15
TEROS 11/12
3.3 THEORY
The following sections explain the theory of VWC, temperature, and EC (TEROS 12 only)
measured by TEROS 11/12.
3.3.1 VOLUMETRIC WATER CONTENT
TEROS 11/12 sensors use an electromagnetic field to measure the dielectric permittivity
of the surrounding medium. The sensor supplies a 70-MHz oscillating wave to the sensor
needles, which charge according to the dielectric of the material. The charge time is
proportional to substrate dielectric and substrate VWC. The TEROS 11/12 microprocessor
measures the charge time and outputs a raw value based on the substrate dielectric
permittivity. The raw value is then converted to VWC by a calibration equation specific to the
substrate (
Section 4.1
).
3.3.2 TEMPERATURE
The TEROS 11 uses a small thermistor mounted on the PCB near the lower needle. The
TEROS 11 temperature measurement is optimized to be accurate when the TEROS 11 is
fully buried in soil or substrate. The TEROS 12 uses a thermistor in the center needle to take
temperature readings and responds faster to temperature changes and is better suited for
needle-only insertion into greenhouse and nursery substrates. The TEROS 11/12 sensors
output temperature is in degrees Celsius unless otherwise stated in the data logger program,
such as in preferences in the ZENTRA software.
NOTE: Even though the sensor head is white, in direct sunlight, the temperature measurement may read high. Use
caution when installing the sensor with the sensor head in the sun.
3.3.3 ELECTRICAL CONDUCTIVITY (TEROS 12 ONLY)
EC is the ability of a substance to conduct electricity and can be used to infer the amount
of ions that are present in solution. EC is measured by applying an alternating electrical
current to two electrodes and measuring the resistance between them. Bulk EC is
derived by multiplying the inverse of the resistance (conductance) by the cell constant
(the ratio of the distance between the electrodes to their area). TEROS 12 sensor bulk
EC measurements are normalized to EC at 25
°
C. The bulk EC measurement is factory
calibrated to be accurate over the range found in the vast majority of soil and growth
substrates.
EC measurements above 10 dS/m are sensitive to contamination of the electrodes by skin
oils, etc. Follow cleaning instructions in
Section 4.2
if measuring the EC of salty soils.
3.3.4 CONVERTING BULK EC TO PORE EC (TEROS 12 ONLY)
For many applications, it is advantageous to know the EC of the solution contained in the
soil pores (
p
), which is a good indicator of the solute concentration in the soil. Traditionally,
p
is obtained by extracting pore water from the soil and measuring
p
directly, a time-
consuming and labor-intensive process. The TEROS 12 sensor measures the EC of the bulk