tensiometer style, no maintenance, soil tension/suction/potential sensor


The TEROS 21 is a soil water potential sensor manufactured by METER Group (formally Decagon Devices).

The TEROS 21, also known as the MPS-6, is the latest technology in soil water potential measurements. It performs measurements similar to a tensiometer but without any of the difficulties of tensiometers. The TEROS 21 has zero maintenance - install in your soil, connect to a data logger or handheld meter, and start measuring. No water columns and no refilling is required.

It supersedes old technology, such as tensiometers and gypsum blocks, because it is more reliable, accurate, requires no maintenance, and it is digital. The TEROS 21 is a must have for anyone who is serious about soil water potential measurements.

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:
  • Supersedes redundant soil water potential sensors such as tensiometers and gyspum blocks.
  • Dramatically Improved Accuracy - Accuracy 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 -5 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 ZL6 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 ZL6 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.


more details

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.


how to install TEROS 21 video



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 characterizes 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.


whole system monitoring solutions

Sap Flow MeterEdaphic Scientific is a one-stop shop for a whole system monitoring solution. We provide plant and soil monitoring systems for researchers and growers.

Our systems not only support soil sensors, but related sensors such as sap flow, dendrometers, weather parameters, and more.

At Edaphic Scientific we want to work with you from the start of your project through to its completion. We can provide:

  • Assistance with project and experimental design
  • Procurement of all monitoring equipment, including sensors, data loggers and data management software. Edaphic Scientific is a one-stop shop where we can source and find any necessary equipment for your project from our preferred suppliers or third party suppliers
  • Installation and training
  • On-going assistance with data interpretation and equipment maintenance
  • Data correction and analysis, including statistical analysis with the R-package
  • Report and publication preparation including tables, figures, graphs, and manuscript writing


advanced data collection and management solutions

Edaphic Scientific recognises the need for flexible and adaptable sensor and data logging solutions for experimental or environmental monitoring projects.

Data can be downloaded directly in the field from data loggers. A direct connection between the data loggers and your computer, via a USB cable, can be used for manual downloading of data.

Alternatively, data can be downloaded over the internet on your iPhone, iPad or desktop computer with the Eagle.io cloud-based, data management software solutions. Through this remote based downloading capabilities, you can download, view and manage your data, and system, anywhere in the world and at anytime.


feature specification
Measurement Principle Frequency domain with calibrated ceramic discs, thermistor
Measurement Range Soil Water Potential: -5 to -100,000 kPa Soil Temperature: -40° to 60°C
Resolution Soil Water Potential: 0.1 kPa Soil Temperature: 0.1°C
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
Power Requirements 3.6 - 15 VDC, 0.03 mA quiescent, 10 mA max during 150 ms measurement
Output SDI-12
Cable Length 5 m (standard) | 75 m (maximum)
Sensor Dimenions 9.6 cm (l) x 3.5 cm (w) x 1.5 cm (d)

manual & docs

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