IL30841A

IL30841A - A capacitor for use in testing the moisture of the soil

Info

Publication number: IL30841A
Application number: IL30841A
Authority: IL
Original assignee: Siemens Ag
Priority date: 1967-11-07
Filing date: 1968-10-09
Publication date: 1971-11-29
Also published as: FR1590868A; CH486698A; IL30841A0; GB1233789A; AT288057B

Description

A capacitor for use testing the of the SIEMENS invention relates to testing the moisture content of more particularly by means of a capacitance The water as moisture in the soil is antly bound by capillary action in minute and the The moisture b capillary action is reduced by evaporation or by water absorption by It is replenished by precipitation or by ground water which rises by capillary The absorptio capacity of the depends primarily upo the capillary upon the pore size and upon the distribution of the pores and thus upon the structure of the sandy soils low water while and clayey soils have high water Soil moisture content may determined by means of a apaoitance measurement vfherein there is employed as a measurin capacitor a cylindrical capacitor in which the wall of the electrode There is between electrodes of the measuring capacitor which same the soil to be tested and which determines the capacitance At least for measurements over relatively long the soil introduced must be in contact with the soil be tested through the apertured wall of outer For the filled measuring capacitor is buried and the unfilled capacitor is pushed into the soil to be each the degree of is not constan throughout the entire internal spaoe of the In are not known and are identical with the porosity and the pore size of the to be The soil introduced between the electrodes of the capacitor therefore a water due to capillary of unknown The measurement values acquired with this soil as dielectric therefore cannot be calibrated and are not comparable with other measurement The problem exists so designing a capacitor measurement results which can be calibrated and reproduced are According to the present invention there is provided a capacitor for use in testing the moisture content of comprising first and second electrode which structures are arranged to act respectively as the opposed of the between which extends a porous dielectric having exposed external surface regions tacting the soil to be wherein the first electrode structure comprises a central electrode which is Coaxially surrounded by a helix constituting the second the helix being disposed in a helical groove in the Reference will now be by way of example to the accompanying diagrammatic in Figure 1 shows a side elevation of a capacitor embodying the with associated electrical and Figure 2 shows a view of the corresponding to line of Figure The illustrated for testing the moisture The dielectric is brought with the soil for the purpose of the The dielectric 3 being porosity and pore the capillary and hence the water of the interior of the capacitor are vely There is set between the dielectric the soil to be teoted a resultant capillary suction force whose magnitude depends only upon the ratio of the capillary forces of the to the capillary forces of the The dielectric absorbs water from the soil when its moisture content or gives up water thereto when the soil moisture content The moisture value obtained with the capacitance measurement is therefore dependent only upon the porosity and the pore size of the It may be calibrated if the porosity and the pore size of the soil tested are The electrode 1 is electrode encased by the dielectric 3 and surrounded coaxially by the electrode 12 which is helical The helical electrode 12 is held in a helical groove formed in the dielectric The inner electrode 1 is encased by the dielectric material of the dielectric known porosit and pore size is a ceramic material such as is commercially obtainable under the name Trade or These ceramics can supplied in various porosities and pore so that the dielectric 3 chosen to be of similar porosity and pore size to the soil to be on the measuring capacitor is a housing 4 i which is disposed a measuring bridge for determining the The circuit arrangement Of the measuring bridge are formed by the capacitors 7 and the measuring capacitor by the damping resistors 8 and The measurement value can be read from the instrument a fully transistorised difference amplifier being provided to increase the reading This difference amplifier may also be disposed in the housing The measuring bridge is supplied from a frequency generator 11 which generates a current at a frequency of about 100 At that electrode which is not at earth potential is preferably coated with an electrically nonconducting which may be a for example Trade It is to be noted tha in the illustrated capacitor the electrode constituted by the helix 12 may be coated with electrically nonconducting in the same way as the shaped electrode in order to protect it from corrosion For the sake of the clarity of the this layer of coating material is not With the illustrated intimate contact with the soil can be and a subtantially complete screening takes the outer regions of the dielectric lying tiall entirely within the electric A change of content can be determined without unacceptable time lag and with high measuring accuracy independently of the establishment of a state of equilibrium within the measuring Such a capacitor therefore particularly suitable as an source for control loop for controlling soil moisture insufficientOCRQuality

Claims

1. A capacitor for use in testing the moisture content of soil, comprising first and second electrode structures, whic structures are arranged to act respectively as the opposed "plates" of the capacitor, between which extends a porous dielectric having exposed external surface regions for contacting the soil to be tested, wherein the first electrode structure comprises a central rod-shaped electrode which is coaxially surrounded by a helix, constituting the second electrode structure, the helix being disposed in a helical groove in the dielectric*

2. A capacitor as claimed in claim 1, wherein the dielectric is a ceramics material.

3. · A capacitor as claimed in claim 1 or 2, wherein one or each of the first and second electrode structures is provided with a coating of an electrically non-conductive material.

4. A capacitor as claimed in claim 3» wherein the electrically non-conductive material is a plastics.

5. · A capacitor as claimed in any preceding claim, wherein the porosity and the pore siae of the dielectric are preselected to be similar to the porosity and pore size of the soil to be tested.

6. A capacitor, for use in testing the moicture content of soil, substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawing.

7. A capacitor as claimed in any preceding claim, connected as a actual-lvalue source for a control loop arranged to control automatically the degree of moisture in a quantity of soil.