METHOD AND APPARATUS FOR THE MEASUREMENT OF MOISTURE OR WATER CONTENT

Abstract: Method and apparatus for the measurement of moisture or water content electrically so that, in the material to be studied, an electrical field is generated by means of the electrodes of the measurement head (1) and that the moisture or the water content is determined as the effect of the material to be studied on the electrical properties (Rm, Cm) of the measurement head (1). The objective of the invention is to avoid mechanical weighing or standardization of a sample taken out of the material to be measured, to be performed before the measurement operation proper, as well as the extra mechanical equipment required for such operations. The problem has been solved by performing the measurement so that the degree of filling of the measurement head (1) is changed and the capacitance (Cm) of the measurement head, varying along with the quantity of the material to be studied, is measured and, when the capacitance reaches a predetermined value (Ca), the resistance (Rm) of the sensing element (1) is measured, by means of which resistance the moisture of the material to be studied is determined. The invention can be applied especially to the moisture-measurement of grain, but also to the measurement of moisture of seeds, hay, soil, peat, sand, wood, chips, and sawdust, as well as for moisture measurements of various powders in production processes. The invention can also be applied to the measurement of the water content, e.g., of silage, water-containing emulsions and mixtures.

Description: Method and Apparatus for the Measurement of Moisture or Water Content The present invention relates to a method for the measurement of the moisture or water content of a material in bulk form, especially of grain, electrically so that, in the material to be studied, placed within the area of measurement, an electrical field is generated by means of the electrodes of the sensing element and that the moisture or the water content is determined as the effect of the material to be studied on the electrical properties of the sensing element. The electrical moisture-measurement methods most commonly used in measurements of the type examined are based either on resistance measurement, wherein the effect of moisture on the resistance of the material to be studied is observed, on capacitance measurement, wherein the moisture changes the dielectric constant and, as a consequence of that, the capacitance of the sensing element, or on impedance measurement, wherein the effect of moisture both on the resistance of the sensing element and on the capacitance is observed. In all of these modes of measurement, in order to obtain a precise value of moisture of the material to be studied, a sample must be used that has been brought into a certain state before the process of measurement proper. This can be accomplished, for example, by weighing or by volume measurement, or by pressing the sample with a known force against the measurement electro- des. The sample-standardization step to be performed separately before the measurement proper makes the determination of the moisture content laborious and slow and deteriorates the precision of the measurement. From the U.S. Patent 3,79^ . 911, Dickey-John Corporation, a grain moisture meter is known in which the separate step of weighing of the grain sample has been avoided by hanging the measurement cell used in capaeilive measurement, by means of a spring scale device, onto the frame of the meter. After the correct quantity of grain has been poured into the measurement cell, the weight of the sample closes a switch that starts a digital measure- ment circuit, and the moisture value of the grain is obtained from the display of the meter. Even though the solution suggested partly eliminates the above problems related to the standardization of the grain sample, one of the drawbacks of the system is the necessary extra mechanics of the spring scale used in the system, which is laborious to manufacture and expensive. Thus, the object of the present invention is, as compared with the prior-art technology, to provide a precise and rapid method especially suitable for the measurement of moisture in grain, in which method the sample does not have to be brought in advance into a standard state for the measurement and in whose performance no extra mechanical units, such as scales, are required. The problem has been solved in the invention by starting from a method of the type described in the introduction so that the quantity of the material to be studied and present within the area of measurement is changed and the capacitance of the sensing element, varying along with the quantity of the material to be studied, is measured and that, when the capacitance of the sensing element reaches or by-passes a predetermined value, the resistance of the material to be studied is measured, by means of which resistance the moisture or water content of the material is determined. Thus, in the method of the invention, the measurement of the capacitance of the measurement capa¬ citor corresponds to the establishment of the degree of filling of the sensing element, e.g. weighing, and the resistive mode of measurement is used in the moisture measurement proper. The method can be carried out without moving mechanical parts completely electronically, and the determination of the moment of measurement can be made very precise, because the capacitance measurement can be made highly sensitive, e.g. as bridge measurement. In the measurement of grain, it has also been noticed that the method additionally has a favourable effect com- pensating variations in the hectoliter weight. The invention also relates to a moisture meter for carrying out the method, which moisture meter is characterized in what is stated in claim 4. The invention will be described below by means of an exemplifying embodiment with reference to the attached drawing, wherein Figure 1 illustrates the dependence of the electrical properties of a typical sensing cup on the moisture of grain and on the quantity of grain present in the sensing cup, and Figure 2 shows the wiring diagram of one embodiment of a moisture meter in accordance with the invention. By way of example, Fig. 1 illustrates the dep^endence of the cap^acitance Cm and resistance Rm of a grain sample that contains wheat from the moisture per- centage of the grain. The frequency of measurement used is 20 MHz. The measurement sensing-element is a cup- shaped measurement capacitor whose capacitance as empty is 18 pF. The curves drawn with full lines correspond to a sample quantity of 100 g, and in the case shown with broken lines, the quantity of sample is 200 g. In Fig. 1, moreover, a straight line C = Ccl parallel to the moisture axis has been drawn, which intersects the capacitance curves representing the sensing-element filling quantities of 100 g and 200 g at points a and b, which, in the example case, correspond to moisture values, of 40 % and 18 $.. On the straight line thus drawn, a scale can be formed, between the points a and b by interpolation and outside that range by extrapolation, in which scale every filling quantity of the sensing element corresponds to a certain moisture percentage. By measuring the filling quantity, e.g. by weighing the sample when the capacitance C = 3 pF has 6AD ORIGINAL Q MPi been reached, it would thus be possible to obtain the moisture percentage by using the scale that was prepared. In stead of this, the foundation of the invention is the idea of avoiding the weighing by preparing a new resis- tance scale by connecting the points c and d of the resistance curves, corresponding to the values of -\0 % s 100 g and 18 % 3 200 g, by means of a straight line. On the straight line obtained, it is now possible, by inter¬ polation between the points c and d and by extrapolation outside those points, to form a substantially linear resistance scale, which directly indicates the moisture percentage. The novel principle of measurement that can be derived from the example is made possible by the circum- stance that the change in weight corresponding to the capacitance on the moisture scale is larger than the change in weight corresponding to the same resistance value, in the case of Fig. 1 more than twice as large. When the principle of measurement suggested above is applied, the measurement can take place in the following way. Grain is poured into a cup-shaped measure¬ ment head and during the filling the capacitance of the measurement head is being measured. As, when the quan¬ tity of grain increases, the increasing capacitance reaches a certain predetermined value, the resistance of the measurement head is measured, which is a function of moisture. The final grain moisture percentage can then be obtained by looking at the table, or directly by processing the measured value, e.g., by means of a microprocessor. The switching-over to the resistance measurement and the measurement of the resistance take only a few milliseconds, so that the pouring can be con¬ tinued without interruption until the measurement head is full. The quantity of material within the area of measurement may, of course, also be changed in some other way besides by pouring, and, correspondingly, in stead of filling, the measurement head may also RIGIN emptied. The quantity of sample affecting in the electrical field of the sensing element can also be varied by pushing an appropriately designed measurement head into the material or by pulling it out of the mate- rial if this procedure is better from the point of view of the material to be studied. The electronic measurement of the filling quantity involved in the invention, of course, gives different filling quantities depending on the moisture-, but at least when the dielectric constant of the material to be measured when completely dry is clearly higher than 1, like in the case of grains, being higher than 2, experience has proved that an adequate precision of resolution is obtained within the entire measurement range normally used. Besides the measurement of moisture of grain, discussed here as an example, the method is also suitable for many other applications, such as for the measurement of moisture of seeds, hay, soil, peat, sand, wood, chips, and sawdust, as well as for moisture measurements of various powders in production processes. The method can also be used for the measurement of the water content, e.g., of silage, water-containing emulsions and mixtures. In Fig. 2, a wiring system for the method described above is shown. In this wiring system, the sensing element is a measurement capacitor 1, whose cap c acitance Cm and resistance Rm are affected by the material to be studied. For the measurement of the capacitance C , a capacitance bridge 2 is connected between the electrodes of the measurement capacitor 1 by means of a change-over contact .. The reference capacitance C of the bridge is, at the most important point of the measurement area, compensated so as to be in balance with the resistance of the sample. The other side of the change-over contact k is connected to the resistance-measurement circuit 3- The resistance-mea¬ surement circuit 3 based on the measurement of the OMPI Q-value of the oscillation circuit comprises an oscil¬ lation circuit consisting of an inductance 1 and of a capacitor C, whose capacitance C is charged via the resistance R from a voltage source, a contact k which switches the circuit onto oscillation, a voltage com¬ parator 4, as well as a counter and display unit 5 - The operation of the circuit 3 is controlled by relays, by the intermediate of their contacts 1 and k , the coils 6 and 7 of the relays being connected to the output of the capacitance bridge 3- The contact members shown as relays in Fig. 2 may be accomplished, e.g., as reed relays or by means of low-ohmic FET-contacts. As values of the components of the oscillation circuit may be chosen R = 2 M. ., C = about 1000 pF, and L = about 1 uH, while the capacitance C of the sensing element is about 50 pF. When the measurement is started, the contacts . and ? are in the positions shown in Fig. 2 and the capacitance bridge measures the capacitance of the sensing element 1. As the filling degree of the sensing element 1 is changed, its capacitance C is also changed. As the capacitance Cm reaches a value C a., 3 which, together with the p r reset caυ * acitance value Cref.., balances the bridge, the relay 6, connected to the output of the bridge, changes the capacitance measurement over to resistance measurement by connecting the circuit 3 to the measure¬ ment capacitor 1. At the same time, the contact k of the relay 7 connects the capacitor C, that was charged via the resistance R, onto the oscillation circuit, whereby a damped oscillation is produced. The number of the oscillations until a certain db-damping is proportional to. the Q-value of the entire oscillating system. The capacitance of the measurement capacitor increases as the moisture increases, and its parallel resistance is reduced. The parallel capacitor C is, however, so large that the parallel resistance appears as the main variable. The damped oscillation is introduced into OMPI a comparator 4, whose reference voltage U „ is taken via a voltage distributor, not shown in the drawing, from the same source as the voltage that starts the damped oscillation, whereby a pulse sequence is obtained out of the comparator 4, which pulse sequence has been cut off after the oscillation has been attenuated down to the desired number of decibels. The pulses of the sequence of pulses are fed into the counter and display unit 5. Instead of a counter and a digital display, the damped sequence of pulses may be integrated by means of completely passive components with the aid of a sample- holding circuit, and the indication of moisture can be presented on the scale of a mA-meter. Alternatively, the sequence of pulses may be passed to treatment by means of a microprocessor, whereby a temperature-corrected indication can be obtained directly on the display as moisture percentage.

Claims: WHAT IS CLAIMED IS: 1. A method for the measurement of the moisture or water content of a material in bulk form, especially of grain, electrically so that, in the material to be studied, placed within the area of measurement, an elec¬ trical field is generated by means of the electrodes of the sensing element and that the moisture or the water content is determined as the effect of the material to be studied on the electrical properties of the sensing element, c h a r a c t e r i z e d in that the quantity of the material to be studied and present within the area of measurement is changed and the capacitance (C ) of the sensing element, varying along with the quantity of the material to be studied, is measured and that, when the capacitance (C ) of the sensing element (1) reaches or by-passes a predetermined value (C_), the resistance (Rm) of the material to be studied is measured, by means of which resistance the moisture or the water content of the material is determined. 2. A method as claimed in claim 1, c h a r ¬ a c t e r i z e d in that a cup-shaped sensing element is used and the quantity of the studied material present within the measurement area is changed by filling or emptying the cup of the sensing element. 3. A method as claimed in claim 1, c h a r ¬ a c t e r i z e d in that the quantity of the studied material present within the measurement area is changed by inserting the sensing element into the material to be studied. 4. An electrical apparatus for the measurement of moisture or water content, especially for the measure¬ ment of the moisture of grain, which device includes a sensing element (1), which produces an electrical field in the material to be studied and whose electrical properties (C , R ) are affected by the material to be studied and present within the measurement area, resistance-measurement circuit (3) for the measurement of the resistance ( _) of the sensing element, and apparatuses for the indication of the moisture-dependent value obtained, c h a r a c t e r i z e d in a capacitance-measurement circuit (2) measuring the capacitance (C m ) of the sensing element (1) and acti¬ vating the resistance mesurement when the capacitance (C ) of the sensing element (1), dependent on the quantity of the material to be studied, reaches or by-passes a predetermined capacitance value (C a.). 5. An apparatus as claimed in claim 4, c h a r a c t e r i z e d in that the capacitance- measurement circuit (2) consists of a capacitance bridge that compares the capacitance (C ) of the sensing element ' (1) with a reference capacitor (C f ). 6. An apparatus as claimed in claim -\ for the measurement of moisture or water content, c h a r ¬ a c t e r i z e d in that the resistance-measurement circuit (3) consists of an oscillation circuit (L,C) to • be connected to the sensing element (1) and of a counter (5) counting the pulses of the damped oscilla¬ tion of the oscillation circuit (L,C,C ,R ) damped by the resistance (R ) of the sensing element, down to a predetermined level. 7. An apparatus as claimed in any of the preceding claims for the ' measurement of moisture or water content, c h a r a c t e r i z e d in that the capacitance-measurement circuit (2) and the resistance- measurement circuit (3) are connected to the sensing element (1) by means of contact means (6, k.) performing the changing-over and controlled by the capacitance- measurement circuit (2). 8. An apparatus as claimed in claim 7 for the measurement of moisture or water content, c h a r a c - t e r i z e d in that the contact means (6, k..) are accomplished as reed-relays or by means of low-resistance FET-contacts.

Inventors: WILSKA MATTI GUNNAR
Applicants: WILSKA MATTI GUNNAR
Application year: 1982
Main classification: G01N27
IPC codes: G01N27, G01N33