by Frikkie Koegelenberg, Pr Eng and Hendrik Jordaan
The pulse delay measurements method by means of Time Domain Reflectometry (TDR), as well as the frequency delay method, are vested in the principle of measuring the dielectrical constant of materials. The dielectrical constant of a material is the measuring of the capacity (electrical permissiveness) of a non-conductive material to conduct high frequency electromagnetic waves or pulses.
We thank the ARC Agricultural Engineering in South Africa for making their manual on soil water sensors available to the readers of ProAgri Zambia.
The dielectrical constant of dry soil varies between two and five, while that of water is 80 at frequencies of between 30 MHz and 1 GHz. Research results have shown that the measuring of a soil water medium’s dielectrical constant reflects an accurate measurement of the soil’s water content.
Relatively small differences in a soil’s water content result in large differences in the electromagnetic characteristics of the soil water medium. The water content of a soil can therefore be determined solely by determining the dielectrical constant of a soil.
The time domain reflectometry technology for soil water content determination is vested in cable testers such as the Tektronix 1 502B. This equipment was originally used for testing the breaks and joints in subsurface cables. Various manufacturers therefore use the apparatus to conduct a high frequency transversal electromagnetic wave next to a cable, which is connected to parallel conductive probes. The parallel conduction probes (two or three) are inserted into the soil and serve as wave conductors. The wave conductors reflect the transversal electromagnetic wave back to the cable tester, where it is reflected on an oscilloscope. The time taken for the signal to be reflected (time delay) is measured accurately by the cable tester.
With the length of the cable and wave conductors known, the reproductive speed of the transversal electromagnetic wave can be calculated. The dielectric constant is inversely related to the reproductive speed of the electromagnetic wave; A faster reproductive speed delivers a lower dielectrical constant and therefore a lower soil water content.
A higher dielectric constant will therefore be an indication of a higher water content in the soil. Wave conductors inserted into the soil consist of two or three parallel stainless-steel probes arranged approximately 50 mm apart. The wave conductors are usually inserted vertically, horizontally or at an angle of 45° into the soil. A screened-off parallel connector cable conducts the electro magnetic volumetric wave between the wave conductors and the cable tester.
The TDR instrument measures the average volumetric water content (%) over the length of the wave conductors. The sphere of influence of an instrument around the wave conductors at the measuring point has a diameter of approximately 1,5 times the spacing of the parallel probes. The wave conductors are permanently installed on the side of a profile hole with conductors, which lie on top of the soil surface. Care must be taken to disturb the soil as little as possible. It is the only method to obtain readings at different depths in one position with the aid of the TDR. Horizontally installed wave conductors give a depth specific reading while wave conductors installed at an angle of 45°, give an integrated larger volume reading, both in the horizontal and vertical directions.
Hand TDR meters consisting of a wave conductor probe, can be used like a neutron moisture meter to determine the water content with the aid of access tubes in the soil.
The following TDR equipment is currently available (in no specific order of preference):
Aquaflex SE 200 soil water meter; Campbell Scientific’s CS616-L wave conductor meter; Hydrosense from Degacon; Trime from IMKO; Tektronix TDR; Gro point & Water point from ESI Environmental Sensors.
• Measurements are determined quickly. Soil water content can be determined at different depths simultaneously. Readings are taken within one minute.
• The TDR measuring technique is very accurate if the apparatus is properly installed and calibrated.
• Accurate and dependable readings can be taken near the soil surface. Measurements as shallow as 100 mm to a depth of 5 m is possible.
• Research results show that the dielectrical constant is independent of the gross density of the soil.
• Continuous readings and data storage with the aid of data loggers are possible.
• The wave conductors should be installed very carefully to ensure contact along the entire length of the probes. Vacuums along the probes cause faulty readings. The probes must also remain parallel, or else the wave conductors do not function correctly.
• Wave conductors cannot readily be used in stony soils, and special precautionary measures must be taken. The access tubes of the probe are installed with a paste of the same soil.
• Cable test apparatus is essential for analyses of the wave patterns.
• Soil brackishness influences the attenuation of the electromagnetic pulse in the soil. The higher the salt content, the lower the accuracy of the TDR. Research is currently being done to find suitable isolating materials for the probes to make them suitable for taking readings in brackish soils.
• TDR equipment is very expensive.
Next month we shall discuss frequency delay measurement by means of capacitance. Visit www.arc.agric.za for more information.