Range: 0 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
NOTE: WP4C will display updated measurements approximately every 5 min until stopped
Length 24.1 cm (9.5 in)
Powder painted aluminum
|Sample cup capacity||
15 mL (0.5 oz) full
3.2 kg (7.1 lb)
20 x 2 alphanumeric LCD with backlighting
Chilled-mirror dew point sensor
Minimum 5 °C
Standard RS-232 to USB cable (included)
Manufactured under ISO 9001:2015
METER WP4C – 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.
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 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.