The functional principle is consistent with that of an impedance measurement.

In a typical test, the sample is placed in contact with two electrodes (the dielectric sensor). When a sinusoidal voltage is applied, the charge carriers inside the sample are forced to move: positively charged particles migrate to the negative pole and vice versa. This movement results in a sinusoidal current with a phase shift.

A low voltage AC signal (input) is applied at one electrode. The response signal detected at the other electrode (output) is attenuated and phase shifted.

Dielectric sensor:

• Alignment of dipoles
• Mobility of ions

C  =  ε_r  C₀
ε_r  =   ε_r’ -  i  ε_r’’

• Electronic polarization  
  This happens to atoms if the applied electrical field has influence on the electron density around the nucleus
  
  
• Atomic polarization  
  Deformation of electron clouds, formation of positive and   negative charged areas
  
  
• Orientation polarization  
  Alignment of permanent and induced dipoles
  
  
• Ionic dislocation polarization  
  Includes ion conductivity and conductivity via space charge and boundary layers
  → ion conductivity dominates at low frequencies 

Frequency:

Water molecules, for example, are permanent dipoles, which rotate to follow an alternating electric field.
These mechanisms are quite lossy – which explains why food heats in a microwave oven.       

Atomic and electronic mechanisms are relatively weak, and usually constant over the microwave region. Each dielectric mechanism has a characteristic “cutoff frequency.” As frequency increases, the slow mechanisms drop out in turn, leaving the faster ones to contribute to ε'. The loss factor (ε_r'') will correspondingly peak at each critical frequency. The magnitude and “cutoff frequency” of each mechanism is unique for different materials.