MPS-2 & MPS-6
3
THEORY
The MPS-2 and MPS-6 use the same principle, but instead mea-
sure 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 depends on the
amount of water present in the ceramic disc pore spaces. Measuring
the dielectric permittivity of the ceramic discs allows you to resolve
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 MPS-2 and MPS-6 has a wide pore size
distribution and is consistent between discs, giving each disc the
same moisture characteristic curve. Thus, you infer the water poten-
tial with the moisture characteristic curve after measuring the water
content of the ceramic.
Equation 1 represents the component variables for determining total
soil water potential (Ψ
t
):
Ψ
t
= Ψ
p
+ Ψ
g
+ Ψ
o
+ Ψ
m
(1)
The subscripts p, g, o, and m are pressure, gravitational, osmotic,
and matric respectively. Of these four components, only Ψ
o
and Ψ
m
are significant and often measured in soil. Ψ
o
arises from dissolved
salts in the soil, and only becomes important if a semi permeable
barrier is present that prevents ionic movement (e.g. plant roots,
cell membranes). Ψ
m
arises from the attraction of water to the soil
particles and is the most important component of water potential
in all but the most salt affected soils. The MPS-2 and MPS-6 both
measure the matric potential of the soil (Ψ
m
). In highly salt affected
soils, it may be necessary to quantify Ψ
o
independently if you take
measurements of soil water potential relating to biological activity.
3.2
Temperature Measurement
The MPS-2 and MPS-6 use a surface-mounted thermistor to take
temperature readings. The thermistor is located underneath the sen-
sor overmold. The MPS-2 and MPS-6 output temperature readings
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