Temperature Influences During Measurements; Vapour Pressure Influence On Pf/Wc - UMS T5 User Manual

Useful notes

6.3 Temperature influences during measurements

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 R = Gas constant (8,31J/mol K)
M = Molecular weight
p = Saturation vapour pressure at soil temperature
o
(from Scheffler/Straub, Grigull)

6.4 Vapour 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 vapour pressure which antagonizes the water
tension.
Temperature
in °C
Pressure
change per
Kelvin in [hPa]
 
R
 


M
p = Vapour pressure
4 10 16
0,6 0,9 1,2

  
T
 
 
ln

  
o
20 25
1,5 1,9
38/48
30 50 70
2,5 7,2 14