E-Zine October 2012
Capacitance and radar contact level measurement systems generally consist of a sensor assembly located above the material surface that extends downwards into the material where the level is to be measured. The electrical properties of many liquids and solids can vary with composition and operating conditions. Level measurement systems that are used to measure the level of materials that form distinct interfaces and that have constant (relevant) electrical properties tend to produce more reliable and more accurate level measurements.
Capacitance and radar contact level sensors are in direct contact with the material and are exposed to the materialís vapors and general environment. The sensors generally contain one or more probes that require a seal where the probe enters the vessel. This seal can come in direct contact with the material when the level of the material is excessively high.
These technologies measure level continuously, however they do so at essentially one point in the vessel. This generally does not pose a problem for liquid applications where the gas/liquid interface is horizontal.
However, in solids applications, material entering and leaving the vessel affect the solid/gas interface. As such, this interface is typically not horizontal, so a single level measurement may not representative if the amount of material in the vessel. In these applications, the level measurement reflects the level at one point in the vessel. Level measurements can change rapidly when the material level that is sensed changes rapidly. In some applications, multiple level measurements may be needed to provide a more accurate indication of the inventory of material in the vessel.
For example, a rat-hole may form as solid material leaves the vessel. If the level measurement reflects a point in the rat-hole, the measured level will decrease (as expected). However, if the material remaining on the sides of the vessel falls and fills in the rat-hole, the level will abruptly (and unexpectedly) increase. Sensors should be located such that the measured level represents the actual level while avoiding rat-hole affects. Multiple sensors may be needed if an appropriate location cannot be found.
In addition, most capacitance and radar contact level sensors cannot accurately measure distances that are close to the sensor itself. Sensors are typically installed to allow the transmitter to disregard measurements at these distances.
Excerpted from The Consumer Guide to Capacitance and Radar Contact Level Gauges