Soil water potential lab instrumentation


The potential for error when measuring soil water potential

Measuring soil water potential is never easy. Traditional methods such as pressure plates or filter paper have always been problematic. Not only are they extremely time consuming, but both methods have issues with accuracy. That’s why we developed the WP4C.

Simply accurate. Simply fast. Simple to use.

As world experts in water potential and soil suction, it wasn’t enough to engineer an instrument that delivered consistent accuracy. We also designed it to be easy to use and take only minutes to register measurements, even in dry soils.

How a chilled mirror results in stone cold precision

When it comes to soil water potential, measurements don’t get any more precise. That’s because the WP4C measures two key components of water potential (matric and osmotic) using fundamental thermodynamics and a finely-tuned calibration. Not only is this a first principles method that every other method gets calibrated from, but it’s also been published extensively.

The WP4C measures water potential by determining the relative humidity of the air above a sample in a sealed chamber (conforms to ASTM D6836). Once the sample comes into equilibrium with the vapor, relative humidity is determined using the chilled mirror method. This involves chilling a tiny mirror until dew starts to form. At the dew point, the WP4C measures both mirror and sample temperature within 0.001 °C. This allows for unparalleled accuracy in the -0.1 MPa to -300 MPa range so you can have full confidence in sample readings.

Measuring soil water potential has never been simple. Until now.

The WP4C is a complex instrument due to its versatility, but extremely easy to use with sample sizes up to 7 ml. Simply fill half of the cup with soil, leaves or seeds, and then equilibrate the sample.

Only using partial moisture release curves? Get what you’ve been missing.

Now, generate full, high-resolution moisture release curves across the entire moisture range by combining WP4C data with HYPROP data. No other method generates a curve with this much detail, or with this little effort.

If you only need the dry end of the curve, the HYPROP FIT Software can be used to plug in water potential data collected by the WP4C for fitting different water retention models (i.e., van Genuchten, van Genuchten Bimodal, Fredlund & Xing, Brooks & Corey).

Combine the WP4C with other LABROS instruments for a complete soil analysis. Add the PARIO for soil particle size analysis, and use data from the HYPROP and the KSAT to generate a hydraulic conductivity curve. 

Satisfies the need for speed

The design of the WP4C is incredibly efficient in a number of ways. To start, you don’t have to spend time teaching your technicians. Just watch a simple video before you begin making measurements. Similarly, it allows for fast equilibration because of sophisticated temperature control. And a final feature that makes for a total time saver: it makes measurements on its own, so you’re free to attend to other things.

Top marks in soil measurement

Unparalleled accuracy. Simplicity of use. Fast speeds. The WP4C meets the highest of instrumentation standards and more. As a result, it instills complete trust and eliminates confusion, which combine to add up to considerable time savings.

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  • Precise mode
  • Chilled mirror dew point technique
  • Fast equilibration
  • Unparalleled accuracy in the -0.1 MPa to -300 MPa range
  • Durable and easy to clean
  • Easy to calibrate with saturated salt solutions
  • Conforms to ASTM D6836
  • Use with HYPROP to create a full soil moisture release curve



±0.05 MPa* from 0 to -5 MPa;
1% from -5 to -300 MPa


-0.1 to -300 MPa*

Measurement time

In precise mode:
10-15 minutes for most soil samples
20 minutes for plant tissue samples

In fast mode:
<5 minutes (reduced accuracy)

Temperature control

15 to 40 °C (± 0.2 °C)

Sensor type

1) Chilled-mirror dew point sensor
2) Infrared temperature sensor

Operating environment

5 to 40 °C (41 to 104 °F)


1 year, parts and labor

Sample cup capacity

7 mL recommended (11 mL full)


3.2 kg (5.2 kg shipping weight)


24.1 x 22.9 x 8.9 cm (9.5 x 9.0 x 3.5 in)

Case material

Powder-painted aluminum

Data communication

RS232A compatible
8-data bit ASCII code
9600 baud
no parity
1 stop bit


110 V to 220 V AC 50/60Hz


20 x 2 alphanumeric dot-matrix LCD with backlighting

Interface cable

Standard RS232 serial cable (included)


Samples wetter than -0.5 MPa will have increasing percentages of error. Some users may be able to make useful measurements to -0.1 MPa using techniques outlined in the user manual.

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 or problem? Our support team can 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.

Phone US: +1 509-332-5600
Phone Europe: +49 89 12 66 52 0


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