ES2566775B2 - Resistive method and system for measuring moisture in a fibrous material, and computer program - Google Patents

Abstract

Resistive method and system for measuring moisture in a fibrous material, and computer program. # The method comprises, sequentially and automatically: #a) apply alternating voltage input signals of different frequencies between electrodes (E1 , E2) in contact and / or inserted in a sample of fibrous material (M); # b) obtain a plurality of values of the circulating current through the sample (M); # c) calculate the equivalent electrical resistance of the sample ( M) from the application of a statistical function on two or more of the current values obtained in b) and / or on values calculated from them; and # d) determine, by calculation, the degree of humidity of the sample (M) from the determined equivalent electrical resistance value. # The system and the computer program are both adapted to implement the method of the invention.

Description

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DESCRIPTION

Resistive method and system for measuring moisture in a fibrous material, and computer program.

Technology Sector

The present invention concerns, in a first aspect, a resistive method for measuring moisture in a fibrous material, in general wood, from the value of the equivalent electrical resistance of a sample of fibrous material determined from the measurement of the current flowing through it when applying an electric voltage on it, and more particularly to a method that comprises performing multiple measurements of current flowing through the sample by applying on the same alternating voltage signals of different frequencies, and applying a statistical function on the measured values to calculate the equivalent electrical resistance.

A second aspect of the invention concerns a system adapted to implement the method.

A third aspect of the invention concerns a computer program adapted to implement the method.

State of the prior art

In accordance with the Construction Products Directive (EU regulation 305/2011), the CE marking of structurally classified rectangular section sawn timber is carried out subject to the provisions of Annex ZA of the harmonized standard EN 14081- 1 in force. The implantation of the CE Marking in the sawmills generates a lot of particular casulstica, which is why to harmonize the interpretations of the harmonized standard EN 14081-1 and coordinate the supervision of its implementation by the national Notified Bodies (those in charge of supervision) , an advisory committee was created, known by the acronym GNB-CPD SG 18.

The conformity assessment system that results from application to structural sawn timber is set by the National Notified Bodies that are responsible for evaluating and approving the correct implementation (reflecting its approval by issuing a certificate) of the Control Manual of Factory Production (CPF). This manual, written by the manufacturers, must establish the internal processes and methodologies that ensure the perfect control of your production.

In its Position Paper NB-CPD / SG18 / 07/051 on the harmonized standard EN 140811: 2005 (of November 2, 2007), GNB-CPD SG 18 establishes that for each batch of wood classified structurally, both by visual means (the most common in Spain) and by automated systems, you must ensure that, at least once, the following variables are recorded: Wood species, Origin, Dimensions, Quality class (according to the standard or classification system followed) , Moisture content (if classified in dry).

It is also indicated in said position document that the competence of the classification staff and the calibration of the humidity meters shall be monitored at least annually. Analyzing with a little attention what is established in the EN standard

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14081-1: 2011 regarding the measurement of moisture content, it is noted that in paragraph 5.1.1 it is quoted verbatim "The moisture content shall be determined in accordance with EN 13183-2 (resistive measure) or EN 13183-3 (capacitive measurement) The accuracy of the humidity measuring device must be ± 2% ".

This requirement established in the harmonized standard EN 14081-1: 2011 regarding the quality of the measurement of the xylohygrometers (evaluated by precision) is very difficult to comply with in practice by most of the devices currently in use since, of agreement with the only major European study in this regard (carried out by 'Forsen and Tarvainen' in the year 2000 on commercial equipment: Forsen, H. and Tarvainen, V. (2000) "Accuracy and functionality of hand held wood moisture content meters ", VTT publications, no. 420. 95 pp. ISBN 9-3)," the accuracy of moisture meters (with a 95% confidence interval) evaluated in laboratory tests on well-conditioned material (without internal gradients of humidity) is ± 1.5% to ± 2.5% for resistance devices and ± 2.5% to ± 4.0% for capacitance devices.The accuracy determined in industrial tests is ± 2% at ± 5% for resistance devices and ± 3% to ± 5% for capacitance devices ". In this same work, the authors quote verbatim that "The majority of resistance moisture meters show systematic deviations from their measurement with respect to the actual values due to the incorrect use of internal calibration curves".

From the reading of the above, a first problem can already be seen: that currently existing commercial devices do not have the precision required in their measure to be used in the control of wood moisture in accordance with EN 14081-1. But the first cause of the problem has also been identified, at least in resistance devices (the most common): that the internal calibration curves of the device are not suitable for the species measured.

To understand the reason for this problem and its possible solution, it should be noted that the internal calibration curve of the large majority of European appliances corresponds to that of spruce wood, and that the shape of this curve is very different from the corresponding to, for example, that of pine wood. The species selectors that are usually seen in commercial devices the only thing they do in practice (except for very advanced models) is to move the pattern curve; which only "translates" the problem of ralz: that the shape of the internal standard curve does not fit those of the different species. For this reason, if you really want to improve the precision of the measurement of resistance xylohygrometers working on one or a few specific species, it is necessary to know and apply the appropriate curves to convert the electrical resistance measurements into moisture measurements.

If we continue with the analysis of the contents of the harmonized standard EN 14081-1: 2011 regarding the measurement of moisture content, we note that in section 6.3.2 2.1 it is quoted verbatim that "All weighing, measuring and test shall be calibrated and regularly inspected in accordance with documented procedures, criteria and frequencies. " In a footnote in this same section it is cited that "The calibration of moisture meters shall be controlled at least annually." From the reading of the above it follows that the manual of the CPF (Factory Production Control) must contain procedures for annual calibration and periodic verification of moisture measuring devices.

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From the combination of the two requirements referred to in standard EN 14081-1: 2011 (5.1.1 and 6.3 2.2.1) it follows that in the CPF there must be not only documented procedures for the calibration of the devices but also In them, the precision of the measurement taken in the species to be marked must be evaluated. That is, there is no room for general calibrations "for all species of a scale" but true calibrations for the wood for which the CE Marking is declared and established.

As regards the use of moisture meters by the resistance method, the harmonized standard EN 14081-1: 2011 calls for the contents of the standard EN 13183-2: 2002, which establishes clearly and decisively how these devices should be used to produce a quality measure. Thus, in section 4 it is quoted verbatim that "This method is valid for wood that has an approximate moisture content between 7% and 30%. Certain types of protection treatments, fireproofing or chemical or thermal treatments they can affect the accuracy of the measurement and require a particular calibration of the instrument depending on the type of treatment ". We clearly see how the need to carry out particular calibrations according to the characteristics of the material to be evaluated is revealed.

But this same norm in its section 6, establishes that "before carrying out the measurements, the electric resistance xylohygrometer must be verified according to the instructions provided by the supplier of the device" and that "to check the precision of the equipment, resistance boxes can be used of calibration ".

From the reading of all of the above it follows that in the light of the CE Marking, in the measure of the moisture content of the structural sawn timber, devices calibrated annually must be used, with precision better than ± 2% and that to achieve this it is necessary to perform calibrations adapted to the characteristics of the substrate to be measured (wood species, applied treatment, etc.). It is also mentioned that resistance boxes can be used to carry out the verifications.

The root of the problem with resistance meters is evidenced by the European study cited above, and carried out by 'Forsen and Tarvainen' in 2000 on commercial appliances, by establishing that "the majority of resistance moisture meters show systematic deviations from their measurement with respect to the actual values due to the incorrect use of internal calibration curves ". For this reason, in order to carry out a calibration of the resistance moisture meters, the use of resistance boxes is not enough, but also, it is necessary to know for each material the correct calibration curves (which relate the electrical resistance for each species of wood measured by the device with the humidity value). This report demonstrates that a large part of the moisture meters do not meet the requirements to allow the Ce marking of the wood since they are above 2% accuracy in measurement and have not been calibrated according to the curves corresponding to each of the wood species they measure.

At this time it is convenient to point out that the vast majority of "calibration certificates" that are seen on the market are not such and that, at most, they can be considered "verification" certificates. This is so! because the only thing that issuers of such certificates do is to verify that the equipment continues to measure similarly to its factory output (following the instructions provided by the

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manufacturer of the device), that is, according to its internal calibration curve (remember that in most of the equipment corresponds to that of the spruce). These "calibration" certificates provide general data valid for all species or, at most, for each of the scales; without indication of the precision obtained by measuring with the device on specific wood species. For this reason, to calibrate a resistance xylohygrometer, it is not enough to have calibrated resistance boxes, but it is also necessary to have the curves that relate the measure of the electrical resistance to the moisture value for each species of wood as well as knowing the precision of such curves which, in short, is the cause - together with the quality of the measurement of the electrical resistance made by the apparatus - of the precision of the measurement taken by the apparatus.

From all of the above, it follows that few measurement systems currently used are capable of responding to European regulations.

The method of measuring electrical resistance is, for practical reasons, the most widely used in practice to determine the degree of moisture in the fields of wood and construction materials, although it has the disadvantage that, although when the content of Humidity is low resistance changes obtained with the change of moisture content are considerable, when the moisture content is high (for example in the case of wood for humidity above 60%) the variation of the electrical resistance with the change of humidity is very low, so that in such cases the measurement values are significantly less accurate, when performed with the state of the art devices. Also, in the resistive technique, very high resistances must be measured (especially for low humidity values) and the appearance of parasitic effects not attributable to the particular measurement can be very high, which can lead to significant errors in the measurement made with state of the art devices.

That is why it is necessary to offer an alternative to the state of the art that allows to implement a precise and adjustable resistive measurement procedure to the type of wood (and in general to any kind of fibrous material), offering the necessary accuracy and confidence to allow the CE marking of the wood and offer adequate guarantees at all levels within the production and distribution channels of the wood.

Some of the commercial devices for the resistive measurement of moisture in wood with the above-mentioned drawbacks are, for example, those of the Veto companies (
http://www.veto.cl/index.php?page=shop.product details & flypage = flypage.pbv.tpl & produ ct id = 239 & category id = 91 & option = com virtuemart & ltemid = 743 & lang = es) and Gann Messu. Regeltechnik GmbH (for example the hydrometer model HT 65:
http: //www.gann. from / Produkte / ElektronischeFeuchtigkeitsmessger% C3% A4te / StandardLine / HydrometteHT 65 / tabid / 101 / language / en-US / Default.aspx).

It should also be noted that there are patent documents that protect methods and systems for measuring humidity, based on resistive procedures, but also suffer from the aforementioned lack of precision in the measurements obtained, some of which are cited below. Note that all of them are in a time horizon greater than 30 years, which is a clear indication that what they propose is very likely to be improved.

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For example, patents US3331020 (published in 1967) and US4408128 (published in 1983) both dedicated to the measurement of resistance and its application in obtaining moisture in various types of materials, including wood and wheat grains. Due to the electronic components and stages of the electronic design used in said circuits, its application to the measure of high resistance values, such as certain wood species, makes it impossible to obtain the precision required by the new regulations of CE marking.

Specifically, US3331020 refers to a circuit based on semiconductor devices (transistors) and defines its novelty compared to previously used vacuum lamps given the recent invention at that time of integrated circuit technology by Jack Kilby in 1958 This circuit is mainly based on the use of two NPN type transistors and variable resistors on which to apply a current source that circulates through the resistance to be measured. This patent does not present any applicability today, given the advanced state of the art and the existing electronic circuits. On the other hand, the rudimentary measurement procedure is clearly very inaccurate; In fact, precision values are not detailed in the patent, speaking only of a linear operating scale of up to 100 MOhm, a very low value considering the high resistance values that must be measured in the case of moisture in wood below 15% (depending on the type of wood).

In relation to the US4408128 patent, also based on the measurement of moisture in wood by means of electrical resistance, it proposes a system that proposes the conversion of the measure of electrical resistance in magnitude of moisture in wood by means of the use of integrated circuits that include the compensation of temperature, wood type compensation curve, log conversion, etc. All this adds great complexity to the system. On the other hand, as in the previous patent, the adjustment curves shown reach maximum values of 120 MOhm, and therefore, offer a reliable value in the humidity measurement above 10% (higher even depending on the type of the species) that is outside the humidity and resistance ranges that need to be measured in order to comply with the measurement standard. Precision values are not offered in this patent either, so a comparison is not possible but in view of the measurement circuits raised in cascade it follows that the precision will be low given the many resistance values used (this introduces errors given the inaccuracy in their value), the use of analog switches that generate current losses sometimes of the same order as the current to be measured, and in general the high circuitry required.

Currently, the instruments that measure with the accuracy required by the CE marking standard are specialized instruments for measuring high resistance, as is the case of the IET1865 Mega-ohmmeter. These are large-scale instruments characterized by the use of complex internal circuitry and as a major disadvantage, it should be noted that in order to make proper use of them, the user is required to have electronic knowledge. This makes its use in applications related to the timber industry unfeasible.

Likewise, the German patent documents DE3306460-A1, DE3306460-A and DE3306460-C make proposals that are mainly based on obtaining the electrical resistance through a very poorly developed analogue measurement circuit and where the value of the measurement of humidity is given by the direct comparison between the

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resistance value obtained with that of certain compensation tables that are variable and programmable, although without the use of any algorithm to automatically and accurately determine the electrical resistance value.

On the other hand, the US4259633 patent details a moisture measurement procedure also based on electrical resistance. This patent proposes a measurement method where a variable resistance is placed coupled to the measurement probe to adjust a possible displacement in the voltage measurement (bias) to zero. It is not indicated that apart from this compensation performed through electronic circuitry, a procedure for measuring multiple data is taken iteratively, as the electronic scheme shows, there is no type of processor or digital system capable of storing and processing The value of the measure is based solely on a resistance switch that through the circuitry performs certain compensations and offers a value based on direct measurement without any processing, iterative acquisition and stabilization. In fact, it is indicated that resistance switching must be done manually through the device operator.

The adjustment proposed in US4259633 is carried out by correcting the magnitude of the electrical voltage applied to the wood, in order to compensate for anomalous properties of the wood, in particular inverse tension effects inherent to each type of wood. It is not indicated in US4259633 or make a variation of the frequency of the applied voltage or take into account any additional measure to that obtained once the magnitude of the electrical voltage has been corrected to obtain the moisture measure of the wood.

In summary, as indicated above, it is necessary to offer an alternative to the state of the art that allows to improve the accuracy of the known resistive measurement methods and systems / devices.

Explanation of the invention

The present invention has the purpose of constituting the aforementioned alternative to the resistive methods of the state of the art, and concerns, in a first aspect, a resistive method for the measurement of moisture in a fibrous material, generally wood, which It comprises, sequentially:

a) apply an electrical voltage between two or more electrodes in contact and / or inserted in a sample of fibrous material;

b) obtain the value of the circulating current through said sample and caused by the application of said electrical voltage:

c) determining the equivalent electrical resistance of said sample of fibrous material from the value of the feedback obtained; Y

d) determine, by calculation and depending on the fibrous material, the degree of moisture in the sample of fibrous material from the equivalent equivalent electrical resistance value.

Unlike the known methods, the one proposed by the first aspect of the invention automatically comprises:

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- in said step a), applying at least a plurality of alternating input voltage signals of different frequencies and also applying at least one continuous input voltage signal;

- in said step b), obtaining a plurality of values of the current flowing through the sample of fibrous material (M) during the application of said plurality of input alternating voltage signals and obtaining at least one value of the circulating current between the two or more electrodes (E1, E2) during the application of said continuous voltage signal; Y

- in said step c), calculate the equivalent electrical resistance from the application of a statistical function on two or more of the current values obtained in b) for two or more corresponding signals of alternating input voltage of different frequencies and / or on values calculated from them and for the signal or signals of continuous input voltage and / or on one or more values calculated from them.

As regards the plurality of values of the circulating current through the sample of fibrous material, the method of the first aspect of the present invention comprises obtaining them from measurements, in general of electrical voltage, performed directly on said or other electrodes in contact with the sample of fibrous material, or indirectly without contact with the sample of fibrous material.

For an example of realization, the method of the first aspect comprises performing said measurements in different areas of the sample of fibrous material and / or at various depths.

Preferably said statistical function is an average function, although, in a complementary or alternative way, the use of other statistical functions is also possible.

According to an example of embodiment, the method comprises iteratively applying a calculation algorithm on the plurality of current values obtained, until obtaining values considered statistically significant for at least the said statistical function, and applying the statistical function, in the stage e), on said statistically significant values.

According to one embodiment, at least a part of the plurality of alternating voltage signals are square pulse signals and step a) comprises applying said square pulse signals of different frequency sequentially.

Preferably, at least the plurality of alternating voltage signals form part of a single frequency variable voltage signal, and the method comprises applying, in step a), said single voltage signal varying its frequency during its application, according to a sweep of frequencies within a given frequency range which, according to an embodiment example, varies up to substantially 1 MHz.

Alternatively, although less preferably, the alternating voltage signals are not part of a single voltage signal but are independent signals from each other.

For another example of embodiment, the method comprises, in step a), also varying the magnitude of said single voltage signal, during its application, by incremental steps.

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The method proposed by the first aspect of the invention comprises, for another example of realization, prior to step a), performing an adjustment process to determine as a minimum a suitable value, at least as regards magnitude, of the input voltage signals to be applied between the two or more electrodes, which causes circulation by the sample of fibrous material, between the two or more electrodes, of a current with a value that is within a measurable, direct or indirectly, where said suitable value is initially used in step a) for at least the first pulse of alternating voltage signal to be applied. In general, the value of the circulating current is obtained indirectly from electrical voltage measurements, so, in that case, the said measurable range refers to electrical voltage values.

Said adjustment process comprises applying between the two or more electrodes an alternating voltage signal with an initial value, referring at least to magnitude, and obtaining, if current circulates, the value of the circulating current through the sample of fibrous material, and :

i) if the circulating current has a value within the measurable range, determine that said initial value is the appropriate value; or

ii) if the current has a value outside said measurable range, increase, if it is below the measurable range, or decrease, if above, said initial value of the alternating voltage signal, apply it on the two or more electrodes and obtain again, if current flows, the value of the circulating current through the sample of fibrous material, and if the circulating current has a value within the measurable range determine that said value is the appropriate value, and if not! repeat iteratively, step ii) until the appropriate value is determined.

Preferably, the alternating voltage signal applied in the adjustment process is a square pulse signal.

The adjustment process also advantageously comprises selecting a suitable measuring range from a plurality of measuring ranges provided by the variation of at least one variable resistance, said selection being carried out, during the application of the alternating signal of the process of adjustment, varying the value of said variable resistance from a minimum value that provides the lowest measurement range to a higher value that provides the highest possible measurement range without generating saturation voltage on the variable resistance), where the measurement range suitable is the one used during stages a) and b), and, in general (for the reasons stated above), it is a measuring range related to electrical voltage measurements.

For an example of carrying out the method proposed by the first aspect of the present invention, step a) comprises applying at least part of said alternating voltage signals of different frequencies sequentially in order to detect frequency or resonance frequencies of the sample of fibrous material from the current values obtained in step b), such resonance frequencies being understood as those that have a greater influence on the response of the fibrous material (and which are due for example to the type of material and / or imperfections in its internal structure), that is, in the value or values acquired by the circulating current, both in terms of polarity and magnitude, as will be explained later, this frequency or resonance frequencies will be taken into account in the

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obtaining the final value of the humidity by means of a correction dependent on the frequency that has been detected as more influential.

Said alternating voltage signals applied in a) in order to detect resonance frequencies of the sample of fibrous material are, for a variant of said realization example, sinusoidal signals.

According to an example of embodiment, step e) of the method proposed by the first aspect of the invention comprises applying at least one average to the current values obtained, excluding those that show a deviation greater than a certain value, and using the average value obtained for the calculation of the equivalent electrical resistance.

Said averaging comprises, for a variant of said embodiment example:

- apply previous averages, each to a respective group of current values obtained, including at least one group of current values of polarity opposite to that of the other groups, and calculate the standard deviation of each of said groups;

- select the average current values of those of said previous averages relative to groups whose standard deviation is below a threshold value (for example 10% of the average value); Y

- iteratively apply averaging to the average current values selected for successive groups of current values obtained, as well as the calculation of their standard deviation, and take the average value thereof as the measurement value once the standard deviation of these average current values is less than a threshold (for example 2% of the average value).

The method of the first aspect of the invention comprises, in general, in step d), calculating the degree of moisture in the sample of fibrous material for a temperature of 25 ° C, or substantially 25 ° C, applying an algorithm that it relates some constants inherent to the type of fibrous material with the equivalent resistance value determined in c), for a temperature of 25 ° C, or substantially 25 ° C.

After that, the method comprises applying a temperature compensation to the moisture degree value calculated to obtain a moisture degree value compensated to adjust to the actual temperature conditions of the fibrous material sample.

Additionally, if at least one resonance frequency of the fibrous material sample has been detected as a result of applying the aforementioned alternating voltage signals, the method comprises applying a correction factor to the moisture degree value calculated or temperature compensated based on information on properties of the sample of fibrous material related to moisture, which is inferred from the value of the resonance frequency detected.

A second aspect of the present invention concerns a system for measuring moisture in a fibrous material, generally wood, by a resistive method, where the system comprises:

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- an electronic measuring circuit configured and arranged to apply an electrical voltage between two or more electrodes to be placed in contact and / or inserted into a sample of fibrous material, and to be obtained, by direct or indirect measurement (in general from measurements electric voltage), the current flowing through said sample and caused by the application of said electric voltage; Y

- processing means in connection with or forming part of said electronic measuring circuit and which are configured and arranged to determine the equivalent electrical resistance of said sample of fibrous material from the current value obtained and to determine, by calculation, the degree of moisture in the sample of fibrous material from the equivalent equivalent electrical resistance value.

Unlike the known systems, the system proposed by the second aspect of the present invention:

- it comprises control means that control said electronic measurement circuit so that, automatically, it applies a plurality of alternating voltage input signals of different frequencies between said two or more electrodes, and so that it obtains a plurality of values of the current flowing through the sample of fibrous material during the application of said plurality of input alternating voltage signals (preferably to obtain several current values for each input alternating voltage signal), and

- the processing means are configured and arranged to calculate the equivalent electrical resistance by processing two or more of the current values obtained for two or more corresponding alternating voltage signals of different frequencies, where said processing comprises the application of a function statistics on two or more of the current values obtained and / or on values calculated from them following steps a, b and c of the method proposed by the first aspect of the invention.

The system proposed by the second aspect of the invention is adapted to implement the method of the first aspect.

According to an example of realization, the system of the second aspect of the invention comprises a device, preferably portable, which houses both the electronic circuit and the processing means and the control means.

For an exemplary embodiment, the system comprises communication means provided to establish a two-way data communication between the processing means and a remote computing system.

According to a variant of said embodiment, the system of the second aspect of the present invention also includes said remote computing system.

It should be noted that, although both the method of the first aspect of the present invention and the system of the second aspect have been described (and claimed) limited to the application of electrical input voltage signals at the two or more electrodes,

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alternatively, it is possible to replace such input voltage signals with input signals (electrical or electromagnetic) that are not voltage but cause, in an equivalent manner, the circulation of current by the sample of fibrous material, such as the case of signals AC input.

A third aspect of the present invention concerns a computer program that includes code instructions that when executed on a computer (or any other kind of computer equipment) implement at least steps e) and d) of the method of the first aspect .

Brief description of the drawings

The foregoing and other advantages and characteristics will be more fully understood from the following detailed description of some examples of realization with reference to the attached drawings, which should be taken by way of illustration and not limitation, in which:

Fig. 1 shows, in a simplified manner, the equivalent circuit that includes the measurement system proposed by the second aspect of the invention and the equivalent resistance of the wood sample Rx, for an embodiment.

Fig. 2 schematically shows the system proposed by the second aspect of the invention, for an exemplary embodiment.

Figure 3 shows the block diagram of a prototype of the measurement circuit that implements the system proposed by the second aspect of the invention, for an embodiment.

Figure 4 is an approximation graph (continuous line) for estimating moisture in wood (Y axis) once the electrical resistance value (X axis) is obtained, according to the experimental values obtained by the calibration laboratory (points), for the Radiata Pine (PR) at 25 ° C.

Figure 5 is a graph of evolution over time of the resistive value in two different samples of initially dry pine, subsequently moistened and allowed to dry, obtained by means of the experimental prototype of the measurement circuit of Figure 3.

Figure 6 is a graph of evolution over time of the resistive value in three different samples of initially dried spruce, subsequently moistened and allowed to dry, also obtained by the measuring circuit of Figure 3.

Detailed description of some examples of realization

Figure 2 shows schematically, and in a simplified way, the system proposed by the second aspect of the invention, for an example of an embodiment for which it comprises a portable device D that houses inside the electronic measuring circuit Cm, to the processing means P and to the control means MC, where the circuit Cm applies the input alternating voltage signals (as explained in a previous section) on the wood sample M through two or more electrodes E1, E2 inserted in it, under the control of the control means MC, and supplies the current values obtained from the current measurements made through the electrodes E1, E2 to the processing means P, so that this process them

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according to the method of the first aspect of the present invention. Although they have been illustrated as independent elements, the control means MC and the processing means P may constitute the same entity, for another example of realization.

The method, system and computer program proposed by the present invention have been experimentally implemented in their more elaborate embodiments, described in a previous section, implementing the measurement procedure developed by the present inventors which is based on the calculation of the humidity of the wood by two main steps: determination of the equivalent electrical resistance and calculation of the corresponding relative humidity by means of compensation algorithms based on the type of wood and the temperature at which the measurement is made. This procedure conforms to standard UNE-EN 13183-2 of 2004 that details how to perform the measurement in its methodological aspects. The invention raises the control of the tensions to be applied to the wood, as well as the introduction of measurement correction factors based on the temperature and the type of wood. The procedure provides a reliable and high precision measurement, with a short stabilization time.

Therefore, in order to comply with the UNE-EN 13183-2 methodological regulation of the measurement, in the aforementioned experimental implementation, first all tests have been carried out following the indications of the same in reference to measuring distances, types of tips of measurement, depth of measurement, orientation of the wood, etc. In this way, the measurement procedure requires the insertion of two points into the wood at a certain depth, which will be referred to as a probe or electrodes on which the voltages object of this invention are applied. This probe performs equivalent resistance measurements with various levels of voltage and current to check the response of the wood and based on the results, it applies an algorithm that extracts the value in relative humidity. The scope of this invention is fixed in obtaining the equivalent resistance value of the wood in a fast and precise manner, and can be used in a very wide range of equivalent resistance of the wood (from mega ohms to tens of giga ohms) without loss of precision throughout the entire measurement range.

As the main basic elements for obtaining the measurement, it is based on a system that allows the adjustment in, initially, five measuring ranges, which will be automatically selected depending on the result of the measurement process so that it is possible to obtain the value of the measurement within the maximum voltage range (V) and that, therefore, will allow us more precision in the result. The resistance measurement that can be obtained for each of the five ranges is as follows: [2M-47M], [4.16M-100M], [42M-1G], [420M-10G], [2.1G- 50G].

Figure 1 illustrates the equivalent circuit that includes the measurement system proposed by the second aspect of the invention and the equivalent resistance of the wood sample Rx, where the system is represented in a simplified manner incorporating an input voltage Vin (variable to provide the signals of alternating voltage of different frequencies as explained in a previous section), a variable resistance Rf that provides the measuring ranges and an amplifier A, which provides, at its output, the output voltage Vout. The equivalent resistance of wood Rx is calculated according to the formula: Rx = (Rf * Vin) / (Vout / PGA_gain) where PGA_gain is the gain of amplifier A, in general a programmable amplifier.

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Once the measuring probe is located and connected through conveniently shielded cables to avoid external influences, the procedure applied consists in successively applying a series of voltage pulse trains on the probe, reading the current values simultaneously that in each case circulates between the two tips of the probe, storing the values and processing them through a digital system (microprocessor or similar). The application process is as follows:

1. The measuring system is fed to a voltage Vcc, so the application of square voltage pulses is started on the wooden sample, starting with a voltage value of Vcc / 2. Experimentally it has been proven that the currents circulate better through the wood if they are initially low and gradually increase, therefore, the first thing that the algorithm should perform is the selection of the lowest resistance measurement range [2M-47M] and quickly increase the range to the maximum [2.1G-50G]. Proceeding in this direction and not the opposite or starting with intermediate values is essential to improve the final precision in the measurement and apply a stabilizing current in the reorganization of the internal structure of the wood that it suffers when passing a current through it. . Next, the selectable value of Rf is set to its maximum value and the voltage Vout is measured for a Tsat time of the order of milliseconds. If Vout is a value close to the saturation voltage (Vsat), the Rf value is changed and measured again, repeating the process until a Rf value is obtained that does not generate a saturation voltage at the output or is traversed all possible Rf values without success.

2. Depending on the result of the measurement, if the current flowing (calculated from the voltage measurement by Ohm's Law) is a value that cannot be determined because it is very low, voltage 0 is increased, 5V (if as a result a very high current value is obtained, the voltage 0.5V is reduced and the same procedure is detailed below). This process is repeated until a measurable current or voltage value is obtained, at which time it is tried again to modify the range to a higher value to verify that the saturation voltage is still exceeded, otherwise, a value close to saturation than a very low Vout value, so that the range that greater Vout provides without reaching saturation is set. A Tstab time of the order of one second is expected.

3. Once the appropriate values of Vcc and Rf have been set, a pulse frequency scan that varies between 100Hz and 1MHz is performed with incremental steps of the order of log10 (applied frequency). With this scan it is possible to check the behavior of the wood at different frequencies since the internal structure of the wood behaves differently depending on the frequency.

4. With the use of this technique, multiple values of the current measurement that circulates through the sample are obtained and which will then be treated by the electrical resistance estimation algorithm.

5. Additionally, to obtain more data another frequency sweep is applied using a sine wave to know the behavior of matter and the resulting current. This may allow to detect resonance frequencies that

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They will facilitate to know details about the structure of the material and therefore, to relate to the type of wood that is measured.

6. As the current data flowing through the sample is obtained at the voltage set for each of the measured frequencies, an average of the values is applied except for those values that have shown a deviation greater than 50% of the measure. If there is a significant resonance value, it will be stored so that it can be part of the algorithm for calculating wood moisture based on the electrical resistance value obtained. For each section of 100 measurements its average and standard deviation are calculated, the sections with a standard deviation less than 10% over the average of the section are selected, repeating this process as many seconds as necessary, for successive groups of measured current values, as well as the calculation of its standard deviation, that is to say that of the average values of each average section, until the standard deviation of the average values of each section is less than 2% on the average of such average values. During the process of measuring by sections it is necessary to take at least one section in the opposite direction to the rest of the measurements (current flowing in the opposite direction) and compare its mean and standard value, which contributes to obtain a more stable value and precise.

7. Given the electrical resistance value, determined from the average current value finally obtained, the next step is to obtain the moisture value equivalent to 25 ° C taking into account the type of material in question, applying an algorithm which relates constants inherent in the type of fibrous material with the equivalent resistance value determined in e), for a temperature of 25 ° C, or substantially 25 ° C. In order to carry out this process, it is necessary to have the calibrated compensation tables, consisting of 255-point tables showing the humidity value corresponding to a given electrical resistance, and a temperature of 25 ° C. These tables must be prepared in the laboratory and are unique for each wooden pipe you wish to analyze. Given that it is a very long and complicated process, the 255-point table has been developed with an algorithm for estimating the humidity curve based on taking 6 resistance and humidity measurements. The algorithm generates an approximation to the humidity (y) -resistance (x) curve of the form y = axb + c, where y is the humidity value at 25 ° C, x is the resistance value obtained according to previous calculations, since , b, c are the constants of the material that has been previously calibrated (different for each material).

8. If a resonant frequency was detected in the frequency sweep process, a correction factor based on this frequency is applied, which means that the resistance value is slightly corrected, which in turn modifies the humidity value as a function of the value of this frequency, since this value indicates the existence of certain properties of the wood that inform about the possibility of this containing more moisture.

9. The moisture value obtained for 25 ° C is then compensated, depending on the temperature. The temperature compensation is done by a displacement equation based on the temperature and the resistance value, since at extreme values the compensation is slightly different.

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10. This process is applied iteratively for at least 100 measurements, with the average value being filtered and averaged, the value finally shown as moisture of the wood sample.

The present inventors do not know of any proposal that incorporates an algorithm or measurement process as sophisticated as that of the present invention and that considers so many factors (change of Vcc, change of Rf, application of square pulses, sine wave, inversion of sense of the current in the probe, estimation algorithm based on multiple measures with elimination of extremes and standard deviation analysis, stabilization of the measurement on the average of values, etc.).

To determine the validity of the procedure, a specific electronic circuit has been designed that implements the system proposed by the second aspect of the invention and allows the application of the necessary voltages and currents, as well as obtaining the measures for the calculation of resistance electric For the final moisture estimation, the applied algorithms have been tested thanks to the development of an electronic acquisition and processing plate that allow verifying the moisture value of the wood sample. The detailed explanation of each of the parts is as follows:

- Sensing circuit: Different conditioning and sensing stages have been studied and tested experimentally to measure the humidity of the wood and the conditioning circuit to be used has been determined. This part is responsible for generating, in the corresponding times, the voltages and currents necessary to apply the procedure outlined above. This circuit has been possible thanks to the new semiconductor technologies that allow to capture very low currents (of the order of femto-amps), together with the use of high-precision ADC converters and microcontrollers that manage the entire process of data storage and processing, asl as its low energy consumption that makes them suitable for applications powered by batteries.

- Acquisition and processing circuit: It is the part of the xylohygrometer responsible for linking the electrical value of the measure with the estimation of the resistance and subsequent calculation of the equivalent humidity by means of the compensation algorithms proposed in the invention. The acquisition and processing system is based on a microprocessor capable of reading the analog signal generated by the sensing circuit, establishing the measurement value according to the value and compensation curves (based on humidity and temperature value), controlling the measurement process, storing the information and communicate with external teams for sending information and / or reprogramming compensation curves.

As for the new circuit implemented, it can be said that it provides numerous advantages over the circuits for measuring humidity based on the resistive method.

To arrive at the final design of the electronic circuit, first of all different prototypes were implemented whose main objective was to improve and extend the resistive measurement interval guaranteeing compliance with the regulations and thereby achieving the CE marking requirements. Specifically, three prototypes were implemented until the final circuit was reached.

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Once the main objective was reached (correct measurement of very high values of electrical resistance), experimental studies were carried out that allowed us to offer advantages over the common commercial instruments used today. The influence of the tension applied to the wood was studied (range, polarity, time of establishment ...), the consumption was minimized to be able to have instruments with lasting autonomy and the instrument of benefits that do not exist today in the the field of the timber industry. Among some of the benefits, the instrument was equipped with different external communications to give it greater versatility and expand its possible fields of application. Among the communications implemented, which the user can choose as they see fit, are Zigbee, GSM Ethernet and USB communications. It is also possible to use the instrument as data storage equipment ("datalogger") by using an SD card. These characteristics are of special interest in the case of the maintenance of buildings or places where it is desired to monitor the humidity values in the wood over time, for example, the roofs of National Heritage buildings, and thus avoid displacements to the measurement site.

Figure 3 shows the block diagram of the definitive measurement circuit coupled to the resistance of the wood sample Rx. This is formed by the following stages or blocks: C1: voltage or current supply, C2: resistance-voltage converter (which includes amplifier A), C3: digital analog converter, C4: microprocessor and C5: display. It should be noted that although the measurements made by the block C2 are voltage, the microprocessor C4 calculates the current values associated with such measured voltage values and processes the calculated current values asl. In addition, the implemented meter includes the possibility of wireless or wired external communication, expanding the field of possible applications.

As you can see, the measurement system / circuit is also characterized by its modularity, which is why its design, assembly and experimental testing has been carried out in stages.

In order to demonstrate the validity of the system and the measurement method proposed by the present invention, numerous experiments were carried out that have allowed the goodness of the measurement system to be evaluated. In the first phase, the objective was to make a correct measurement of the electrical resistance, for which experimental measurements were made using the VRS-100 resistive decay box with certified calibration and resistive values similar to those that could be found in the wood.

Once the measurement procedure on the resistive decay box was validated, measurements were carried out directly on wood following the procedure described in the UNE-EN13183-2 standard for pieces of wood located in a climate chamber at a certain humidity and temperature, For different types of wood. Moisture-resistance calibration tables provided by CIFOR-INIA and others existing in the bibliography were used. As an example, the graph in Figure 4 shows the results obtained for moisture estimation. The figure shows the approximation of the humidity-resistance curve where given a series of points experimentally obtained in the calibration laboratory through a climatic chamber for a type of wood at known humidity and temperature, the complete adjustment curve is obtained to provide of the moisture estimate for any electrical resistance value that the meter obtains.

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To validate the measurement system proposed by the present invention, three comparisons have been made. The first consisted in comparing the measured value with that of a resistive box of decades calibrated and certified by an official laboratory. Table 1 shows some of the results obtained with Rth being the actual resistance of the resistive decay box used as a pattern, Rexp the resistance reading provided by the circuit proposed by the present invention, Rf corresponds to the resistance located in the loop of feedback of the amplifier of Figure 1 and Er (%) corresponds to the error made in the measurement (error between the value indicated by the box of decades and the measurement calculated by the circuit). It can be seen how the result of the reading (Rexp) for the same resistance value Rth pattern improves the accuracy of the measurement by varying the resistance value Rf (this invention precisely raises the dynamic adjustment of Rf to improve accuracy) and also achieves The main objective of this invention is that the error made in the measure is less than 10% in resistive value, which guarantees compliance with the new regulations.

The second comparison consisted of comparing the values obtained by the system object of this patent with those obtained by a commercial megaohimeter IET1865 (
http://www.ietlabs.com/1865-megohmmeter.html). Table 2 shows the results obtained in the measurement of resistances of the same box of decades as in the first case Rexp being the value of the measurement obtained by this equipment and Er the error between the megaohmmeter and the box of decades. As can be seen, in most cases the error is similar to that of table 1, which indicates the validity of the proposed system considering the huge differences between equipment (size, weight, measurement time, cost).

 Rth (M)

 Rexp (M) Rf (M) Er (%)

 10

 9,994 1 0.1

 100,001

 99.8 1 0.2

 100,001

 99.56 10 0.4

 1000,183

 1000 10 0.0

 1000,183

 1005 100 0.5

 10016,717

 10120 100 1.0

 10016,717

 10080 500 0.6

 10016,717

 9920 1000 1.0

 99890.14

 10630 1000 6.9

Table 1

 Rth (M)

 Rexp (M) Er (%)

 10

 10,011 0.1

 100,001

 100,187 0.2

 1000,183

 1003 0.3

 10016,717

 10034 0.2

 99890.14

 95558 4.3

Table 2

Table 3 corresponds to the results obtained with another instrument of high resistance, in this case portable and in some way comparable to that proposed by the present invention, specifically it is the "Material Moisture Gigamodule" model of the German manufacturer Scanntronik (
http://www.scanntronik.de/English/Produkt Materialfeuchte Gigamodul eng.php).

It shows how the errors obtained in the measurement of resistance, if compared with the results of Table 1, increase considerably.

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 Rth (M)

 Rexp (M) Er (%)

 9.99589

 10 0.04

 100,048

 103.28 4.7

 1046

 1071 6.7

 10024

 10964 9.4

 99280

 102347 3.1

Table 3

It can be seen, therefore, that the system proposed by the second aspect of the invention offers a suitable solution for high ranges of resistance equivalent to low ranges of humidity. It is precisely in these ranges where maximum accuracy in measurement is required and where current commercial instruments intended for measuring moisture in wood present problems of accuracy. This system is characterized by its simplicity, low consumption and notable improvement in the accuracy of the measurement in a greater range of resistance, reaching 100 GQ with an error of 6.9% equivalent to an error less than 1% in humidity From the wood

On the other hand, it should be noted that all these results refer to electrical resistance measurement since they are those that can be reliably compared. On the other hand, they are meaningless if a correct resistance-wood conversion is not performed where it is very important to have the relevant characterization curves. As previously stated, it is necessary to have the appropriate curves for each type of wood, for which it is necessary to have data experimentally. Thus, if there are no minimum errors and own curves associated with each type of wood, its application in the wood sector is not possible if it is required to comply with the CE marking regulations. Therefore, the adjustment curves are also important and commercial devices do not publish them, hence it is not possible to compare results with the equipment that offers the result of measurement in humidity since it is very difficult to know if the error is due to the system measurement or adjustment curve.

In any case, to check the operation of the circuit by measuring the resistance of the wood, numerous tests were carried out to guarantee the repeatability and reproducibility of the measurements. Among the measurement tests, some of them were carried out continuously for several days. These tests consist of taking a first resistive value, moistening the samples and observing their evolution. Figure 5 shows the modification of the resistive value for two types of wood (radiata pine and pinaster pine). The results obtained coincide with the expected behavior.

Figure 6 shows the results obtained in three samples of the same type of wood, specifically the fir. In this case the tendency of variation in the three samples, as expected, is very similar, although in the case of Fir 2 a greater difference is observed. Basically this behavior is due to the fact that, despite being exactly the same type of wood and identical samples, the situation of the electrodes on the wood was different. That is why it is vital to establish a measurement method that complies with the CE marking regulations, guaranteeing the required accuracy, repeatability and reproducibility.

As a summary, the advantages offered by the present invention, in its different aspects and examples of realization, are the following:

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1. New procedure in the way of obtaining the measurement of electrical resistance of the wood, based on multiple iterations with variations in voltage, which allow an algorithm to be established to give the equivalent resistance more adjusted to the real value of the humidity of the sample .

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2. New compensation algorithm for the different types of wood when obtaining the moisture equivalent depending on the resistance value obtained, which also incorporates the temperature as compensation factor.

10 3. Higher speed of stabilization in the measurement, being able to perform a reliable measurement in

less than a second

4. Accuracy obtained below 2%, which allows its use in the CE marking of the wood used in construction.

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5. Possibility of developing low cost measuring equipment and battery powered as portable measuring equipment.

6. It allows its utilization at an industrial level in an intensive and robust way.

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7. It facilitates the annual calibration of the measurement procedure by means of the re-parameterization of the measurement algorithms, which guarantees compliance with European regulations in this regard.

A person skilled in the art could introduce changes and modifications in the examples of realization described without departing from the scope of the invention as defined in the appended claims.

Claims (25)

5 10 fifteen twenty 25 30 35 40 Four. Five fifty 1. Resistive method for measuring moisture in a fibrous material, comprising, sequentially: a) apply an electrical voltage between two or more electrodes (E1, E2) in contact and / or inserted in a sample of fibrous material (M); b) obtain the value of the circulating current through said sample of fibrous material (M) and caused by the application of said electrical tension; c) determining the equivalent electrical resistance of said sample of fibrous material (M) from the current value obtained; Y d) determine, by calculation and depending on the fibrous material, the degree of moisture in the sample of fibrous material (M) from the determined equivalent electrical resistance value, the method being characterized in that it automatically comprises: - in said step a), applying at least a plurality of alternating input voltage signals of different frequencies and also applying at least one continuous input voltage signal; - in said step b), obtaining a plurality of values of the current flowing through the sample of fibrous material (M) during the application of said plurality of input alternating voltage signals and obtaining at least one value of the circulating current between the two or more electrodes (E1, E2) during the application of said continuous voltage signal; Y - in said step c), calculate the equivalent electrical resistance from the application of a statistical function on two or more of the current values obtained in b) for two or more corresponding signals of alternating input voltage of different frequencies and / or on values calculated from them and for the signal or signals of continuous input voltage and / or on one or more values calculated from them. 2. Method according to claim 1, characterized in that it comprises obtaining said plurality of values of the circulating current by the sample of fibrous material (M) from measurements made directly on said (E1, E2) or other electrodes in contact with the sample of fibrous material (M), or indirectly without contact with the sample of fibrous material (M). 3. Method according to claim 1, characterized in that said statistical function is an average function. 4. Method according to claim 1, 2 or 3, characterized in that it comprises iteratively applying a calculation algorithm on the plurality of current values obtained in b), until obtaining values considered statistically significant for at least said statistical function, and apply said statistical function, in step c), on said statistically significant values. 5 10 fifteen twenty 25 30 35 40 Four. Five fifty Method according to any one of the preceding claims, characterized in that at least part of said plurality of alternating voltage serials are square pulse signals and because step a) comprises applying said square pulse signals of different frequency sequentially. Method according to any one of the preceding claims, characterized in that at least said plurality of alternating voltage serials form part of a single frequency variable voltage signal, where the method comprises applying, in step a), said single signal of voltage varying its frequency during its application, according to a frequency sweep within a given frequency range. 7. Method according to claim 6, characterized in that said determined frequency range varies up to substantially 1 MHz. 8. Method according to claim 6 or 7, characterized in that it comprises, in step a), also varying the magnitude of said single voltage signal, during its application, by incremental steps. 9. Method according to any one of the preceding claims, characterized in that it comprises, prior to step a), performing an adjustment process to determine at least one suitable value, at least as regards the magnitude, of the signals of input voltage to be applied between said two or more electrodes (E1, E2), which causes circulation by the sample of fibrous material (M), between the two or more electrodes, of a current with a value that is within a measurable range, directly or indirectly, where said suitable value is initially used in step a) for at least the first pulse of the alternating voltage signal to be applied. 10. Method according to claim 9, characterized in that said adjustment process comprises applying between the two or more electrodes (E1, E2) a signal! of alternating voltage with an initial value, referring at least to magnitude, and obtaining, if current flows, the value of the circulating current through the sample of fibrous material (M), and: i) if the circulating current has a value within the measurable range, determine that said initial value is the appropriate value; or ii) if the circulating current has a value outside said measurable range, increase, if it is below the measurable range, or decrease, if above, said initial value of the alternating voltage signal, apply it on the two or more electrodes (E1, E2) and obtain again, if current circulates, the value of the circulating current through the sample of fibrous material, and if the circulating current has a value within the measurable range determine that said value is the appropriate value, and if It is not so repeated iteratively, step ii) until the appropriate value is determined. 11. Method according to claim 10, characterized in that said alternating voltage signal applied in the adjustment process is a series of square pulses. 12. Method according to any one of claims 9 to 11, characterized in that the adjustment process also comprises selecting a suitable measuring range from a plurality of measuring ranges provided by the variation of at least one variable resistance (Rf), being carried said selection, during the application of said signal! alternates the adjustment process, varying the value of said variable resistance 5 10 fifteen twenty 25 30 35 40 Four. Five fifty (Rf) from a minimum value that provides the lowest measurement range to a higher value that provides the highest possible measurement range without generating saturation voltage on the variable resistance (Rf), where said suitable measurement range is used during stages a) and b). 13. Method according to any one of the preceding claims, characterized in that step a) comprises applying at least part of said alternating voltage signals of different frequencies sequentially in order to detect resonance frequencies of the sample of fibrous material from of the current values obtained in step b). 14. Method according to claim 13, characterized in that said alternating voltage signals applied in a) in order to detect resonance frequencies of the sample of fibrous material (M) are sinusoidal signals. 15. Method according to any one of the preceding claims, characterized in that said step c) comprises applying at least one averaging to the current values obtained, excluding those that show a deviation greater than a certain value, and using the average value obtained for the calculation of the equivalent electrical resistance. 16. Method according to claim 15, characterized in that said averaging comprises: - apply previous averages, each to a respective group of current values obtained, including at least one group of current values of polarity opposite to that of the other groups, and calculate the standard deviation of each of said groups; - select the average current values of those of said previous averages relative to groups whose standard deviation is below a threshold value; Y - iteratively apply averaging to the average current values selected for successive groups of current values obtained, as well as the calculation of their standard deviation, and take the average value thereof as the measurement value once the standard deviation of these average current values is less than a threshold. 17. Method according to any one of the preceding claims, characterized in that it comprises, in step d), calculating the degree of moisture in the sample of fibrous material (M) for a temperature of 25 ° C, or substantially 25 ° C , applying an algorithm that relates constants inherent in the type of fibrous material with the equivalent resistance value determined in e), for a temperature of 25 ° C, or substantially 25 ° C. 18. Method according to claim 17, characterized in that it comprises applying a temperature compensation to the calculated moisture degree value to obtain a compensated moisture degree value to conform to the actual temperature conditions of the fibrous material sample (M). 5 10 fifteen twenty 25 30 35 40 Four. Five fifty 19. Method according to claim 17 or 18 when it depends on 13, characterized in that if at least one resonance frequency of the sample of fibrous material (M) has been detected as a result of applying said alternating voltage signals, the method comprises applying a correction factor to the value of moisture degree calculated or compensated in temperature based on information on properties of the sample of fibrous material related to moisture, which is inferred from the value of said resonant frequency. 20. Method according to any one of the preceding claims, wherein said fibrous material (M) is wood. 21. System for measuring moisture in a fibrous material, using a resistive procedure, where the system comprises: - an electronic measuring circuit (Cm) configured and arranged to apply an electrical voltage between two or more electrodes (E1, E2) to be placed in contact and / or inserted into a sample of fibrous material (M), and to obtain, by direct or indirect measurement, the current flowing through said sample of fibrous material (M) and caused by the application of said electrical tension; Y - processing means (P) in connection with or forming part of said electronic measuring circuit (Cm) and which are configured and arranged to determine the equivalent electrical resistance (Rx) of said sample of fibrous material (M) from the current value obtained and to determine, by calculation, the degree of moisture in the sample of fibrous material (M) from the equivalent electrical resistance value (Rx) determined, the system being characterized because: - the system comprises control means (MC) that control said electronic measurement circuit (Cm) so that, automatically, apply a plurality of alternating voltage input signals of different frequencies between said two or more electrodes (E1 , E2), and so that it obtains a plurality of values of the circulating current through the sample of fibrous material (M) during the application of said plurality of signals of alternating input voltage, - and because said processing means (P) are configured and arranged to calculate the equivalent electrical resistance (Rx) by processing two or more of the current values obtained for two or more corresponding alternating voltage signals of different frequencies, where said processing comprises the application of a statistical function on two or more of the current values obtained and / or on values calculated therefrom following the steps a, b and c according to claim 1. 22. System according to claim 21, characterized in that it is adapted to implement the method according to any one of claims 1 to 20. 23. System according to claim 21 or 22, characterized in that it comprises a device (D) that houses both the electronic measuring circuit (Cm) and the processing means (P) and the control means (MC). 24. System according to claim 21-23, characterized in that it comprises communication means provided for establishing a two-way communication of data between the processing meters (P) and a remote computer system. 5 25. System according to claim 24, characterized in that it also comprises said remote computer system. 26. Computer program that includes code instructions that when executed in a computer implement at least steps c) and d) of the method according to any one of claims 1 to 20.