SWT4 User Manual v3.1
Useful notes
39
6.2 Temperature influences
If the sensor is not powered continuously the voltage should be
switched on for no longer than 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 (1 bar, 1000 hPa):
o
M
T
R
ln
= Water tension
R = Gas constant (8,31J/mol K)
M = Molecular weight p = Vapor pressure
p
o
= Saturation vapor pressure at soil temperature
(from Scheffler/Straub, Grigull)
6.3 Vapor pressure influence
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 opposes the water tension.
Temperature in °C
4
10
16
20
25
30
50
70
Pressure change per Kelvin in
[hPa]
0.6
0.9
1.2
1.5
1.9
2.5
7.2
14
6.4 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
SWT4 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.
