Articles
WATER CONTENT MEASUREMENTS IN SOIL AND ROCKWOOL SUBSTRATES: DIELECTRIC SENSORS FOR AUTOMATIC IN SITU MEASUREMENTS
Article number
304_23
Pages
209 – 218
Language
Abstract
The water content and ionic conductivity of soil or rockwool substrates can be derived from their dielectric properties.
Up to now accurate measurements have only been possible with the help of advanced laboratory equipment.
There are two methods; measurement in Frequency Domain and in Time Domain.
For the Time Domain method the most popular instruments use Reflectometry (TDR). The main disadvantage of TDR is the need for graphical interpretation of the data.
In addition long electrodes of at least 15 cm are required.
The method is not well suited for automation.
With the Frequency Domain method a sinusoidal current is fed through an impedance formed by two electrodes with the soil as the dielectric.
The voltage developed across the electrodes is a measure for the dielectric properties of the soil.
Many Frequency Domain instruments have been developed at various laboratories.
All instruments having their own shortcomings.
At TFDL a research program on the dielectric behaviour of soil has been carried out and possible sources of error have been identified.
Circuitry has been developed that deals effectively with these errors.
It can cope with an ionic conductivity of up to 10mS/cm.
Sensors and systems including a simple low-cost hand-held meter have been developed.
Electrode length ranges from less than 2 cm upward.
More than ten years of practical experience are available with these sensors in a broad range of applications.
Two recent applications are presented; The measurement of water content and ionic conductivity in rockwool substrates and the automatic measurement with 160 sensors of soil water content in the Rhizolab in Wageningen.
Up to now accurate measurements have only been possible with the help of advanced laboratory equipment.
There are two methods; measurement in Frequency Domain and in Time Domain.
For the Time Domain method the most popular instruments use Reflectometry (TDR). The main disadvantage of TDR is the need for graphical interpretation of the data.
In addition long electrodes of at least 15 cm are required.
The method is not well suited for automation.
With the Frequency Domain method a sinusoidal current is fed through an impedance formed by two electrodes with the soil as the dielectric.
The voltage developed across the electrodes is a measure for the dielectric properties of the soil.
Many Frequency Domain instruments have been developed at various laboratories.
All instruments having their own shortcomings.
At TFDL a research program on the dielectric behaviour of soil has been carried out and possible sources of error have been identified.
Circuitry has been developed that deals effectively with these errors.
It can cope with an ionic conductivity of up to 10mS/cm.
Sensors and systems including a simple low-cost hand-held meter have been developed.
Electrode length ranges from less than 2 cm upward.
More than ten years of practical experience are available with these sensors in a broad range of applications.
Two recent applications are presented; The measurement of water content and ionic conductivity in rockwool substrates and the automatic measurement with 160 sensors of soil water content in the Rhizolab in Wageningen.
Authors
M.A. Hilhorst, J. Groenwold, J.F. de Groot
Keywords
Online Articles (42)