TEROS 21

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TEROS 21

Soil Water Potential Sensor

The TEROS 21 matric potential sensor is incredibly easy to use and surprisingly affordable.

Full-range soil water potential sensor
Accurate. Easy to use. No recalibration.
Onboard temperature measurement

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Overview / Features

Do not settle for less

When it comes to measuring water potential (or soil suction), it’s hard to find a sensor that meets your every need. You’re either forced to deal with lowered accuracy or high-maintenance hassles (plus getting soaked on cost). That’s why we invented TEROS 21.

Broad soil applications. Dependable accuracy.

To say TEROS 21 is more accurate than competitor sensors doesn’t do it justice. That’s because unlike competitor models, we calibrate each and every sensor for you using a process that we have spent years refining and perfecting, so the TEROS 21 water potential sensor can come to a fixed water potential. The result: a long-term monitoring solution you can finally trust.

A true full-range water potential sensor that’s low maintenance and low cost

The TEROS 21 water potential sensor is incredibly easy to use. It requires no maintenance, and it’s accurate enough for most applications. In fact, the TEROS 21 provides an even more accurate soil moisture picture than measuring water content alone. A water content sensor only shows the percentage of water in the soil, but add a TEROS 21 water potential sensor, and you’ll know if that water is available to plants and where it will move. Plus, unlike water content, matric potential isn’t dependent on soil type, so you can compare moisture between different sites. Not only that, the TEROS 21 is surprisingly affordable, and the new Gen 2 version boasts an improved circuit, a more robust microprocessor, and an improved measurement range. It now measures all the way from near saturation to air dry (0 to −100,000 kPa) making it the world’s first true full range water potential sensor.

The only worry-free soil water potential sensor

Ease of use isn’t something you normally associate with water potential measurement devices. Until now. That’s because TEROS 21 is plug and play in a number of ways. First, once it’s in the ground, the durable epoxy coating ensures long-lasting usage. Second, no maintenance is involved. That means no refilling. And no worrying about frozen conditions. Lastly, the TEROS 21 water potential sensor is also easy to integrate into systems (SDI-12 compatible) so it can be used with third-party loggers. All this adds up to saving you time and a lot of unnecessary labor.

Soil water measuring that’s immeasurable in value

Accurate. Easy to use. Affordable. The TEROS 21 water potential sensor outperforms in every aspect because we specifically designed it to save you time, hassle and money.

Feature summary

Easy to use
Improved accuracy comes from the six-point factory calibration
Tough, long-lasting body
No recalibration
Low salt sensitivity
Affordability
Excellent range (sensitivity from 0 kPa all the way to air dry [-100,000 kPa])
Onboard temperature measurement
Plug and play capability
Use with the ZL6 for remote access to data on the cloud
SDI-12 compatible

Specifications: TECHNICAL SPECIFICATIONS

Measurement Specifications

Water Potential

Range: 0 to −100,000 kPa (1.70 to 6.00 pF)

Resolution: 0.1 kPa

Accuracy: ±(10% of reading + 2 kPa) from −100 to −5 kPa

NOTE: TEROS 21 Gen 2 can read up to 0 kPa when on a wetting path. The air entry of the soil limits the performance of the sensor to 0 kPa on the drying curve.

NOTE: TEROS 21 is not well calibrated beyond -100 kPa. For more information on using the TEROS 21 beyond this range, see Section 3.3.3 in the user manual

Temperature

Range: -40.00 – 60.00 °C

Resolution: 0.10 °C

Accuracy: ±1.00 °C

Dielectric Measurement Frequency

70 MHz

Communication Specifications

Output

DDI serial or SDI-12 communication protocol

Data Logger Compatibility

METER ZL6, EM60, and Em50 data loggers or any data acquisition system capable of 3.6- to 15-VDC power and serial or SDI-12 communication

Physical Specifications

Dimensions

Length: 9.6 cm (3.8 in)

Width: 3.5 cm (1.4 in)

Height: 1.5 cm (0.6 in)

Sensor Diameter

3.2 cm (1.3 in)

Operating Temperature Range

Minimum: -40.00 °C

Maximum: 60.00 °C

NOTE: Sensors may be used at higher temperatures under certain conditions; contact Customer Support for assistance.

Cable Length

5 m (standard)
75 m (maximum custom cable length)

NOTE: Contact Customer Support if a nonstandard cable length is needed.

Connector Types

3.5-mm stereo plug connector or stripped and tinned wires

Electrical and Timing Characteristics

Supply Voltage (VCC to GND)

Minimum: 3.6 VDC

Maximum: 15.0 VDC

Digital Input Voltage (Logic High)

Minimum: 2.8 V

Typical: 3.6 V

Maximum: 5.0 V

Digital Input Voltage (Logic Low)

Minimum: -0.3 V

Typical: 0.0 V

Maximum: 0.8 V

Power Line Slew Rate

Minimum: 1.0 V/ms

Current Drain (During Measurement)

Minimum: 3.0 mA

Typical: 5.0 mA

Maximum: 16.0 mA

Current Drain (While Asleep)

Minimum: 0.0 mA

Power Up Time (DDI Serial)

Maximum: 50 ms

Power Up Time (SDI-12)

Typical: 175 ms

Measurement Duration

Typical: 175 ms

Other

Product Awards

Winner of the 2022 AE50 Innovation Award

Compliance

EM ISO/IEC 17050:2010 (CE Mark)
EN 55011:2016 / A1:2017 (RCM Mark)

Prop 65 warning

EU Declaration of Conformity

GSA

View GSA details

Support / FAQ

TEROS 21 Quick Start

Quickstart Guide

PDF, 1.4MB

TEROS 21 Manual (Gen 2)

Manual

PDF, 1.1MB

TEROS 21 Integrator Guide (Gen 2)

Integrator Guide

PDF, 0.61MB

TEROS 21 Manual (Gen 1)

Manual

PDF, 1.1MB

TEROS 21 Integrator Guide (Gen 1)

Integrator Guide

PDF, 0.6MB

VIDEO: How to Install TEROS 21 Sensors

Instructions

URL, 0MB

TEROS 21 Firmware Updater Instructions

Firmware

PDF, 1MB

METER Splice Kit Repair Instruction Video

Instructions

URL, 0.0 kb

Sensor wire splicing guide (complete method)

Instructions

PDF, 5MB

Sensor wire splicing guide (quick method)

Instructions

PDF, 0.9MB

TEROS 21 FAQs

Are there water potential case studies for plants and turf available?: There are a lot of case studies about different types of plants, specifically about optimal water potential ranges. There was a paper done by Dr. Sterling Taylor on this topic, and there are also some studies being done by BYU scientists with in situ water release curves in turfgrass. One of our scientists wrote an article (find it here) and gave a webinar (find it here) about some of the turfgrass work being done at BYU.

What sensor might be appropriate in more arid environments when the soil water potential might be very low much of the year?: One of the better sensors for measuring in really dry conditions is a thermocouple psychrometer. The problem is they are not as commercially available and difficult to find. But if you can find one, they are a really useful tool for arid environments.

Can the relationship between soil moisture and water potential in the sensor range of accuracy be used to infer water potential from soil moisture readings in drier conditions?: This is actually a common approach taken by many people. You can try and develop that relationship in situ and infer what the drier condition water potentials are. There are functions available such as different van Genuchten functions to try and fit those data.

How is the sensitivity of capacitance sensors to the resistivity of pore water (chemical composition)?: Capacitance sensors will be affected by salt concentrations in soils when they become higher. Typically, we start seeing issues when the saturated extract EC is higher than 3 dS/m. This can be hard to correct for since the sensors don’t measure EC. If you had another sensor nearby measuring the bulk EC of the soil, you could potentially make a correction for this.

How is the temperature sensitivity of the capacitance method in the wet side (between 20 to 50 C)? Are there any compensation equations for your sensors?: The temperature sensitivity in the wet range for the TEROS 21 is low. Because there is more water in the ceramic, the temperatures swings don’t have much of an impact on the measurement. I would expect the readings between -10 and -300 kPa to have low sensitivity to that temperature range. Having said that, there is a great paper on temperature compensation for the TEROS 21 that works well. Here is the reference: L. Walthert and P. Schleppi (2018). Equations to compensate for the temperature effect on readings from dielectric Decagon MPS-2 and MPS-6 water potential sensors in soils. J. Plant Nutr. Soil Sci. 2018, 000, 1–11 (article link).

Are water potential sensors available that measure at 2" and 5" at the same time?: Currently, there is not a profile-type water potential sensor. The only way would be to place individual sensors at the desired measurement depths. A profile probe could be a powerful tool for this measurement and is something we may approach in the future.

Is there a paper you can refer me to concerning the effects of digging a trench on the soil at a site?: I don’t have a specific paper to refer to on this topic. The concern with large trenches is the way it affects water movement through the soil near the sensor. Depending on how the trench is repacked you can wind up with preferential flow paths which will result in faster water migration through the soil profile. For more information on this topic, see our article: "5 Ways Site Disturbance Impacts Your Data."

Which is the best sensor for measuring water potential lower than -1 atmosphere for research purposes?: For water potential below -1 atm (-100 kPa), a solid matrix sensor like the TEROS 21 is going to be more appropriate.

What does the -9990 error code or "Sensor value is temporarily out of range" mean?: Water potentials below -2,000 kPa exceed the detection limits of the TEROS 21. When water potential is below -2,000 kPa, the TEROS 21 will report an error code (-9990), and an error message will appear (Sensor value is temporarily out of range).

How can you measure capillary water potential?: Capillary water potential is tied to matric potential. So if you are measuring matric potential with a tensiometer or a TEROS 21, you are essentially measuring the effect of the capillaries or those different pore sizes. You can also use the HYPROP. The WP4C will also work assuming the soil has a negligible osmotic potential.

Do matric potential sensor readings include osmotic potential?: This depends on what type of instrument you are using to measure the potential. For example, tensiometers, granular matric sensors, and the TEROS 21 ONLY measure matric potential. So these sensors are blind to osmotic potential. Laboratory instruments like the WP4C measure both osmotic and matric potential. But in terms of field sensors, there aren’t any that give both components.

How can you measure kPa or MPa? And what tools can you use for container production?: kPa and MPa are really just a preference. You convert between the two by moving the decimal point. In containers, you can use tensiometers which are highly accurate in the wet range but not in the dry range. Matric potential sensors such as the TEROS 21 also work well. They aren’t as accurate as a tensiometer in the wet end, but they give you a better range and require less maintenance.

What are important considerations when thinking about measuring water content and water potential in peatlands (with organic soils)?: Variability of your substrate is a big one. There is a lot of variability in soils as well, but we have better mechanisms to capture and account for variability in mineral soils. Good substrate-to-sensor contact is critical and trickier to accomplish (good installation), but it is achievable. You will most likely require a custom calibration for water content.

Would you agree that with the impact of soil moisture on the atmosphere, measuring water content alone is not enough?: It depends on your specific goals. If you are studying the impact of soil water on atmospheric impact then you would need water potential. There are plenty of cases where water content alone is sufficient if you also have information about your soil.

If I use the TEROS 21 to measure soil water potential when planning irrigation, do I need to know the soil types?: No. With the TEROS 21 you just need to know the matric potential limits of your plants, and you do not need to worry about soil type.

What is the matric potential?: Matric potential is the force that would need to be exerted to move a water molecule from the surface of a soil particle. For example, a matric potential of -100 kPa would require a force of -101 kPa to pull that water molecule off of the soil particle. It is one component of the total water potential. Learn more about the different components of water potential here.

Resources / Publications

Resource links

Case studies

Selected Publications

Listed below are examples of cited publications for the TEROS 21 soil matric potential sensor. This list is not exhaustive. The MPS-6 matric potential sensor was renamed TEROS 21 in 2015, but they are both the same sensor.

2020

Wang, Hao, Ankit Garg, Shan Huang, and Guoxiong Mei. “Mechanism of compacted biochar‑amended expansive clay subjected to drying–wetting cycles: simultaneous investigation of hydraulic and mechanical properties.” Hydrology, (2020). (Article link).
Holdo, Ricardo M., Daphne A. Onderdonk, Annabelle G. Barr, Meshak Mwita, and T. Michael Anderson. “Spatial transitions in tree cover are associated with soil hydrology, but not with grass biomass, fire frequency, or herbivore biomass in Serengeti savannahs.” Journal of Ecology 108, no. 2 (2020): 586-597. (Article link).
Kukal, Meetpal S., Suat Irmak, and Kiran Sharma. “Development and Application of a Performance and Operational Feasibility Guide to Facilitate Adoption of Soil Moisture Sensors.” Sustainability 12, no. 1 (2020): 321. (Article link).
Rukhaiyar, Saurav, Shan Huang, Haihong Song, Peng Lin, Ankit Garg, and Sanandam Bordoloi. “A New Intelligent Model for Computing Crack in Compacted Soil-Biochar Mix: Application in Green Infrastructure.” Geotechnical and Geological Engineering 38, no. 1 (2020): 201-214. (Article link).
Torres-Sanchez, Roque, Honorio Navarro-Hellin, Antonio Guillamon-Frutos, Rubén San-Segundo, Maria Carmen Ruiz-Abellón, and Rafael Domingo-Miguel. “A Decision Support System for Irrigation Management: Analysis and Implementation of Different Learning Techniques.” Water 12, no. 2 (2020): 548. (Article link).
Ravi, Sridevi, Tim Young, Cate Macinnis-Ng, Thao V. Nyugen, Mark Duxbury, Andrea C. Alfaro, and Sebastian Leuzinger. “Untargeted metabolomics in halophytes: The role of different metabolites in New Zealand mangroves under multi-factorial abiotic stress conditions.” Environmental and Experimental Botany 173 (2020): 103993. (Article link).

2019

Baker, Kathryn V., Xiaonan Tai, Megan L. Miller, and Daniel M. Johnson. “Six co-occurring conifer species in northern Idaho exhibit a continuum of hydraulic strategies during an extreme drought year.” AoB Plants 11, no. 5 (2019): plz056. (Article link).
Fidantemiz, Yavuz F., Xinhua Jia, Aaron LM Daigh, Harlene Hatterman-Valenti, Dean D. Steele, Ali R. Niaghi, and Halis Simsek. “Effect of water table depth on soybean water use, growth, and yield parameters.” Water 11, no. 5 (2019): 931. (Article link).
Genc, Derya, Jeramy C. Ashlock, Bora Cetin, and Paul Kremer. “Development and Pilot Installation of a Scalable Environmental Sensor Monitoring System for Freeze–Thaw Monitoring under Granular-Surfaced Roadways.” Transportation Research Record (2019): 0361198119854076. (Article link).
Haghverdi, Amir, Brian Leib, Robert Washington-Allen, Wesley C. Wright, Somayeh Ghodsi, Timothy Grant, Muzi Zheng, and Phue Vanchiasong. “Studying crop yield response to supplemental irrigation and the spatial heterogeneity of soil physical attributes in a humid region.” Agriculture 9, no. 2 (2019): 43. (Article link)
Kadioglu, Hakan, Harlene Hatterman-Valenti, Xinhua Jia, Xuefeng Chu, Hakan Aslan, and Halis Simsek. “Groundwater Table Effects on the Yield, Growth, and Water Use of Canola (Brassica napus L.) Plant.” Water 11, no. 8 (2019): 1730. (Article link).
Nielsen, Kristoffer T., Per Moldrup, Søren Thorndahl, Jesper E. Nielsen, Mads Uggerby, and Michael R. Rasmussen. “Field-scale monitoring of urban green area rainfall-runoff processes.” Journal of Hydrologic Engineering 24, no. 8 (2019): 04019022. (Article link).
Rashid Niaghi, Ali, and Xinhua Jia. “New Approach to Improve the Soil Water Balance Method for Evapotranspiration Estimation.” Water 11, no. 12 (2019): 2478. (Article link).
Todesco, Flora, Simone Belmondo, Yoann Guignet, Liam Laurent, Sandrine Fizzala, François Le Tacon, and Claude Murat. “Soil temperature and hydric potential influences the monthly variations of soil Tuber aestivum DNA in a highly productive orchard.” Scientific Reports 9, no. 1 (2019): 1-10. (Article link).
Shaikh, Janarul, Sanandam Bordoloi, Sudheer K. Yamsani, Sreedeep Sekharan, Ravi R. Rakesh, and Ajit K. Sarmah. “Long-term hydraulic performance of landfill cover system in extreme humid region: Field monitoring and numerical approach.” Science of the total environment 688 (2019): 409-423. (Article link).
Zhang, Xufeng, Arseniy Andreyev, Colleen Zumpf, M. Cristina Negri, Supratik Guha, and Monisha Ghosh. “Thoreau: A fully-buried wireless underground sensor network in an urban environment.” In 2019 11th International Conference on Communication Systems & Networks (COMSNETS), pp. 239-250. IEEE, 2019. (Article link).

2018

Eliades, Marinos, Adriana Bruggeman, Hakan Djuma, and Maciek W. Lubczynski. “Tree water dynamics in a semi-arid, Pinus brutia forest.” Water 10, no. 8 (2018): 1039. (Article link).
Karagoly, Yahya, Snehasis Tripathy, Peter John Cleall, and Talib Mahdi. “Suction measurements by a fixed-matrix porous ceramic disc sensor.” In Proceedings of the 7th International Conference on Unsaturated Soils, Hong Kong. 2018. (Article link).
Ket, Pinnara, Chantha Oeurng, and Aurore Degré. “Estimating Soil Water Retention Curve by Inverse Modelling from Combination of In Situ Dynamic Soil Water Content and Soil Potential Data.” Soil Systems 2, no. 4 (2018): 55. (Article link).
Suchoff, David H., Penelope Perkins-Veazie, Heike W. Sederoff, Jonathan R. Schultheis, Matthew D. Kleinhenz, Frank J. Louws, and Christopher C. Gunter. “Grafting the indeterminate tomato cultivar Moneymaker onto Multifort rootstock improves cold tolerance.” HortScience 53, no. 11 (2018): 1610-1617. (Article link).
Suchoff, David H., Jonathan R. Schultheis, Matthew D. Kleinhenz, Frank J. Louws, and Christopher C. Gunter. “Rootstock improves high-tunnel tomato water use efficiency.” HortTechnology 28, no. 3 (2018): 344-353. (Article link).
Suchoff, David H., Christopher C. Gunter, Jonathan R. Schultheis, Matthew D. Kleinhenz, and Frank J. Louws. “Rootstock effect on grafted tomato transplant shoot and root responses to drying soils.” HortScience 53, no. 11 (2018): 1586-1592. (Article link).
Walthert, Lorenz, and Patrick Schleppi. “Equations to compensate for the temperature effect on readings from dielectric Decagon MPS‐2 and MPS‐6 water potential sensors in soils.” Journal of Plant Nutrition and Soil Science 181, no. 5 (2018): 749-759. (Article link).

2016

Guéry, S., J. D. Lea-Cox, M. A. Martinez Bastida, B. E. Belayneh, and F. Ferrer-Alegre. “Using sensor-based control to optimize soil moisture availability and minimize leaching in commercial strawberry production in Spain.” In International Symposium on Sensing Plant Water Status-Methods and Applications in Horticultural Science 1197, pp. 171-178. 2016. (Article link).
Navarro-Hellín, Honorio, Jesús Martínez-del-Rincon, Rafael Domingo-Miguel, Fulgencio Soto-Valles, and Roque Torres-Sánchez. “A decision support system for managing irrigation in agriculture.” Computers and Electronics in Agriculture 124 (2016): 121-131. (Article link).

2016

Genc, Derya, Jeramy Ashlock, Bora Cetin, Kristen Cetin, Masrur Mahedi, Robert Horton, and Halil Ceylan. “Analysis of In Situ Soil Thermal and Hydraulic Data from a Subgrade Sensor Network under a Granular Roadway.” Journal of Cold Regions Engineering (2014). (Article link).