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Construction of Time-domain Reflectometry (tdr) Sensor for Measuring Soil-water Content

A. B. M. Z. Hossain
Scientific Officer
Bangladesh Ricc Research Insritute, Regional Station, Kushtia, Bangladesh

M. M. Alam
Scientific Officer
IWM Division, Bangladesh Ricc Research Institute, Cazipur, Bangladesh

G. K. Bhuiyan
Scientific Officer
FMPHT Division, Bangladesh Ricc Research Institute, Gazipur, 1701, Bangladesh

Abstract/ Executive Summary:

Field monitoring of volumetric soil water content (VWC) is critical for a variety of applications. This study was conducted by constructing of several TDR sensors for measuring soil-water content. However, the Calibration of these sensors is more sensitive to variations in soil properties for time domain reflectometry (TDR), which is generally regarded as the best electronic means of VWC measurement and which has a relatively robust calibration. The three-wire TDR sensor of length 0.1m and wire spacing 3 cm (centre to centre) constructed by the locally available materials' work well in the laboratory. Volumetric soil-water content measured by TDR and that measured by gravimetric method is found comparable within 2%. However, TDR does not measure volumetric water content accurately at low soil-water content (≥0.10).

Keywords: Time-domain reflectometry (TDR), Dielectric Constant (c), Volumetric water content (VWC)

Location: BRRI

Start Date:

End Date:

Program Area: Resource Development and Management

Commodity: Water management

Objectives:

1) To construct TDR sensor by locally available materials; and

2) To verify TDR method to measure soil-water content with the constructed sensors.

Methodology:

An experiment was conducted using silt loam soil. Air-dry soil was packed uniformly in separate polyvinyl chloride (PVC) cylinder, 8.7 cm in diameter and 14 cm length with the base closed with polyethylene sheet. The soil was then spread on a clean tray. Water was added in the soil by using a hand sprayer and mixed properly. This soil was packed uniformly in PVC cylinder in 3 to 4 steps. In this way, 31 soil samples, ranging water content from air dry to water saturated, were prepared in the laboratory. The soil sample was compacted in the PVC column properly in each case. One TDR sensor was inserted from the top of the cylindrical soil samples vertically and good contact was maintained between the soil and metal surfaces of the rods. Formation of cracks a.nd air gaps around the rods can pose significant problems in the measurement. So, necessary precautions were taken in this regard. The TDR was controlled by a computer through a data logger. TDR measured the electronic constant of the soil sample and data logger recorded the value. The TDR reading was collected from the data logger by a computer. The verification of the accuracy of the TDR method in measuring the volumetric soil-water content was checked by thermo- gravimetric method. Thirty-one small aluminium pots were weighed by electronic balance. After TDR measurement, wet soil collected from each cylindrical soil column in the pot and the pot was weighed again. Similarly, 31 soil sample were collected and dried in an oven at 105? for 24 hours to obtain the weight of dry soil. The volumes of the pots were also calculated by measuring their diameter and height by using a slide calipers.

Source of Information: Int. J. Sustain. Agril Tech. 2(3): 14-20, April 2006

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

The constructed parallel three-wire sensors work well for use of TDR. The signals for the sensors show clear reflections from the beginning and end of each sensor. The time-domain reflectometry measures the bulk dielectric constant of soils by measuring the time of travel of an electromagnetic pulse through a sensor. The dielectric constant is strongly dependent on the volumetric water content of the soil by a non-linear relationship. A linear relationship is found between volumetric water-content measured by TDR and that measured by gravimetric method. The overestimation in TDR measurement occurs allow soil-water content (<0.01).

Knowledge Management: Journal