Electrical capacitance between two plates is the electrical charge divided by the voltage difference needed to maintain this charge. A capacitance level sensor measures the capacitance between the center rod and the column in Fig. 11.12. When the liquid level is different in the two columns, the capacitance in them will also be different.
The total capacitance for a given clearance between the rod and the column is expressed by the equation
Vapor has a dielectric constant appreciably less than liquids, so the application of Eq. 11.2 shows that as the liquid level in the probe chamber rises, so does the capacitance between the rod and the column. This capacitance is sensed and usually converted into an electric current varying between 4 and 20 mA. A comparison of the dielectric constant of oil and ammonia, for example, suggests that if the probe chamber has accumulated oil, that the sensed level will be in error.
There are two types of capacitance level indicators, the single-point and the continuous. Single-point probes indicate the presence of liquid at one level, and thus are comparable to the float switch. This application of the capacitance principle fails to capitalize on its full capabilities, because circuitry is available to open or close a contact at multiple values of the probe output. A typical set of level indicators is shown in Fig. 10.13 applied to a low-pressure receiver where the capacitance level probe not only manages the normal control level, but signals
two extreme positions to activate the low-level and high-level alarm. The highand low-level cutouts are usually part of a separate system, perhaps float switches, so that the plant is protected in the event of a failure of the capacitance level controller.
An advantage of the capacitance level controller is that a setting of a level can easily be adjusted by changing the pickoff point in the 4 to 20 mA range that triggers the action. Changing the level setting with a float switch is not so simple, because the connections of the float chamber on the vessel must be relocated. Capacitance probes have no moving parts in the sensor to wear out, so they are not subject to sticking and failure of electrical contacts in the probe.