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Enabling better global research outcomes in soil, plant & environmental monitoring.

WP4C Water Potential Meter

Fast, Accurate Water Potential Measurements
Use the WP4C to measure water potential in 5 to 10 minutes. Range: -0.1 to -300 MPa*

The WP4C can be used to measure the water potential of any porous material. It's typically used to analyse soil, soilless substrates, plant material, and seeds, but it has been used to measure a variety of unusual things, including apples, bricks, oil, and insect carapaces.

Easy Operation
The WP4C is a benchtop instrument. To measure water potential, you place the sample in a round sample cup (4cm in diameter and 1cm tall) and set the cup in the lexan sample drawer. You slide in the drawer and turn the knob to seal the chamber. After about 5 minutes, the WP4C will beep and show your final water potential reading.

Precise Mode
Measurements can be inaccurate if they’re made too soon. When run in Precise Mode, the new WP4C checks to make sure the sample has fully come to equilibrium before displaying a final reading.

Faster Equilibration
The WP4C’s new nickel Teflon sample chamber coating is hydrophobic. Less water absorption in the sample chamber means faster equilibrium.

Finely Tuned Calibration
The WP4C sensor is durable and easy to clean. The instrument can be calibrated using saturated salt solutions (mix your own or order single-sample vials of standards on the accessories tab). New algorithms let you fine-tune calibration for excellent accuracy (±0.05 MPa or better).

The WP4C can be used to measure the water potential of any porous material. It’s typically used to analyse soil, soilless substrates, plant material, and seeds, but it has been used to measure a variety of unusual things, including apples, bricks, oil, and insect carapaces.


ACCURACY RANGE: ±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 dewpoint sensor, 2) Infrared temperature sensor
OPERATING ENVIRONMENT: 5 to 40 °C (41 to 104 °F)
SENSOR TYPE: Frequency domain with calibrated ceramic discs, thermistor
OUTPUT: RS232 (TTL) with 3.6 volt levels or SDI-12 communication protocol
SAMPLE CUP CAPACITY: 7 mL recommended (15 mL full)
WEIGHT: 3.2 kg (5.2 kg shipping weight)
DIMENSIONS: 24.1 x 22.9 x 8.9 cm (9.5 x 9.0 x 3.5 in)
CASE MATERIAL: Powder-painted aluminium
DATA COMMUNICATIONS: RS232A compatible, 8-data bit ASCII code, 9600 baud, no parity, 1 stop bit.
POWER: 110 VAC to 220 VAC, 50/60 Hz
DISPLAY: 20 x 2 alphanumeric dot-matrix LCD with backlighting
INTERFACE CABLE: Standard RS232 serial cable (included)
WARRANTY: One year, parts and labour


How Does it Work? Chilled Mirror Dewpoint Technique

The WP4C measures water potential by determining the relative humidity of the air above a sample in a closed chamber (an AOAC-approved method; also conforms to ASTM 6836). Once the sample comes into equilibrium with the vapour in the WP4C’s sealed chamber, the instrument finds relative humidity using the chilled mirror method. This method entails chilling a tiny mirror in the chamber until dew just starts to form on it. At the dewpoint, the WP4C measures both mirror and sample temperature with 0.001°C accuracy. This allows the WP4C to deliver water potential readings with unparalleled accuracy in the -0.1 MPa to -300 MPa range


The WP4C will continue to measure sample water potentials to 0 MPa, but since its accuracy is ±0.05 MPa, readings above -0.05 MPa have errors of 100% or more. -0.1 MPa is usually the the practical limit for good accuracy. At these water potentials, special methods should be used to maximise accuracy. Read about these methods in the WP4C manual.

Limits of the Method

At high water potentials, the temperature differences between saturated vapour pressure and the vapour pressure inside the sample chamber become vanishingly small. Because of this, the WP4C can now resolve temperatures to a thousandth of a degree. To read water potentials in the wet range (0 to -0.1 MPa), we recommend German manufacturer UMS’s fast response tensiometers and HYPROP moisture release curve instrument.

Note: the WP4C range goes to 0 MPa, but readings of samples wetter than -0.1 MPa will have an increasing, and typically unacceptable, percentage of error. Some users may be able to make useful measurements in samples wetter than -0.1 MPa using special techniques. For more information, see the WP4C User Manual.

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Nardini, A., Ramani, M., Gortan, E. and Salleo, S. (2008b). Heterogeneity of gas exchange rates over the leaf surface in tobacco: an effect of hydraulic architecture?
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American Society for Testing and Materials (2003), Standard Test Methods for Determination of the Soil Water Characteristic Curve for Desorption using a Hanging Column, Pressure Extractor, Chilled Mirror  Hygrometer, and/or Centrifuge. Designation D 6836-02. in 2003 Annual Book of ASTM Standards, American Society for Testing and Materials, Philadelphia, PA.

Anderson, C.E. and Stormont, J.C. (2003), Laboratory Measurement of Soil Moisture at Capillary Potential Greater than 1500 kPa, Transportation Research Board Annual Meeting, Washington D.C.

Duniway, M.C., Herricka, J.E. and Monger, H.C. (2007), ‘The High Water-Holding Capacity of Petrocalcic Horizons‘, Soil Science Society of America Journal, vol. 71, pp. 812-819.

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