SOIL WATER POTENTIAL (kPa)
Sensor 1
Sensor 2
Permanant wilting point
Time series TEROS 21 water potential data collected at 20 cm under a dry oak forest in
Switzerland (Walthert 2013). Note the range extends well beyond permanent wilting point.
3.3.4 TEMPERATURE MEASUREMENT
TEROS 21 uses a surface-mounted thermistor to take temperature readings. The thermistor
is located underneath the sensor epoxy. The TEROS 21 output temperature readings in
degrees Celsius unless otherwise stated in preference settings in METER software programs.
If the black plastic body of the sensor is exposed to solar radiation, the temperature
measurement may read high. Exposure of the body also drastically decreases the life
expectancy of the sensor. Do not install the sensor with the body above ground.
3.4 CONSIDERATIONS
TEROS 21 sensors use similar technology to METER water content sensors and are
susceptible to the same constraints. Using TEROS 21 in certain environments will require
additional considerations.
3.4.1 MEASURING IN FROZEN SOILS
TEROS 21 measures the dielectric permittivity of two ceramic discs to measure their water
content and then derive their water potential. The dielectric permittivity of water in the
ceramic discs is 80 compared to a dielectric permittivity of ~5 for the ceramic material or
1 for air. When water freezes to ice, the dielectric permittivity drops to 5 at the frequency
of the sensor measurement, meaning that the sensor can no longer accurately measure
the water in the ceramic. TEROS 21 does not accurately measure water potential in frozen
soil conditions. However, the water potential of the soil under frozen soil conditions can be
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