Capacitance sensors use capacitance

Capacitance

In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...

 to measure the dielectric

Dielectric

A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

 permittivity

Permittivity

In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...

 of a surrounding medium.
The configuration is like the neutron probe where an access tube made of PVC is installed in the soil. The probe consists of sensing head at fixed depth. The sensing head consists of an oscillator circuit, the frequency is determined by an annular electrode, fringe-effect capacitor

Capacitor

A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

, 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
  Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
  
   content of soil
  Soil
  Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
  
  , 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
  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:...
  
   in agriculture around the world. For more information see www.irrigationworld2000.com http://www.irrigationworld2000.com and www.sentek.com.au http://www.sentek.com.au

• Cure monitoring
  Cure monitoring
  Real-time computing of cure monitoring is an essential component for the control of the manufacturing process of composite materials. The rationale for cure monitoring relies on the various physical or chemical properties that can be used to follow the transformation of an initially liquid...
  
   of Composite materials: Dielectric or Capacitance Sensors are used to measure the electrical response of thermoset 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. The pioneering work of Mamishev et al. (University of Washington, Seattle) has laid the bases for the expansion of the use of these sensing structures.

• The capacitance-sensors also can be used for the measurement of level of solid materials e.g. at Cement plant - Hopper or Silo level measurements.