User Manual

253-L and 257-L Soil Matric Potential Sensors
7.2.3.2 Non-Linear Relationship
For more precise work, calibration and temperature compensation in the range
of 10 to 100 kPa has been refined by Thompson and Armstrong (1987), as
defined in the non-linear equation,
])01060.021.34(062.1[01306.0
sss
RTT −+−
2
s
R
SWP =
where SWP is soil water potential in kPa
TABLE 7-3. Comparison of
Estimated Soil Water Potential
and R
s
at 21°C
kPa (Non-
Linear
Equation)
kPa
(Linear
Equation)
(R
s
)
kOhms
–3.7 1.00
–9 –11 2.00
–14 –18 3.00
–20 –26 4.00
–27 –33 5.00
–35 –41 6.00
–45 –48 7.00
–56 –56 8.00
–69 –63 9.00
–85 –70 10.00
–105 –78 11.00
–85 12.00
–92 13.00
–99 14.00
–107 15.00
–115 16.00
–122 17.00
–129 18.00
–144 20.00
–159 22.00
–174 24.00
–188 26.00
–199 27.50
18