CALIBRATION OF TIME DOMAIN REFLECTOMETRY WATER CONTENT MEASUREMENTS IN GROWING MEDIA

Authors:	J.A. Kipp, R.C. Kaarsemaker
Keywords:	Time domain reflectometry, calibration method, rockwool, glasswool, perlite, coir dust, pumice stone
Abstract: Time Domain Reflectometry (TDR) is a widely accepted method to determine volumetric water content () in soil. With TDR, the apparent dielectric permittivity (constant) K_a is measured, which is mainly dependent on the water content of the medium. In growing media this method has hardly been tested. The aim of our study was to test the applicability of TDR for different growing media. A new calibration method is introduced in which different pressure heads are applied and water contents are determined indirectly. Calibration was carried out for the following media: glass- and rockwool, coir dust, pumice stone, and perlite. For glass- and rockwool a complete set of data could be obtained by this calibration method. Water contents in the other media could be measured in the range which is commonly occuring in horticultural practice. For the granual media a linear relationship between and K_a defined in terms of L_a and L seems to be suitable to determine water content in the range of values common in horticulture practice. Separate calibration of these materials is only necessary, when absolute water contents are needed. For glass- and rockwool polinomials fitted better.
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