E-Zine January 2013
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Relative immunity to variation in electrical properties can often be achieved by knowing the range over which the electrical conductivity or dielectric constant can vary in a given application in conjunction with selecting a sensor that is relatively immune to the property changes encountered.
Some materials have electrically properties that can vary widely during operation and/or be below the electrical conductivity limits of the available sensors. In these applications, level measurement systems are available that measure the electrical parameter and compensate for its changes.
Coating on the sensor can effectively change the capacitance value at a given level. These coatings can be temporary, but they more likely build up over time. To avoid periodic maintenance, some capacitance level system designs are available with features that reduce the effect of coating on the measurement. One technique is to measure the admittance of the probe whereby the resistance and capacitance of the probe are used to identify and subtract the effects of coating. Another technique is to use an additional active shield to quantify the effects of coating. High frequency excitation of the capacitance probe can also be used to reject the effects of some coatings.
Capacitance level measurement systems for solids applications are similar to those for liquids except that the sensor may be different. Solids applications pose the additional complication in that variations in the material bulk density can affect its electrical properties and hence the level measurement.
Excerpted from The Consumer Guide to Capacitance and Radar Contact Level Gauges