The properties of a wet material may be expressed explicitly in term of the relative permittivity e = e '- je , where e ' is the dielectric constant of the material and e " is the loss factor. Losses in the material may be expressed in term of the conductivity , because e "=s /w e 0 , where w =2p f is the angular frequency of applied field at a frequency f and e 0 is the dielectric constant of vacuum. The permittivity is a measure of the polarization which the material is undergone in an applied electromagnetic Held. In water varies with frequency to give dispersion which is known as Debye dispersion. The dispersion and absorption observed for the water is caused by the frequency dependence of the polarisation of water molecules .As the frequency of the applied field increases the molecules are unable to reorient completely before the field reverses. This type of relaxation is called the polar relaxation. Dependence between the permittivity of water and the frequency of electromagnetic field is shown in fig.1.

It should be noted that the dielectric constant of water changes slowly over the region of dispersion with an associated absorption band centred at a frequency equal to 1/2 where is the relaxation time. The orientation of water molecules is considerably affected by thermal agitation and the corresponding dielectric properties of water are therefore temperature dependent, as shown in fig.2.

The principle of microwave moisture content measuring method is based on the fact that the permittivity of water is much higher than that of most dry substances. A small amount of water causes significant changes of the permittivity of the wet material which can be detected using even a conventional microwave measuring equipment. From the data presented in Fig.1 and 2 it is obvious that the characteristics of dielectric materials containing water vary with frequency and temperature. However, in practice the frequency dependence may be neglected, while any changes of the material temperature should be monitored and compensated.

The following formulae may be used to relate the electromagnetic wave propagation parameters to those of wet materials (e " / e ' < 0.1):

where a-the attenuation constant , b - the phase constant, r - the voltage reflection coefficient at the surface of the wet material.

Despite the complexity of the physical structure of wet materials, usually there exists a simple linear relationship between the moisture content and the attenuation and phase shift of an electromagnetic wave passing through the sample, at least in a limited range of moisture content.

Because the dielectric losses in the wet material are predominantly due to the moisture itself, the relationship between the attenuation and the water content may be written in a simple form:

N = 8,7a Wr L

where

N - microwaves attenuation:

W - studied material moisture;

r - material density;

L - material thickness;

a - water absorption coefficient.

As the microwave absorption by dry material coefficient practically is equal to zero, theoretically there are no limits to the increase of accuracy of moisture measurement on microwaves by absorption method. In the practice, for the accuracy limit determination it is necessary to take into account: the influence of material temperature on the value of absorption coefficient (see Fig.2), material density instability and inaccuracy of material thickness specification between transmission and reception aerials. Still, these problems can be easily overcome. As the temperature dependence of absorption coefficient is known, or may be designated for choosing diapason of the electro-magnetic radiation, it is easily compensated by putting into operation in moisture meter of additional material temperature correction channel. Stability of parameter L can be provided in a simple way by aerials placement on a large distance, at which its relative fluctuations (vibrations, various deformations, etc.) will be insignificant. Pouring density of the mould mixture is a function of moisture, and at conveyer measurements it may be integrated to acceptable fluctuations. In laboratory conditions "L" and "p" can be easily specified by experimental conditions.

 

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