Tutorial: Conductivity sensors
There are three liquid characteristics that are often thought of together, although they are different and use different sensor technology. Conductivity, ORP, and pH all have to do with specific chemical characteristics, but measuring each in a process stream requires its own approach. Conductivity is simply the ability of a liquid to conduct electricity, which is related directly to the concentration of ions in the liquid. While pH is concerned specifically with hydrogen ions, conductivity does not differentiate.
Conductivity is the inverse of resistance, and measured in Siemens/cm. Usually units are milli-Siemens or micro-Siemens depending on the range. If you were to take an old fashioned multi-meter and reverse the resistance scale so zero is on the left, you’d have the idea.
Simple conductivity sensors use two or four electrodes with a known surface area that are placed in the liquid or built into a tube or vessel a specified distance apart. If conductivity is low (high resistivity), the electrodes will typically be close together. If conductivity is high, they can be farther apart. While this is an effective approach, it is prone to inaccuracies if there is sediment or bubbles that effectively insulate part of an electrode. Moreover, electrodes in the solution are subject to corrosion if the product is aggressive.
More sophisticated sensors eliminate actual liquid to sensor contact by using a pair of separate coils positioned around a section of non-conductive tubing. One coil has a current passing through it. Conductive liquid passing through the excited coil becomes a transformer secondary, which becomes a primary for the second coil. The amount of current thus induced in the second coil indicates conductivity. While that is a very simple explanation of the technology, the advantage is that there is no electrode exposed to the liquid, which eliminates those related drawbacks mentioned earlier. This design can be configured as a probe or built as a spool section of pipe which can be placed in the process stream.
Conductivity can be used to determine the concentration of a solution, as long as the dissolved substances are known. However, since it is indiscriminant, the sensor cannot tell the difference between acid and alkali. In some cases, this is not an issue. For example, conductivity sensors are standard equipment with CIP (clean in place) systems in food and pharmaceutical processing plants. (See related Control Engineering articles from March, 2007 and April, 2007 .) Cleaning solutions used by the system can be acid or alkali, and conductivity sensors are used to ensure solutions have correct concentration for adequate action during cleaning cycles, and again after rinsing to make sure the water is free of chemicals.
Like any instrumentation technology, it’s important to choose the right range based on the anticipated conductivity. A very wide ranging device will have lower accuracy for low conductivity liquids, so you need some idea of the levels you need to cover.
The following companies manufacture conductivity sensors:
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—Peter Welander, process industries editor, PWelander@cfemedia.com ,
Control Engineering Weekly News