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How it works
is given the relative permittivity value of 1. All other materials
have a measured relative permittivity value compared with that
of the perfect vacuum.
Some examples of approximate relative permittivity values at
20°C are:
Material
Relative
permittivity
Perfect vacuum
1
Air
1.0006
Dry wood
2
– 6
Dry silica sand
2.5
– 3.5
Quartz
4.5
Granite
7
– 9
Water
80.4
When these and other materials are combined together to form
a soil or substrate, the WET150 detects the overall mixed bulk
permittivity which is formed from their individual permittivity
contributions, depending upon their relative quantity in the mix
.
Converting permittivity to water content
The relationship between soil permittivity (
Ɛ′
) and its equivalent
water content (θ), depends upon the mix proportions of its
constituent materials (sand, organic matter, water, air, etc). This
relationship between soil permittivity and water content is also
non-linear.
However, for any given soil it can generally be assumed that the
permittivity contribution from the soil particles is constant, so the
permittivity measured by the WET150 is only affected by
changes in water content. Also, by using the square-root of
permittivity (called the
refractive index
), it is possible to further
simplify the relationship with water content as:
√Ɛ′ = 𝑎
0
+ (𝑎
1
∙ 𝜃)
