Water & Nutrient Management:
Soil Moisture-Based Irrigation Systems

Tensiometer

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Working Principle

When a sealed water-filled tube is placed in contact with the soil through a permeable and saturated porous material, water (inside the tube) comes into equilibrium with the soil solution (i.e., it is at the same pressure potential as the water held in the soil matrix). Hence, the soil water matric potential is equivalent to the vacuum or suction created inside the tube.

Description

The tensiometer consists of a sealed water-filled plastic tube with a ceramic cup at one end and a negative pressure gauge (vacuometer) at the other. The shape and size of the ceramic cup can be variable and the accuracy depends on the gauge or transducer used (about 0.01 bar). Typically the measurement range is 0-0.80 bar, although there are low-tension versions (0-0.40 bar) designed for coarse soils.

Advantages

• Direct reading
• Up to 4 inch measurement sphere radius
• Continuous reading possible when using pressure transducer
• Electronics and power consumption avoidable
• Well-suited for high frequency sampling or irrigation schedules
• Minimal skill required for maintenance
• Not affected by soil salinity, because salts can move freely in and out across the porous ceramic cup
• Inexpensive

Drawbacks

• Limited soil suction range (<1 bar)
• Relatively slow response time
• Requires intimate contact with soil around the ceramic cup for consistent readings and to avoid frequent discharge (breaking of water column inside)
• Especially in swelling or coarse soils, the ceramic cup can loose contact with soil, thus requiring reinstallation
• Requires frequent maintenance (refilling) to keep the tube full of water, specially in hot dry weather

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This page was last updated on June 15, 2010.