Capacitance sensors (or Dielectric sensors) use

capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized ar ...

to measure the

dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...

permittivity In electromagnetism, the absolute permittivity, often simply called permittivity and denoted by the Greek letter ''ε'' (epsilon), is a measure of the electric polarizability of a dielectric. A material with high permittivity polarizes more in ...

of a surrounding medium. The configuration is like the neutron probe where an access tube made of PVC is installed in the soil; probes can also be modular (comb-like) and connected to a logger. The sensing head consists of an oscillator circuit, the frequency is determined by an annular electrode, fringe-effect

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

, and the dielectric constant of the soil. Each capacitor sensor consists of two metal rings mounted on the circuit board at some distance from the top of the access tube. These rings are a pair of electrodes, which form the plates of the capacitor with the soil acting as the dielectric in between. The plates are connected to an oscillator, consisting of an inductor and a capacitor. The oscillating electrical field is generated between the two rings and extends into the soil medium through the wall of the access tube. The capacitor and the oscillator form a circuit, and changes in dielectric constant of surrounding media are detected by changes in the operating frequency. The capacitance sensors are designed to oscillate in excess of 100 MHz inside the access tube in free air. The output of the sensor is the frequency response of the soil’s capacitance due to its soil moisture level.

Applications

* One application for such a device is measuring the

water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

content of

soil Soil, also commonly referred to as earth or dirt Dirt is an unclean matter, especially when in contact with a person's clothes, skin, or possessions. In such cases, they are said to become dirty. Common types of dirt include: * Debri ...

, where the volume of water in the total volume of soil most heavily influences the dielectric permittivity of the soil because the dielectric of water (80) is much greater than the other constituents of the soil (mineral soil: 4, organic matter: 4, air: 1). When the amount of water changes in the soil, a probe will measure a change in capacitance due to the change in dielectric permittivity that can be directly correlated with a change in water content. Capacitance sensors are now widely used in

irrigation scheduling Irrigation scheduling is the process used by irrigation system managers to determine the correct frequency and duration of watering. The following factors may be taken into consideration: * Precipitation rate of the irrigation equipment – how q ...

in agriculture around the world. *

Cure monitoring Curing is a chemical process employed in polymer chemistry and process engineering that produces the toughening or hardening of a polymer material by cross-linking of polymer chains. Even if it is strongly associated with the production of thermo ...

Composite materials A composite material (also called a composition material or shortened to composite, which is the common name) is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or ...

: Dielectric or capacitance sensors are used to measure the electrical response of

thermoset In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer ( resin). Curing is induced by heat or suitable radiation ...

resins In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on natu ...

and matrices of composite materials at specified depth over the sensor surface. The key model involved in the use of these sensors is the electric field model. The correspondence between electrical properties of the material within the field and the measurement (i.e. capacitance) is fundamental in interpreting the readings from the dielectric sensor. * Tip clearance measurement in turbomachinery testing. * The capacitance-sensors also can be used for the measuring of level of some solid materials in structures such as hoppers or silos.

References

{{reflist * Dane J.H., Topp G.C, co-editors 2002. "Methods of Soil Analysis Part 4-Physical Methods." Soil Science Society of America, Inc. Madison, Wisconsin. * Czarnomski N.M., Moore G.W., Pypker T.G., Licata J. 2005. "Precision and accuracy of three alternative instruments for measuring soil water content in two forest soils of the Pacific Northwest". ''Canadian Journal of Forest Research'' 35, 8, pg. 1867. Hydrology Physical chemistry In situ geotechnical investigations Soil physics Measuring instruments Composite materials Analytical chemistry

Applications

See also

References