Capacitance sensors (or Dielectric sensors) use

capacitance Capacitance is the ability of an object 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 are two closely related ...

to measure the

dielectric In electromagnetism, a dielectric (or dielectric medium) is an Insulator (electricity), electrical insulator that can be Polarisability, polarised by an applied electric field. When a dielectric material is placed in an electric field, electric ...

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

of a surrounding medium. The configuration is like the neutron probe where an access tube made of PVC is installed in the

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

; 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 An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...

, fringe-effect

capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...

, 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 The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base u ...

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 is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

content of

, 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 in agriculture around the world. * Cure monitoring of

Composite materials A composite or composite material (also composition material) is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or physical properties and are merged to create a ...

: 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 and ...

resins 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