Tensiometers Then and Now

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TensiometersThen and Now

• Elizabeth Scherling
• BAE 558
• Semester Project
• Spring 2005

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Outline

• Theory
• Components
• Historical Devices
• Applications
• Modern Devices
• Applications
• Calibration
• Maintenance
• Challenges
• Future Tensiometers

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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

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Components I

• Thin-walled porous cup
• Rigid body
• Pressure gauge
• Mechanical
• Electrical pressure transducer
• Hydraulic manometer
• De-aired water

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Components II
Porous Cup
Body
Pressure Gauge
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Historical Devices I

• B. E. Livingston
• 1908
• Earliest account of a tensiometer-like device

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Historical Devices II

• H. E. Pulling and B. E. Livingston
• 1915
• Measured what they called the water supplying
  power of the soil

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Historical Devices III

• C. J. Lynde and H. A. Dupre
• 1913
• First hanging column design
• Measured what they called the capillary lift of
  soil

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Historical Applications

• Irrigation control
• Auto-irrigation for potted plants
• Capillary pressure measurement

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Modern Devices I

• J. M. Hubbell and J. B. Sisson
• 1998
• Advanced tensiometer design

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Modern Devices II

• B. Lebeau, S. Barrington, and R. Bonnell
• 2003
• Micro-tensiometer

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Modern Applications

• Irrigation Control
• Greenhouse
• Farm
• Field
• Vadose Zone Studies
• Transport Research
• Ultimately
• Tensiometers have the potential to save water
  and reduce infiltration contamination

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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.

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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

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Challenges I
Choosing the correct tensiometer
Hubbell and Sisson, 1998
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Challenges II
Weather Conditions
Cloudy day
Sunny day
Hansen and Pasian, 1999
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Challenges III
Limited Matric Potential Ranges
From a study by Hansen and Pasian, 1999

• 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

Low Range
Medium Range
High Range
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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

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Questions
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