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Measuring conductivity is simple, but ...

Blog post Elscolab - Measuring conductivity is simple, but ...

Measuring conductivity is one of the simplest and most robust measurements in industrial processes. Nevertheless, comparative measurements, such as control measurements, often raise lots of questions. You can find the answers below.

Cell constant

The basis of everything is the cell constant or cell factor (for inductive sensors). The cell constant converts the effectively measured resistance between the electrodes to the conductivity value which would have been measured with a (hypothetical) standard measurement cell. This allows you to compare results of measurements with different sensors. The cell constant, often abbreviated as CC, is usually specified on the sensor and in the certificate supplied by the manufacturer. In your measuring converter (transmitter), CC is often set to 1/cm as standard. It is therefore essential to enter the correct cell constant in the measuring converter, and the same applies to the control measurement. With digital sensors, the cell constant or cell factor is stored in the sensor head, and is immediately transferred to the measuring converter when connected.

Temperature coefficient

TC, the temperature coefficient which compensates for temperature, is equally important. This coefficient varies according to the fluid, so the TC of the product you are measuring must be set. For inline measurements that always measure the same product flow, you only need to do this once. For laboratory and portable devices that continually measure different liquids, the TC must be set per measurement in order to obtain correct results. Although the TC of most pure substances is known, this is not the case for mixtures, which therefore might have to be determined separately. It is difficult to overestimate the importance of the temperature coefficient. For most saline solutions, it is around 2% per °C! This means that a small temperature difference can quickly lead to a large error of several percent. For control measurements, the TC of the inline measurement and the control device must be the same to avoid problems.

Reference temperature

The finally important parameter is Tref, the reference temperature, which is closely linked to the temperature coefficient. The TC is used to convert the measured value to the conductivity value at a certain reference temperature. Tref is usually 20 °C or 25 °C, but not necessarily. Again, the reference temperature of the inline measurement and the control measurement must be the same, since a difference of 5 °C with a TC of 2% means an error of 10%.

Keep these tips in mind to make sure your results are good, accurate, and comparable. That avoids pointless discussions and a lot of hassle.

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