METER TEROS 21 - TEROS 21 Measurement Range

10
SYSTEM
TEROS 21 measures the dielectric permittivity of a solid matrix (porous ceramic discs)
to determine its water potential. The dielectric permittivity of air, the solid ceramic,
and water are 1, 5, and 80, respectively. So, the dielectric permittivity of the porous
ceramic discs is highly dependent on the amount of water present in the pore spaces.
Measuring the dielectric permittivity of the ceramic discs resolves a wide range of water
contentmeasurements.
Water content and water potential are related by a relationship unique to a given material,
called the moisture characteristic curve. The ceramic used with the TEROS 21 has a wide
pore size distribution and is consistent between discs, giving each disc the same moisture
characteristic curve. Thus, the water potential can be inferred from water content using the
moisture characteristic curve of the ceramic
Equation 1 gives the component variables for determining total soil water potential(Ψ
t
):
Equation 1
pg mt
WW WWW
=+++
o
where Ψ
p
is pressure, Ψ
g
is gravitational, Ψ
o
is osmotic, and Ψ
m
is matric.
For TEROS 21 applications, Ψ
p
and Ψ
g
are generally insignificant. Ψ
o
arises from dissolved
salts in the soil and only becomes important if a semipermeable barrier is present that
prevents ionic movement (e.g., plant roots or cell membranes). Ψ
m
arises from the attraction
of water to the soil particles and is the most important component of water potential in most
soils. TEROS 21 responds to the matric potential of the soil (Ψ
m
). In highly salt-affected soils,
it may be necessary to quantify Ψ
o
independently if the measurements of soil water potential
are related to biological activity (Section 3.4.2).
3.3.2 MEASUREMENT RANGE
TEROS 21 measures the water content of porous ceramic discs and converts the measured
water content to water potential using the moisture characteristic curve of the ceramic.
Therefore, it is important that the ceramic discs drain over a wide water potential range.
Pore size determines the water potential at which a pore drains (the air entry potential
or bubble pressure), so the ideal ceramic would have pores that range from very small to
relatively large. METER designed the ceramic discs to approach this ideal (Figure4). The
discs have a total pore volume that is weighted toward the larger pores, which drain at water
potentials within the plant-available range (approximately −33 to −1,500 kPa). However, the
TEROS21 measurement range extends all the way to air dry (−100,000 kPa).