What is a capacitive level sensor
What is a capacitive level sensor. The capacitive liquid level sensor measures the height of the page by measuring the change of capacitance. It is a metal rod inserted into the container containing the liquid.
The metal rod is used as one stage of the capacitor, and the wall of the container is the other stage of the capacitor. The medium between the two electrodes is liquid and air.
The dielectric constants of liquid and air are different. If the liquid level changes, the capacitance changes accordingly, so as to measure the liquid level.
When the liquid level rises, the total dielectric constant between the two electrodes of the capacitive liquid level gauge increases accordingly, so the capacitance increases.
Conversely, when the liquid level drops, the value of ε decreases and the capacitance also decreases.
Operating principle of capacitive level sensor
A capacitors in made up of two conductive plates which are separated from each other by a dielectric.
The storage capability of capacitor defined by the capacitance , C, is directly dependent on the plate areas（A）, their distance apart （d）and the dielectric constant of the material between the plates:
Capacitive probes are used in liquids that are non-conductive and have a high dielectric constant（ε）and can be used for continuous level monitoring.
A capacitive probe shown below, consist of an inner rod with an outer shell immersed in the liquid that we intend to measure the level.
The capacitance of the probe is more measured between the inner rod and the outer shell with the aid of a capacitance bridge. In the portion out of the liquid, air serves as the dielectric between the rod and outer shell.
In the immersed section, the dielectric is that of the liquid that causes a large capacitive change as the level of liquid changes. If the tank is made of metal it can serve as the outer shell.
The capacitance change is directly proportional to the liquid. The dielectric constant of the liquid must be known for this type of measurement.
Most materials has their unique permittivity at a certain temperature. There are some examples of materials with their properties shown in Figure 1.1.
Application field
（1）Level detection in silos, tanks and bunkers, for both limit detection and continuous measurement;
（2）Conductive, insulating liquid-chemical industry, oil field, water and sewage treatment;
（3）Conductive and insulating slurry – papermaking, pharmaceutical, water and sewage treatment;
（4）Powder, ash, powder-power plant, metallurgy, cement;
（5）Pellets, coal, grain-power plants, metallurgy, grain;
（6）Interface, two different liquids-oil field, chemical industry.
What are the advantage of capacitive level sensor?
- It is suitable for high pressure, high temperature, strong corrosion, easy crystallization, anti-blocking, anti-freezing, and solid powder and granular materials
- They have know moving parts, easy to clean
- Relatively inexpensive and reliable
- Easy to install and can be adapted easily for different size of vessels
- Good range of measurement, from few cm to about 100 m
- Simple to use
In order to have a clearer understanding of “What is a capacitive liquid level sensor?”, Kacise divides it into three parts for explanation.
