RF Capacitance Level Switch Working Principle
RF Capacitance level controls are based on an electronic device called a capacitor. The capacitor is a device that stores energy. This energy is not stored in the probe; rather, the RF Capacitance level control is merely measuring how much energy can be stored. The amount of capacitance the RF Capacitance level control is measuring is extremely small and is measured in picofarads (1 X 10-12) farads. The capacitor is made up of two conductive plates parallel to each other. Separating the two plates is an insulator.
The amount of energy a capacitor can store is influenced by several things. First, a larger plate area results in more space to store energy. Second, more space between the plates reduces the amount of energy storage. Finally, a higher dielectric constant media can contain more energy than a lower dielectric media. The dielectric is where the actual capacitance is developed. The following chart shows the dielectric constant and conductivity for some sample materials.
Substances are considered either conductive or non-conductive. Non-conductive materials have a dielectric less than 10 or a conductivity less than 10µsiemens/cm. Conductive materials have a dielectric constant greater than 10 or a conductivity greater than 10 µsiemens/cm. Interestingly, there is a similar relationship between dielectric constant and conductivity. Non-conductive substances tend to have low dielectric constants and conductive substances tend to have high dielectric constants.
In a level application, one plate is the probe and the other is the wall of the tank (see following figure). These do not change, nor does the distance between them. The only thing that changes is the dielectric constant. Air has a dielectric constant of one; anything else you measure will have dielectric value greater than one.
When the substance level increases, the dialectic of the substance is replacing the air and causes the capacitance to increase. The preset capacitance value is equal to the set point level wanted and trips a switch when the level is reached. The transmitter creates a linear output in relationship to the capacitance measured.
The substance between the two plates has to be an insulator in order to have a capacitor. When a conductive material is between the plates, an electrical short is created. This, in turn, signals the level transmitter to indicate a high level. A Teflon insulator around the sensor will prevent this from happening, as the figure below demonstrates.
An electrical connection is created through the conductive substance from the tank wall and the Teflon probe. When the level in the tank rises, the capacitor is created by the metal probe rod, the substance being measured and the probe insulator (Teflon), where the sensor rod and substance are the plates and insulator is the dielectric. This means that rather than measuring the dielectric of the substance, the dielectric of the probe where it is covered by the substance is being measured.
Restrictions of RF
- Sensitive to changes in material dielectric (Note: dielectric compensation additives help, but the liquid can stratify.)
- Normally needs field calibration, which requires a change in level
- Dependent on contact with the substance being measured
- Conductive coatings can build up on the sensor and create false readings
Model numbers 651, 681, 660 (Electronics and Housing)
Model CA, CB, CC, CD, CE, CJ (RF Probe)
All RF Capacitance Switches SOR offers consists of two parts - the electronics/housing and the probe. The different electronics and housings serve different application needs while the different probes are used in different process medias. For more information on the probes, please see pages 21-25 of the RF Capacitance Level Controls (Form CAT1100).
651 Single-Point RF Switch
The 651 provides basic, single-point switching for use as an alarm or indicator. It’s virtually immune to process coatings on the probe, making it a useful solution for many tough level applications. This immunity, combined with the absence of any moving parts, makes the 651 well suited for applications that are difficult for other technologies.
681 Single-Point RF Switch with Self Test
Available as an integral or remote-mounted unit, the 681 provides single-point switching, and with its many safe and operation features, is well suited for demanding industrial applications. Its “Self-Check” function constantly monitors circuit and probe integrity. A dedicated relay (line powered) or current shift (loop powered) indicates if the unit is not functioning properly. An optional, adjustable differential provides control of two set points with one relay, which gives the 681 pump and valve control for maintaining correct process levels.
660 Multi-Point RF Switch
The 660 Series provides the options of multiple-point switching plus narrow and wide differential switching. By combining these features, the 660 Series units can be used for a wide variety of control needs. The available switching combinations are designed to provide multiple alarms, pump/valve control, or a combination of alarms and equipment control. The 660 Series makes it possible to combine up to four single-point devices into one package for lower costs and reduced maintenance.