Soil water potential lab instrumentation


The potential for error when measuring 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. 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.

Proven first-principles method based on fundamental physics

The WP4C is so accurate, it’s used to calibrate other measurement methods and has been published extensively. Why? The dew point sensor inside the WP4C is a primary measure of water potential, not some secondary parameter merely correlated with water potential. It measures the combined matric and osmotic potential using fundamental thermodynamics and a finely-tuned calibration. Here’s how it works:

The WP4C determines 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.

Only using partial 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).

Save time and effort

The design of the WP4C is incredibly efficient in a number of ways. To start, you don’t have to spend a ton of time teaching your technicians. Plus, 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.

World-class soil water potential measurement

Unparalleled accuracy. Simplicity of use. Fast speeds. 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. 

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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


View GSA details


Water potential

Range: 0 to –300 MPa
Resolution: NA
Accuracy: ±0.05 MPa from 0 to –5 MPa
1% from –5 to –300 MPa

NOTE: All vapor pressure instruments (including the WP4C) are limited by accuracy in the wet end of the water potential range. The range of 0 to −5 MPa has an accuracy of ±0.05 MPa. For example, a measurement of −0.1 MPa has an accuracy of ±50% of the measurement and a measurement of −1 MPa has an accuracy of ±5%. The WP4C will not measure water potential accurately near field capacity (-0.033 MPa).


Range 15-40 °C
Resolution 0.1 °C
Accuracy ±0.2 °C

Read time

Soil sample:
~10–15 min (precise mode)
<5 min (fast mode)

NOTE: WP4C will display updated measurements approximately every 5 min until stopped

Plant sample:
~20 min


Case dimensions

Length 24.1 cm (9.5 in)
Width 22.9 cm (9.0 in)
Height 8.9 cm (3.5 in)

Case material

Powder painted aluminum

Sample cup capacity

15 mL (0.5 oz) full
7 mL (0.25 oz) recommended


3.2 kg (7.1 lb)


20 x 2 alphanumeric LCD with backlighting

Sensor types

Chilled-mirror dew point sensor
Infrared temperature sensor

Operating temperature

Minimum 5 °C
Typical NA
Maximum 40 °C

Data communications

RS-232A serial
8-data bit ASCII code
9,600 baud, no parity
1 stop bit

Interface cable

Standard RS-232 to USB cable (included)


110–220 VAC
50/60 Hz


Manufactured under ISO 9001:2015
EM ISO/IEC 17050:2010 (CE Mark)
Compatible standard: ASTM D6836-07

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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