E-Zine April 2013
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Radar energy travels almost exclusively in the probe and that its speed is not dependent upon the pressure, temperature, or composition of the fluid. In this sense, varying operating conditions should not affect the performance of these level measurement systems. However, the dielectric constant of some materials can vary with its operating conditions, such as composition and temperature. Therefore, this technology may not be appropriate if the dielectric constant of the material can fall below the minimum requirements for the instrument to operate properly.
Bridging across the probes can occur and cause a measurement that is higher than the actual level in the vessel. When accumulations over time are normal for the process, routine maintenance may be required to keep the system operating. Careful analysis of the sensor design with regard to its ongoing maintenance should be performed.
The material itself can cause the intensity of the reflected radar signal to degrade when the material exhibits poor reflective qualities on the surface of the material such as when contaminants on the surface of the material cause the energy to reflect poorly. Notwithstanding other problems, radar contact level measurement is typically limited to materials with a dielectric constant greater than approximately 1.3 to 1.7. Some users report that they do not use these devices in applications where the dielectric constant is less than 1.9. Liquids with lower dielectric constants generally do not reflect well and may cause the level measurement to be erratic or fail to operate. That said, at least one supplier claims to be able measure the level of material with a dielectric constant of 1.10.
Radar contact level instruments are designed to measure the material level. However, foams that have a relatively large dielectric constant will conduct a significant amount of radar energy. This can cause a reflection that essentially causes the instrument to measure the level of the foam rather than the actual desired measurement surface.
In addition, accuracy can be degraded based upon the surface on which the energy is reflected. For example, level measurement may typically measure the top of a layer of foam due to its ability to absorb radar energy. However, the characteristics of the foam, such as its composition, density, bubble size, and the like, can cause the foam to not absorb the radar energy and measure the level of the material. At other times, the foam might absorb radar energy and measure a point interior to the foam layer. In this application, varying amounts and consistency of foam can cause the measurement to become erratic.
Excerpted from The Consumer Guide to Capacitance and Radar Contact Level Gauges