E-Zine April 2011
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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 be representative of 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 non-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. This is often called the blanking distance.
Excerpted from The Consumer Guide to Non-Contact Level Gauges