DE29521250U1 - Device for measuring the matrix potential in the soil - Google Patents

Description

Device for measuring the matrix potential in soil

The subject of the invention is an attachment for the Time-Domain Reflectrometry (TDR) probe, which is designed to determine the volumetric water content in the soil (European Patent No.: 92402596.8). Using the attachment, the probe can measure the matrix potential in the soil.

The matrix potential is a value for the bond strength between water and soil particles caused by capillary forces in the soil. Knowledge of the matrix potential is of great importance for research in plant physiology, soil science and hydrology as well as for agriculture and forestry. Several methods are known for determining it: tensiometer, psychrometer and gypsum block as well as neutron probe, gravimetric water content and time-domain reflectrometry (TDR). All of these methods either have a narrow measuring range (tensiometer) or application options that depend on soil properties (gypsum block, psychrometer) or they require a characteristic curve of the relationship between water content and matrix potential (pF curve) of the soil to be examined (neutron probe, gravimetric water content and time-domain reflectrometry (TDR)) (for summary evaluations see Philip W. Rundel and Wesley M. Jarrell (1990): Water in the environment, in: R.W. Pearcy, J. Ehleringer, H.A. Mooney and P.W. Rundel (eds.): Plant Physiological Ecology, Chapman and Hall, London, New York, 29-41).

The TDR probe directly determines the volumetric water content in the soil (G.C. Topp and J.L. Davis (1985) : Measurement of Soil Water Content using Time-domain Reflectrometry (TDR)—A Field Evaluation. Soil Sei. Soc. Am. J., Vol. 49, 19-24). In order to obtain the matrix potential, the measured values must be converted using the pF curve of the soil.

Determination is not only time-consuming (usually over four weeks), but also complicated and therefore expensive. Furthermore, shrinkage and swelling of the soil can create small gaps between the TDR measuring rods and the soil, causing significant measurement errors (T.J. Dean, J.P. Bell and A.J.B. Baty (1987): Soil Moisture Measurement by an improved Capacitance Technique, Part I. Sensor Design and Performance. Journal of Hydrology, 93, 67-78). These errors are propagated when converting to matrix potential.

The invention is based on the task of converting the TDR probe designed to determine the volumetric water content in the soil so that it directly measures the matrix potential in the soil without using pF curves and thereby eliminates the errors that arise due to the gaps between the TDR measuring rods and the soil.

The object is achieved according to the invention in that the TDR probe is arranged in an attachment according to claim 1. As a result, the probe does not measure the water content in the soil, but rather that in the filling of the attachment. Using the pF curve of the filling, the determined water content can be output directly as the matrix potential of the filling of the attachment using an integrated electronic transformation or can be subsequently converted to the matrix potential of the filling. Since the filling is in contact with the soil to be measured and there is an equilibrium in the matrix potential between the two after a certain time, the matrix potential of the filling is the same as that of the surrounding soil. This gives the matrix potential of the soil directly.

The filling of the attachment is pressed from below with a spring device so that even when dry, no gaps arise between the TDR measuring rods and the filling of the attachment, thus avoiding measurement errors.

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The measuring range and the measuring accuracy of the TDR probe equipped with the attachment are determined by the properties of the filling (pF curve) and can be varied as required.

The TDR probe equipped with the attachment measures the matrix potential directly over a wide range, avoids errors due to gap formation between the TDR measuring rods and the ground and is particularly suitable for continuous measurements.

An embodiment of the invention is explained below with reference to the drawings. Each of these shows a perspective view:

Fig. l: the attachment according to the invention, partially

cut,
Fig. 2: the probe and
Fig. 3: the holder of the attachment.

This makes it clear that the commercially available TDR probe 1 contains two measuring rods 2. These are arranged in an attachment which, from the outside to the inside, consists of a fine nylon net 5, a holder 3 made of plastic, a filling made of a water-permeable material 4 and a spring device 6, 7. Materials that have a low dielectric constant, a stable pF characteristic under the influence of soil solutions and sufficient water permeability can be used as the filling.

The filling 4, the net 5 and the spring device 6, 7 are attached to the TDR probe 1 by the holder in such a way that the propagation area of the magnetic waves from the TDR measuring rods 2 relevant for determining the water content is completely in the filling 4 and that an exchange of water in liquid or gaseous form between the filling 4 and the soil to be measured is ensured.

Claims (8)

Protection claims 1. Measuring device for measuring the matrix potential in the soil with: a measuring cell with a housing which consists at least in sections of a material which allows soil moisture to penetrate into the interior of the housing, at least two electrodes arranged in the housing and spaced apart from one another and a dielectric which fills the space between the electrodes and the inner wall of the housing, an electrical circuit that detects the water content in the dielectric of the measuring cell and uses a previously known relationship between water content and matrix potential of the dielectric used to determine the matrix potential present in the dielectric. 2. Measuring device according to claim 1, characterized in that the measuring cell has a cylindrical housing and two rod electrodes running parallel therein. 3. Measuring device according to claim 1 or 2, characterized in that the dielectric used has a permittivity of less than or equal to 81. 4. Measuring device according to one of claims 1 to 3, characterized in that the electrical circuit detects the water content in the dielectric by measuring the capacitance between the electrodes. 5. Measuring device according to one of claims 1 to 3, characterized in that the electrical circuit measures the water content via a TDR measurement, in which an electromagnetic wave is sent along the electrodes through the dielectric. 6. Measuring cell for measuring the matrix potential in the soil with: a housing which consists at least in sections of a material which allows soil moisture to penetrate into the interior of the housing, at least two electrodes arranged in the housing and spaced apart from each other, and a dielectric with a permittivity of less than or equal to 81, which fills the space between the electrodes and the inner housing wall. 7. Measuring cell according to claim 6, characterized in that the Measuring cell has a cylindrical housing and two rod electrodes running parallel therein. 8. Measuring cell according to claim 6 or 7, characterized in that the housing of the measuring cell is at least partially formed by a nylon net.