Water potential in the lab


What’s new with TEROS 31?

The TEROS 31 tensiometer combines our world-renowned precision technology with the power of ZENTRA Cloud, giving you easier, faster water potential data in near-real time.

And it’s plug and play with the ZL6 data logger, making setup a breeze.

Tiny tensiometer tackles big problems

If you need spot measurements of water potential in soil columns, soil cores, or sampling rings, there aren’t a lot of options. Until now. With the TEROS 31, we put our 25 years of expertise to work in the smallest space possible: a ceramic tip with a surface area of only 0.5 cm2. It’s the only tensiometer in the world small enough and precise enough to perform excellent water potential spot measurements in even the tightest spaces. And now it’s even better. 

Double your data power

The TEROS 31 combines METER world-renowned precision technology with the power of ZENTRA Cloud, giving you easier, faster water potential data in near-real time. And compatibility with the ZL6 data logger means precision is now plug-and-play, making setup a breeze. The TEROS 31 minor footprint allows major advantages over larger tensiometers, such as very little soil disturbance and incredibly fast response time. And, because of its small size, it’s one of the only tensiometers in the world that can extend its measuring range.  

Lab tensiometer with heroic strength and speed

TEROS 31 is more robust than ever before. Technological advancements have made it possible to incorporate a more ruggedized, highly precise pressure transducer, giving you higher data resolution in an almost unbreakable form. And the TEROS 31 tensiometer has a superheroic response time of only 5 seconds for a pressure change of 0 to –85 kPa. It reacts much faster to changing soil conditions because of its small water volume, enabling you to measure even the most minute changes in water potential—something lower-quality tensiometers cannot do. It measures matric potential within the gravitational range where most water movement occurs and into the capillary range, helping you understand whether water will move and where it will go.

Tensiometer measurement range—extended

Most tensiometers have a measuring range of at least 100 to –85 kPa. But the TEROS 31 increases the matric potential measuring range to nearly –150 kPa. How is this possible? Normally, when a tensiometer reaches –85 kPa, the water boils, forming an air bubble. The air bubble expands and contracts with changes in pressure, making the tensiometer unable to measure suction. But the TEROS 31 retards the boiling point, extending the measurement range well beyond normal limits.

Setup in seconds

Now you can spend far less time on complex setup. Just insert the TEROS 31 stereo plug into the ZL6 data logger, and start seeing numbers. ZENTRA Cloud makes it possible to see near-real-time data wherever you are. TEROS 31 can be installed in any position and orientation with a miniature auger (not included) to ensure as little soil disturbance as possible.  For spot measurements, just auger a 5-mm hole and insert the tensiometer. It’s that simple. To save you even more time and effort, bubbles are detectable through the transparent shaft, making it easy to see when it’s time to refill.

Small tensiometer—huge benefits

For more than a quarter of a century, METER has been the leading expert in the development of tensiometers with over 10,000 sold. Nobody in the world makes an instrument this small, this simple, or this precise. Trust the TEROS 31 lab tensiometer for fast, accurate spot measurements in all your tightest spaces.

Request a quote Contact us See TEROS 32 field tensiometer


  • Laboratory tensiometer
  • Easy data visualization in real time: plug and play with ZL6 logger and ZENTRA Cloud
  • Small and fast
  • Little soil disturbance
  • Extended range
  • Install tensiometer in any position or orientation
  • Bubbles easily detectable through the transparent shaft
  • Output signals are balanced
  • Accurate zero-tension reading




Water potential

Range: –85 to +50 kPa (up to –150 kPa during boiling retardation)
Resolution: ±0.0012 kPa
Accuracy: ±0.15 kPa


Range: –30 to +60 °C
Resolution: ±0.01 °C
Accuracy: ±0.5 °C

NOTE: If the sensor unit is not buried, measured temperature may diverge from soil temperature.



DDI serial
SDI-12 communication protocol
TensioLINK communication protocol
ModbusTM RTU communication protocol

Data logger compatibility

METER ZL6, EM60, and Em50 data loggers or any data acquisition system capable of 3.6–28.0 VDC excitation and SDI-12, ModbusTM RTU, or tensioLink communication.



Width: 2.35 cm (0.93 in)
Depth: 1.75 cm (0.69 in)
Height: 4.90 cm (1.93 in)

Tensiometer shaft diameter

5 mm (0.19 in)

Tensiometer shaft length

2, 5, 7, 10, 15, or 20 cm

Operating temperature range

Minimum: 0 °C
Maximum: 50 °C


Ceramic: Al2O3, bubble point 500 kPa
Shaft: PMMA
Sensor unit: PMMA and TPE

Cable length

1.5 m

Cable diameter

0.165 ± 0.004 (4.20 ± .10 mm) with minimum jacket of 0.030 (0.76 mm)

Connector types

3.5-mm 4-pin stereo plug connector

Stereo plug connector diameter

3.50 mm

Conductor gauge

22 AWG/24 AWG drain wire


Supply voltage (VCC to GND)

Minimum: 3.6 V
Typical: 12.0 V
Maximum: 28.0 V

Digital input voltage (logic high)

Minimum: 1.6 V
Typical: 3.3 V
Maximum: 5.0 V

Digital input voltage (logic low)

Minimum: –0.3 V
Typical: 0.0 V
Maximum: 0.9 V

Digital output voltage (logic high)

Typical: 3.6 V

Power line slew rate

Minimum: 1.0 V/ms

Current drain (during measurement)

Minimum: 18.0 mA
Typical: 25.0 mA
Maximum: 30.0 mA

Current drain (while asleep)

Minimum: 0.03 mA
Typical: 0.05 mA
Maximum: 0.90 mA

Power up time (DDI serial)

Minimum: 125 ms
Typical: 130 ms
Maximum: 150 ms

Power up time (SDI-12)

Minimum: 125 ms
Typical: 130 ms
Maximum: 150 ms

Power Up Time (SDI-12, DDI disabled)

Minimum: 125 ms
Typical: 130 ms
Maximum: 150 ms

Measurement duration

Minimum: 60 ms
Typical: 65 ms
Maximum: 70 ms


Manufactured under ISO 9001:2015
EM ISO/IEC 17050:2010 (CE Mark)

California requires the following notice:

⚠️ WARNING: This product may contain chemicals including ethylene glycol, lead, nickel, styrene and cadmium, which are known to the State of California to cause cancer, birth defects or other reproductive harm. For more information, go to www.P65Warnings.ca.gov



Have a question? Our team is ready to help!

We manufacture, test, calibrate, and repair every instrument in-house. Our scientists and technicians use the instruments every day in our product testing lab. No matter what your question is, we have someone who can help you answer it.

Email Europe: sales.europe@metergroup.com
Phone US: +1 509-332-5600
Phone Europe: +49 89 12 66 52 0

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