Sensors and Actuators A 147 (2008) 544–552 Contents lists available at ScienceDirect Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna Calibration of a TDR probe for low soil water content measurements Wojciech Skierucha a,∗ , Andrzej Wilczek a , Olga Alokhina b a Institute of Agrophysics, Polish Academy of Sciences, Doswiadczalna 4, Lublin 20-290, Poland b Karpenko Physico-Mechanical Institute of NAS Ukraine, Naukova 5, Lviv 79601, Ukraine article info Article history: Received 5 February 2008 Received in revised form 28 May 2008 Accepted 17 June 2008 Available online 25 June 2008 Keywords: Soil moisture Dielectric permittivity Time domain reflectometry abstract Time domain reflectometry (TDR) probes are increasingly used for field and laboratory estimation of soil water content. Usual calibration of TDR probes for the determination of soil water content uses two media: air for low and water for high values of dielectric permittivity, although the measured range of dielectric permittivity in soil is much smaller as compared to the range implied by the calibration media. The use of air for calibration of short TDR probes gives calibration errors due to overlapping incident and reflected pulses in the reflectogram, which result in their relative shift in time. This phenomenon, named the convolution effect, can be avoided by the application of selected calibration media. The presented approach minimizes of dielectric permittivity measurement errors by choosing the calibration media with dielectric permittivity values close to the limits of the measurement range and the possibility to use TDR probes of various lengths. The comparison of errors of TDR apparent dielectric permittivity measurement in three sandy soils, based on the probe calibrations in various media, is also presented. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The application of time domain reflectometry (TDR) to mea- sure in situ volumetric water content  (m 3 m -3 ) has now been widely accepted. The main features of TDR technique that gained advantages over standard gravimetric soil moisture measurement method include: small measurement volume, relative ease of oper- ation, fast response, measurements are not destructive, the ability of the probes multiplexing and the measurement automation. TDR multiplexing gives an opportunity to use many probes sequentially, thereby allowing the measurement of temporal and spatial soil water content changes. Since its introduction in the early eighties [27], TDR is now among the most utilized soil moisture measurement techniques in both laboratory and field conditions. The Institute of Agrophysics PAS, Lublin, Poland has its share in the development of methods and TDR technique [11–13,19,21–25]. The TDR soil moisture mea- surement system developed there uses needle pulse for feeding the two rods parallel waveguide sensor, which is a distinctive feature different to commonly used step pulse of other systems originated in the Tektronix cable tester [7,15–17,27]. The shape of the needle pulse resembles the shape of Gaussian curves with the rise and fall times (time distance between 10% and 90% of the signal ampli- tude) equal to 0.25 ns. The reflectogram generated by needle pulse ∗ Corresponding author. E-mail address: w.skierucha@ipan.lublin.pl (W. Skierucha). is much simple to interpret in the calculation of time distances between respective reflections. The time domain reflectometry technique (TDR) consists in measuring the travel time of electromagnetic (EM) pulses along a waveguide of known length. This travel time is related to the dielectric permittivity of the medium the waveguide is inserted in: t = 2L √ ε r c (1) where t is travel time, L is length of the waveguide, ε r is the soil apparent dielectric permittivity, which is equal to the real part of dielectric permittivity when the dielectric loss caused by polariza- tion and conduction are negligible and c is the speed of light in free space (2.9979 × 10 8 ms -1 ). Changes in water content  (m 3 m -3 ) modify the dielectric per- mittivity of soil and affect the travel time of the electromagnetic wave, permitting an indirect determination of the soil’s volumetric water content. The relation  = f(ε r ) is the result of empirical cali- brations [9–11,28] or modeling on the base of three or four phase dielectric mixing models giving physically pragmatic approach [1,2,5,6,18,30]. The TDR measurement error of soil moisture comes from three reasons [20]: (1) calibration error, (2) probe installation and (3) time t error caused by electronics of the measuring device. The calibration error is caused by the influence of soil texture, den- sity, temperature and dielectric loss. In majority of soil conditions the impact of the imaginary part of dielectric permittivity can 0924-4247/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2008.06.015