The present invention relates to a method . for automatically determining the mass content in a block or log of timber, especially round timber, wherein the moisture content of the wood is determined by means of electrodes which are pressed into the wood material in accordance with the introductory portion of claim 1. Further, the present invention relates to a method for determining the moisture content of a wood material, wherein an electric current is conducted through a block or log of timber by means of electrodes which are pres¬ sed into the log of timber, and measure the properties of the electric current in order to obtain an indication of the characteristic electrical properties prevailing within the timber log. The present invention also relates to an arrange¬ ment for determining the moisture content in wood material, which arrangement includes means for transporting a block or log of timber as well as electrode means intended to be pres¬ sed into the timber log in order to provide manner, an electrical connection between the electrode means.

Cellulose and paper mills use large quantities of pulp wood for the production of cellulose and paper, where it is of interest for both the wood material providers as well as the wood material buyers to know the content of "dry" pulp wood, i.e. the true mass content in the wood material which is the object of a transaction.

The problem today is that the incoming pulp wood is bought by weight which is converted to volume, but the volume is an insufficient,, indicator for the constitution of the cellulose, since cellulose furthermore requires additions in the form of chemicals so that the cellulose can form an optimal mass which then is sold as .bales or further refined to paper. Knowledge of the moisture percentage of the pulp wood is also facili¬ tating the tree felling and buying functions since a moisture index facilitates finding the best pulp wood quality, and at the same time a buyer does not have to pay for water but only for pulp fibres.

Various methods are already known for measuring timber. Thus it is possible, by means of optical log measurement arranged, e.g. at a log conveyor, to make an approximation of the .rough volume for an individual log. In this context an operator can also make an ocular inspection and then obtain a subjective opinion of the log's quality. This method only measures the volume of a log but not the degree of moisture therein. Furthermore, a control of the quality at, e.g., a grading table is both time consuming, difficult and impractical.

In handling of wood material a method called pile measuring is further used for cargo on vehicles. Such a measurement is usually committed manually using a gauge stick, by means of which the so called frame volume is determined, after which the measured result is evaluated by approximating "density factors" such as average diameter, twig frequency and twigging, crookedness and piling, after which the fixed volume of the cargo is evaluated by multiplying the frame value with a fixed volume percentage. This method provides an approximative value for the cargo volume but does not provide any evaluation of the moisture- content of the wood material. The process is cumbersome and work consuming and it is diffi¬ cult to adapt to an automatic wood material process.

Nowadays a cargo volume can also be approximated by means of video photographs taken from different directions, combined with a computerized image processing, and in this way building a virtual three-dimensional model of the logs. Nevertheless, this method does not provide any evaluation of the liquid content in the wood material and this method is also cumber¬ some for evaluating the volume of the wood material, since the wood material cannot then be transported by lorry straight to, e.g., a pulping plant, but must be reloaded onto some kind of conveyor.

The known measuring methods discussed above have often been combined with an ocular inspection of the logs in order to remove such logs which are not considered to fulfill the quality requirements. Such an inspection is fully manual and requires rather high experience. Since the inspection is manual an increased throughput speed requires a corresponding increase of the labor staff, and for high amounts of wood material there is a further need to divide the inspection into parallel working stations. Also this method fails to give any reliable estimation of the moisture content for the entire load or for the individual logs, and thus it does not provide any reliable figure of the pulp mass content either.

One also knows different measuring methods based on so called sampling, where certain portions of a larger log quantity are selected, in accordance with some system, for a closer analysis. These selected and analyzed portions, usually full bundles of timber, are then considered to be representative for the whole lot . The results obtained by such an analysis are then utilized in order to evaluate the whole wood material lot on the basis of the measured criteria. Thus, according to one method a respective timber bundle is weighed, after which the volume of the bundle is determined either by individual measuring as discussed above or by lowering the whole bundle into water and determining its volume on the basis of the difference in weight. In practice, such a determination of a volume requires a lot of equipment such as a truck, a weighing bridge, a large water basin, piping, electronics and card readers as well as large areas, as well as time for the actual measuring. In spite of this- the estimate of the moisture content of the wood material is only approximative and due to the sampling one cannot exactly take into consideration such factors like, e.g., the impact of the habitat on the density of the wood material. The method is space consuming and capital intensive and furthermore there will often be a time gap between the time when the pulp wood arrives to the plant and the time when the moisture percentage of the pulp wood is measured, and then quite a lot of moisture vanishes into the air especially in the summer.

In order to determine the general quality of the timber on the basis of a sampling one is forced to analyze the logs separately in order to estimate, on the basis of such an analysis, coefficients which are more or less representative for the whole lot . None of the known methods provide any exact value for the moisture content in the pulp wood, which, on the other hand, is what the cellulose industry would need. Neither will one receive the measured values continuously as a part of a transportation chain, but the evaluation is obtained on the basis of sampled bundles transported from one location to another.

Such a measuring of the moisture content in planks and boards is known, where two adjacent electrodes are applied on the plank surface and the moisture is determined on the basis of the conductivity between the electrodes. However, such a measuring of the moisture is performed as individual measuring operations and it cannot be applied on, e.g., pulp wood where thick wood logs having a layer of bark are received at a large cellulose or paper mill in amounts of 100,000 pieces within a couple of days, and where the measuring of the moisture should be performed on an on-line basis without interruptions and around the clock. Also, the meters discussed above are not capable of measuring the moisture in the depth of the logs .

Finally, methods for measuring the moist content in wood chips in context with, e.g., heating are known, but also these methods are unsuitable for application on pulp wood or the like logs having a solid consistence.

Thus, all known methods for quantification, quality determinations and moisture measurement are laborious and slow, while an industrial process requires a fast and continuous measuring using simple means and providing a high exactness.

Thus, the purpose of the present invention is to provide an arrangement by means of which it is possible to measure the moisture content, preferably defined as a percentage, in a continuous process and in an incoming stream of wood material, especially the pulp wood for a cellulose or paper mill or both.

Further, one object of the present invention is to provide an exact moisture index for incoming pulp wood for cellulose or paper mills on an on-line basis, in order to help the buying function to act optimally; to buy pulp wood having the proper moisture content, to know the moisture content of the pulp wood in order to make the optimal cellulose for each individual need, and to be able to identify the optimal wood material and the optimal felling lots.

One further object of the present invention is to be able to determine the amount of incoming pulp wood as a volume, by means of auxiliary equipment installed in a conveyor. That way a conversion from weight to volume can be avoided, which means a considerable saving of working stages as well as an elimination of possible measuring errors thus arising.

When the data regarding the moisture content in the pulp wood has been collected the data is returned to the respective arriving consignment, indicated for example in cubic meters, and the whole received lot of pulp wood obtains a volume indication and a relevant moisture percentage.

In accordance with the present invention the objects are achieved as disclosed in the appended independent claims. Thus, the method for determining the mass content is characte¬ rized in conveying the timber log along a conveyor through a measuring light-wall, arranging at least one portion of the timber log on a measuring bridge, and pressing electrodes arranged along said measuring bridge into said portion from at least two essentially diametrically opposite sides. Thereafter an electric current is conducted through the respective opposite electrodes and the properties of the electric current through the electrodes are registered. Suitably the operation is- performed so that the timber log is transported through the light-wall whereafter the moisture of the unbarked log is measured prior to sending the log further for barking.

For every opposite pair of electrodes an indication of the relative moisture of the .timber log is calculated on the basis of said electrical properties. The arrangement according to the present invention is especially well suited for measuring the moisture content of unbarked logs, whereby it has been established by, experimental means that the correspondence between the moisture content and the conductivity can be ex¬ pressed in the following manner:

Y = a + Jb * In (x) * D

wherein Y is the relative moisture content expressed in %, x is the resistance expressed in kΩ, D is the diameter of the unbarked log expressed in millimeters and a and b, respectively, are correcting factors. In experiments with spruce it has turned out that a suitable factor a would be about 62,7 while the factor Jb suitably has a value of -4,4.

At the same time information regarding the cross section of the timber log at the respective pairs of electrodes is calculated on the basis. of the measured valued from the light wall, and thus the total volume of the raw log is determined in a manner known per se, for example by means of arrangements which are brought on the market under the trade names "Elmes 3600" or "Banner A-gauge High Resolution Mini-Array" which scan the arriving raw log by means of a carpet of beams of infrared light. After this the moisture indications obtained at each pair of electrodes are combined with the corresponding measured data for the cross section of the timber log at the location of the same respective pair of electrodes. Finally the total relative moisture content for at least the timber log portion which has been arranged on the measuring bridge is calculated as a function of said moisture content indication and said cross section information for respective pairs of electrodes, and thereafter the mass content is calculated as a function of the cross section information and the moisture content.

According to the present, invention the method for determining the moisture content is characterized in that electrodes are pressed in from directions which in relation to the timber log are generally opposite, in order to obtain at least one co¬ operating pair ,of electrodes, after which an electric current is brought to run between the electrodes in the respective pairs of electrodes. Further, the respective cross sectional dimension of the timber log is defined at said pairs of electrodes. Finally the moisture content or an estimate there¬ fore is determined as a function of the respective measured electrical characteristics and the corresponding cross sectional dimension.

In order to perform the measuring operations in accordance with the present invention an arrangement has been developed which is characterized in that the arrangement includes means for determining, along the extent of the timber log, a multi¬ tude of spaced respective cross sections of the timber log, a measuring bridge including at least two electrodes which are essentially opposite in relation to the timber log and which constitute at least a first co-operating pair of electrodes, as well as means for pressing said opposite electrodes into the timber log at' opposite sides of the log.

The respective pairs of electrodes are arranged to determine, in a first cross sectional direction at suitably a multitude of spaced measuring points located along the extent of the timber log, for the respective cross section characteristic electrical measuring properties which depend on the relative moisture content of the timber log. The arrangement taken as an entity further includes processor means for calculating, based on said respective cross sections and . said characteristic electrical properties which have been measured for said cross sections, an estimate for the average relative moisture content of the timber log. Some other characteristics and especially favorable embodiments of the present invention are disclosed in the dependent claims.

The present invention will now be disclosed in more detail as an example and referring to the enclosed schematic principle drawing which discloses a favorable embodiment of the present invention, wherein

Figure 1 in perspective discloses the general structure of an arrangement in accordance with the present invention,

Figure 2 sectionally discloses a timber log which has been arranged on a conveyor and against which measuring means have been arranged in accordance with the present invention, and

Figure 3 discloses an example of an electrode arrangement by means of which an electrode can be pressed into a timber log.

Referring to Figure 1 the present invention includes a con¬ veyor 1 known per se (of which, for clarity, the Figure shows only some rolls Ia which are rotatable by means of a driving motor 2) for transporting a log (which for clarity reasons has been omitted from Figure 1) through the apparatus. The log is transported through a measuring light-wall 3 known per se to a measuring bridge generally referred to by reference 4. The measuring bridge includes a number of individual measuring electrodes 5 so that opposite pairs of electrodes 5a, 5b and 5c, 5d, respectively, can be brought to move towards each other, as clearer indicated more clearly in Figure 2. Said electrodes 5a, 5b, 5c and 5d can then be brought into contact with the surface of the log 7 and suitably to some extent brought to penetrate into the wood material. Electrical cur¬ rent is conducted through the electrodes and they are con¬ nected to an analyzer which is capable of registering currents and voltages. The voltage is favorably AC in the order of 100 to 1000 Volts whereby the frequency suitably is a normal net¬ work frequency of about 50 Hz. Most of the currents run under the bark in the raw log, since the stratum under the bark is the water-bearing portion of a tree. For this reason it is desirable that the bark remains relatively intact so that reliable measuring results can be obtained. It is also of advantage that the raw log is so fresh and in as good shape as possible, since the tree then will preserve its normal water content in relation to the actual raw log. By means of the analyser a value x is determined, which value describes the conductivity of the log, and thus an estimate of the moisture content of the log is attained therefrom through the above correlation.

In practice, the measuring is performed in the respective analyzed cross section suitably by means of two pairs of electrodes having a relative angle of 90°. Herein it is of importance that opposite pairs of electrodes perform the measuring operation simultaneously so that by measuring in direct opposite directions, from the upside downwards and from one side to the other, the raw log can be penetrated and thus all voltages in the measured directions can be obtained.

In the embodiment disclosed in the appended Figures the measuring operation is not started until retaining straps 6 have fixed the raw log to the measuring device, in order to guarantee a full stability for the log. Since the moisture usually is unequally distributed in the raw log. For example, if the log has been stored in the wood for some time on one specific side the moisture will be located at the lowermost part of the log, in relation to the ground. Laboratory experi¬ ments have further shown that moisture will move immediately under the bark in the surface layer of the raw log, i.e. the deeper one penetrates into the log the less moisture there will be. Measurements have also indicated that if the log has been newly felled the most part of the moisture will prevail in the lower end of the log. For this reason the length of the electrodes is favorably such that a conical portion of about 15 to 20 millimeters will penetrate through the bark, since a deeper penetration . brings about the risk for a mis¬ representation of the values.

Figure 1 discloses a multitude of successive electrodes 5aj_...5a-j_ ...5an ... 5d;]_...5dj_...5dn at respective pressing beams 8a ... 8d, which by means of pressing means 9 known per se such a hydraulic cylinders, screw means or linear motors can bring a respective array of electrodes 5a^...5aj_ ...5an to move against the surface of the log 7 in order to execute a measuring operation, and, respectively, to free the electrodes from the log surface in order to render a further transport of the log 7 away from the measuring arrangement possible. Favorably the electrodes 5 are pressed not more than 20 milli¬ meters into the surface of the log, in order to guarantee that each pair of electrodes obtain a sufficient contact with the wood material.

As discussed above, the voltage will be conducted immediately under the bark in the surface layer of the raw log. For this reason the electrodes should be pressed only so deep that they pierce the bark and penetrate about 20 millimeters into the raw log proper. In laboratory tests the force required for pressing in electrodes having a conical end and a base dia¬ meter of 7 millimeters has been in the order of 1500 to 1900 Newton, the number of electrodes have been 40 and the weight of each electrode, including the mechanical equipment, have been about 4 kilograms.

In spite of the fact that rollers equipped with electrodes in some case would facilitate a higher through-put speed it has in practice turned out that ' successively arranged separate electrodes provide a more reliable result. In order to get an exact measuring of the current it must be measured in diametrically opposite directions . For measuring the current under the bark, which during the winter season is icy, a penetration through the bark into the raw wood material is required, which usually is impossible if the measuring is performed by means of continuous rollers . Neither can rollers be used unless the log to its shape is fully round or oval, and also extending twigs and an uneven bark constitute a hindrance. Further it has been observed that continuous rollers are less effective for measuring the current in the actual raw wood material under the bark where most of the current will be conducted.

The electrodes themselves are suitably conically shaped so that they effectively will force the cells of the wood material aside without splitting them. The angle of the penetrating electrode point is favorably about 40°.

The actual measuring operation is suitably performed in two steps after the electrodes have been pressed into the wood material. This means that after initialling the measuring operation a measurement is first made in one direction, for example vertically, and thereafter it is performed in another direction which is angular to the first one, in this case horizontally. The results of the measuring operation are saved in a data base, the electrodes are withdrawn from the log, the log is transported away from the apparatus and the next log is brought in. Since the measured values are in practice slightly influenced by discontinuities in the log it is usually appropriate to exclude the extreme values from the calculations, e.g., 15 % at both ends of the result interval.

Above some favorable embodiments of the present invention have been discussed by means of examples, but for the professional it is clear that the present invention can be varied in many- other ways within the scope of the appended claims.