WP4C Dewpoint Potentiameter

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 analyze soil, soilless substrate, 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 bench-top 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 a lexan sample drawer.  You slide in the drawer and turn a 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 ( order single-sample sized vials of standards directly from Labcell ).  New algorithms let you fine-tune calibration for excellent accuracy (±0.05 MPa or better).

Plug WP4C Data into HYPROP Data Evaluation Software

The HYPROP FIT Software can be used to plug in water potentail data collected by the WP4C for fitting different water retention models (i.e. van Genuchten, van Genuchten Bimodal, Brooks & Corey).

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 vapor in WP4C's sealed chamber, the instrument finds relative humidity using the chilled mirror method.  A tiny mirror in the chamber is chilled until dew just starts to form on it.  At the dewpoint, the WP4C measures 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 probably the the practical limit for good accuracy.  At these water potentials, special methods should be used to maximize accuracy.

Limits of the Method

At high water potentials, the temperature differences between saturated vapor pressure and the vapor pressure inside the sample chamber become vanishingly small.  The WP4C can now resolve temperatures to a thousandth of a degree.  In order to read water potentials in the "tensiometer range".

To read water potentials in the wet range (0 to -0.1 MPa), we recommend the Hyprop moisture release curve instrument.

Accuracy ±0.05 MPa from 0 to -5 MPa
1% from -5 to -300 MPa
-0.1 to -300 MPa*
Measurement Time 10-15 mins for most soil samples
20 mins for plant tissue samples
In fast mode; 5 mins (reduced accuracy)
Temperature Control 15°C to 40°C (± 0.2°C)
Sensor Type Chilled-mirror dewpoint sensor
Infrared temperature sensor
Operating Environment 5°C to 40°C (41°F to 104°F)
Warranty 1 year, parts and labour
Sample Dish Capacity 7 ml recommended (15 ml full)
3.2 kg (5.2 kg shipping weight)
Dimensions 24.1x22.9x8.9 cm (9.5x9.0x3.5 in)
Case Material Powder-painted aluminum
Data Communications RS232A compatible
8-data bit ASCII code
9600 baud
no parity
1 stop bit
Power 110VAC to 220VAC, 50/60 Hz
Display LCD with backlighting
Interface Cable RS232 serial cable
Compatible Standards ASTM D6836-07
*Note: 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.

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Non UK/Ireland Customers

Non UK/Ireland Customers, please contact Decagon Devices at: www.decagon.com

Environmental Catalogue
Size: 2.5 MB (2,581,813 bytes)

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