TensiometersThen and Now

Elizabeth Scherling

BAE 558

Semester Project

Spring 2005

Outline

• Theory• Components• Historical Devices

– Applications

• Modern Devices– Applications

• Calibration• Maintenance• Challenges• Future Tensiometers

Theory

• Tensiometers directly measure soil water matric potential

• Forces responsible for capillary action and water retention

–van der Waals–Electrostatic dipole–Osmotic forces–Surface tension

Components I

• Thin-walled porous cup

• Rigid body

• Pressure gauge– Mechanical– Electrical: pressure transducer– Hydraulic: manometer

• De-aired water

Components II

Porous Cup Body Pressure Gauge

Historical Devices I

• B. E. Livingston• 1908• Earliest account of a

tensiometer-like device

Historical Devices II

• H. E. Pulling and B. E. Livingston

• 1915• Measured what they

called the “water supplying power of the soil”

Historical Devices III

• C. J. Lynde and H. A. Dupre

• 1913• First hanging column

design• Measured what they

called the “capillary lift of soil”

Historical Applications

• Irrigation control– Auto-irrigation for potted plants

• Capillary pressure measurement

Modern Devices I

• J. M. Hubbell and J. B. Sisson

• 1998• Advanced

tensiometer design

Modern Devices II

• B. Lebeau, S. Barrington, and R. Bonnell

• 2003• Micro-tensiometer

Modern Applications

• Irrigation Control– Greenhouse– Farm– Field

• Vadose Zone Studies

• Transport Research

• Ultimately:Tensiometers have the potential to save water and reduce infiltration contamination

Calibration

• The tensiometer must be subjected to known positive and negative pressures.– U-tube manometer– Vacuum chamber

• Recalibration only recommended after long periods of inactivity.

Maintenance

• Gas Evolution– De-air water prior to filling and for subsequent

water replacement• Vacuum, autoclave, or boil

– Check for bubbles frequently in simple designs

– Add water regularly for advanced designs

• Maintain intimate contact between porous cup wall and soil matrix

Challenges I

Choosing the correct tensiometer

Hubbell and Sisson, 1998

Challenges IIWeather Conditions

Hansen and Pasian, 1999

Cloudy day

Sunny day

Challenges III

• Low range: 3 to 6 kPa– Controlled and reliable

• Medium range: 9 to 12 kPa– Not stable or reliable

• High range: 15 to 18 kPa– Not stable or reliable

Limited Matric Potential Ranges

From a study by Hansen and Pasian, 1999

Low Range

Medium Range High Range

Future Tensiometers

• For long duration in situ use:– Cost effective advanced tensiometers

• For large-scale greenhouse use:– Micro-tensiometers with greater control of

large ranges of moisture tension

Questions

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