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Useful notes
7.2
Temperature influences
If the sensor is not powered continuously the voltage should be
switched on 10 seconds before a measurement. In this case, the self
heating is negligible.
The correlation of water tension to water content is temperature
dependent. The influence is low at tensions of 0 to 10 kPa 0 ...
0,6 kPa/K, but high for tensions over 100 kPa:
= Water tension
M = Molecular weight
p
= Saturation vapor pressure at soil temperature
o
(from Scheffler/Straub, Grigull)
7.3 Vapor pressure influence on pF/WC
If the temperature of a soil with a constant water content rises from
20°C to 25°C the soil water tension is reduced for about 0,85 kPa
due to the increased vapor pressure which antagonizes the water
tension.
Temperature
in °C
Pressure
change per
Kelvin in [hPa]
7.4 Osmotic effect
The ceramic has a pore size of r = 0,3 m and therefore cannot block
ions. Thus, an influence of osmosis on the measurements is
negligible because ion concentration differences are equalized
quickly. If the T8 cup is dipped into a saturated NaCl solution the
reading will be 1 kPa for a short moment, then it will drop to 0 kPa
again.
R
M
R = Gas constant (8,31J/mol K)
p = Vapor pressure
4
10
16
0,6
0,9
1,2
T
ln
o
20
25
1,5
1,9
46/58
30
50
70
2,5
7,2
14
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