E-Zine November 2012
Capacitance level measurement sensors are probes that are partially covered by material in the vessel. Rising level tends to cover more of a probe inserted from the top of the vessel.
A capacitor is an electrical component that (theoretically) consists of two conductive plates separated by a distance. The space between the plates must be nonconductive. Increasing the size of the plates increases the capacitance of the component. Similarly, decreasing the effective spacing between the plates increases the capacitance of the component.
For electrically conductive materials, the sensor typically consists of a metal rod covered by a nonconductive coating. In these sensors, the conductive material and rod are separated by the nonconductive coating and form a capacitor. As the level of a conductive material rises to cover more of the sensor, the effective size of the plates increases because more of the probe assembly is covered. This tends to increase the capacitance of the sensor. A capacitance level transmitter is used to measure the capacitance which is indicative of the level to which the sensor is covered and hence, the level in the vessel.
In electrically nonconductive materials, the sensor often consists of a metal probe surrounded by an electrically conductive pipe. The probe and pipe form the capacitor plates that are separated by a distance. The electrical dielectric constant of the nonconductive material is different than that of the gas/vapor in the vessel. As the level of a nonconductive material rises to cover more of the sensor, the effective distance between the plates decreases because the nonconductive material has a higher dielectric constant than the gas/vapor that it displaced. This tends to increase the capacitance of the sensor. A capacitance level transmitter is used to measure the capacitance which is indicative of the level to which the sensor is covered and hence, the level in the vessel.
Click here to read Part 2
Excerpted from The Consumer Guide to Capacitance and Radar Contact Level Gauges
ISSN 1538-5280
|