At first glance, calibrating conductivity sensors may seem straightforward; however, many times, this is not the case. I’d like to share with you some of the various methods for calibrating contacting conductivity sensors and outline some of the potential issues that can accompany such procedures.
There are three main methods for going about calibrating contacting conductivity sensors: 1) with a standard solution, 2) directly in process against a calibrated referee instrument and sensor, and 3) by grab sample analysis. Understanding the fundamentals of each method, and the issues associated with each, can help make the decision on how best to calibrate a sensor.
Calibration with a standard solution consists of adjusting the transmitter reading to match the value of a solution of known conductivity at a specified temperature. This method is best when dealing with process conductivities greater than 100 µS/cm. Use of standard solutions less than 100 µS/cm can be problematic as you run into the issue of contaminating the standard with atmospheric carbon dioxide, thereby changing the actual conductivity value of the standard. For maximum accuracy, a calibrated thermometer should be used to measure the temperature of the standard solution.
If dealing with low conductivity applications (less than 100 µS/cm), in-process calibration against a referee instrument is the preferred method. This method requires adjusting the transmitter reading to match the conductivity value read by a referee sensor and transmitter. The referee instrument should be installed close to the sensor being calibrated to ensure you are getting a representative sample. Best practices for attaining a representative sample across both the process and referee sensors include using short tubing runs between sensors and increasing sample flow. Though usually not as accurate as calibrating against a standard solution, this method eliminates the need to have to remove your sensor from process, and removes any risk associated with contamination from atmospheric carbon dioxide.
The least preferred method of calibrating a contacting conductivity sensor is by analyzing a grab sample. This method entails collecting a sample from process and measuring its conductivity in a lab or shop using a referee instrument. Calibration via grab sample analysis is only ever recommended if calibration with a standard solution is not possible or if there are issues with installing a referee instrument directly in process. The collected grab sample is subject to contamination from atmospheric carbon dioxide and is also subject to changes in temperature when being transported. In other words, there are many potential sources for error in the final calibration results.