By Dave Joseph, Emerson
Conductivity measurement is one of the most ubiquitous in industry so the issue of how to correctly calibrate a toroidal conductivity sensor comes up for a lot of plant personnel. Because the purpose of the measurement is to get information about the total concentration of ions in solution (i.e., the conductivity), the effect of sensor dimensions and windings must be accounted for. The correction factor is the cell constant. Conductivity is equal to the measured conductance multiplied by the cell constant.
There are two basic ways to calibrate a toroidal sensor: against a standard solution, or against a referee meter and sensor. A referee meter and sensor is an instrument that has been previously calibrated and is known to be accurate and reliable. The referee instrument can be used to perform either an in-process or a grab sample calibration. In-process calibration involves connecting the process and referee sensors in series and measuring the conductivity of the process liquid simultaneously. Grab sample calibration involves taking a sample of the process liquid and measuring its conductivity in the laboratory or shop using the referee instrument. No matter which calibration method is used, the analyzer automatically calculates the cell constant once the known conductivity is entered.
The cell constant is also influenced by the nearness of the vessel walls to the sensor, the so-called wall effect. Conductive and non-conductive walls have opposite effects. Metal walls increase the amount of induced current, which leads to an apparent increase in conductance and a corresponding decrease in cell constant. Plastic walls have the opposite effect.
Calibration against a standard solution requires removing the sensor from the process piping. It is practical only if wall effects are absent or the sensor can be calibrated in a container identical to the process piping. The latter requirement ensures that wall effects during calibration, which are incorporated into the cell constant, will be exactly the same as the wall effects when the sensor is in service.
Calibration against a referee – in-process – involves connecting the process and referee sensors in series and allowing the process liquid to flow through both. The process sensor is calibrated by adjusting the process analyzer reading to match the conductivity measured by the referee instrument.
Calibration against a referee – grab sample – is useful when calibration against a standard is impractical or when in-process calibration is not feasible because the sample is hot, corrosive, or dirty, making handling the waste stream from the referee sensor difficult.
If you click HERE, you’ll find a very useful white paper that walks you through the issues related to calibration of a toroidal conductivity sensor and explores each method in some depth. I hope this is useful in simplifying your conductivity measurements.
Please let me know if I can answer any questions. Thanks for stopping by.
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