Water & Nutrient Management:
Soil Moisture-Based Irrigation Systems

Tensiometric Measurement:
Heat Dissipation

Working Principle

The thermal conductivity of water produces heat dissipation, so that a dry material will heat up faster than a wet one. In other words, the heat flow in a porous material is proportional to its water content.

Description

A thermal heat probe consists of a porous block containing a heat source and an accurate temperature sensor. The block temperature is measured before and after the heater is powered for a few seconds. Thereby, block moisture is obtained from the temperature variation. Since the porous block, placed in contact with the soil, is equilibrated with the soil water, its characteristic curve will give the soil water potential. Hence, the sensor must be provided with the calibrated relationship between the measured change in temperature and soil water potential. Measurement range: 0.1-30 bar (less accurate for 10-30 bar range).

Advantages

• Wide measurement range
• No maintenance required
• Continuous reading possible
• Not affected by salinity because measurements are based on thermal conductivity

Drawbacks

• Needs a sophisticated controller/logger to control heating and measurement operations
• Slow reaction time. It does not work well in sandy soils, where water drains more quickly than the instrument can equilibrate
• Fairly large power consumption for frequent readings

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This page was last updated on June 15, 2010.