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Enabling better global research outcomes in soil, plant & environmental monitoring.

TEROS 21 Water Potential Sensor

The TEROS 21 is a matrix water potential sensor that provides long term, maintenance-free soil water potential and temperature readings at any depth without sensitivity to salts. The range of the TEROS 21 goes from field capacity to air dry. Lab and field tests indicate that it can make accurate measurements at water potentials at least as dry as permanent wilting point.

The TEROS 21 features:

  • Dramatically Improved Accuracy – Accuracy (±10% of the reading) comes from the TEROS 21’s six point factory calibration. Compare with confidence to other TEROS 21 sensors or to other measures of water potential.
  • Tough, Long-Lasting Body – Epoxy overmolding will withstand even difficult field conditions. Sensors are rated for long-term research studies.
  • No Recalibration – Other matrix potential sensors have a tendency to degrade over time. The TEROS 21 uses a silica based ceramic material that doesn’t degrade and doesn’t need replacement or recalibration.
  • Insensitivity to Salts -The sensor is accurate in salty environments, a variety of soils, and even in locations where the salinity conditions change over time.
  • Affordability – Breakthrough calibration technique gives the TEROS 21 the accuracy of an individually calibrated sensor at a price comparable to most uncalibrated sensors.
  • Excellent Range – The TEROS 21 has sensitivity from -9 kPa all the way to air dry (-100,000 kPa), with accurate measurements to at least permanent wilting point.
  • Onboard Temperature Measurement  Temperature measurements let you evaluate another important growing condition without adding a sensor.
  • Plug and Play Capability  Just install the sensor, plug it into the Em50 series logger, set the clock and measurement intervals, and start logging data. No programming required.
  • Remote access to data  Use the TEROS 21 with an Em50G data logger and access your data from your office, lab, classroom, or anywhere with an internet connection.
  • SDI-12 Compatible – The TEROS 21 is compatible with nearly all major commercial data loggers through SDI-12, a universal communications protocol.

The Meter TEROS 21 was previously the Decagon MPS-6.


Utilising the TEROS-21 Sensor in a custom IoT (Internet of Things) package

The SNiP-MPS is a ‘Sensor Node Integrated Package’ for LoRaWAN or CAT-M1 communication of real-time soil moisture potential and temperature measurements for continuous soil monitoring.

The base SNiP-MPS integrates an S-NODE and 1 TEROS-21 soil moisture sensor (previously the Decagon MPS-6) to a site’s unique network, communication and power requirements. The S-NODE can support an added 3 TEROS-21 devices.

How it Works

TEROS 21 sensors are composed of a moisture content sensor and a porous substrate with a known moisture release curve. After the porous material has equilibrated with the surrounding soil, the moisture sensor measures the water content of the porous material, and the sensor uses the moisture release curve to translate moisture content into water potential.

Range vs. Accuracy

A sensor’s range depends on the variation in pore sizes in the porous substrate; the wider the range of pore sizes, the bigger the measurement range. Commercially available ceramics are designed to have a uniform pore size, which limits their range. The TEROS 21 uses a ceramic specifically designed with a wide pore size distribution for wider measurement range.

However, a sensor’s accuracy depends on how well the moisture release curve characterises the porous substrate in that particular sensor. The more consistent the substrate is from sensor to sensor, the more accurate each sensor will be. Widely varied pore sizes lead to inconsistency from sensor to sensor, putting these two critical sensor goals in conflict.

Calibration Solution

This conflict can be resolved by individually calibrating each sensor. This has always been a time-consuming and expensive process, however.

The TEROS 21’s accuracy comes from breakthrough calibration methods that allow sensors to be individually calibrated using an automated calibration apparatus. These new techniques make the TEROS 21 the first low cost matrix potential sensor with research-grade accuracy

Accuracy Soil Water Potential: ±(10% + 2 kPa) from -9 to -100 kPa (see manual for additional accuracy specifications past -100 kPa)
Soil Temperature: ± 1°C
Resolution Soil Water Potential: 0.1 kPa
Soil Temperature: 0.1°C
Range Soil Water Potential: -9 to -100,000 kPa
Soil Temperature: -40° to 60°C
– Sensors can be used at higher temperatures under some conditions. Contact us for more details.
Measurement Speed 150 ms (milliseconds)
Equilibration time 10 min to 1 hr depending on soil water potential
Sensor Type Frequency domain with calibrated ceramic discs, thermistor
Output RS232 (TTL) with 3.6 volt levels or SDI-12 communication protocol
Operating Environment -40° to 60°C
– Sensors can be used at higher temperatures under some conditions. Contact us for more details. Water potential measurements will not be accurate below 0°C.
Power 3.6 – 15 VDC, 0.03 mA quiescent, 10 mA max during 150 ms measurement
Cable Length 5m, custom cable lengths available
Cable Connector Types 3.5 mm “stereo” plug or stripped and tinned lead wires (3)
Sensor Dimensions 9.6 cm (l) x 3.5 cm (w) x 1.5 cm (d)
Data Logger Compatibility (not exclusive) Meter/Decagon Em50 Series (rev 2.13+), ProCheck (rev 1.53+), any SDI-12-capable data logger
Warranty One year, parts and labour