the right sensor for your process? Contact our experts, or check out our Conductivity Sensor Guide.