RU2240545C2 - Method of determining wood moisture - Google Patents

Abstract

FIELD: measuring engineering.

SUBSTANCE: method includes varying the value of current by a linear law with respect to the initial value of current, measuring the second voltage, determining diffusion resistance of the specimen from the two values of current and voltage, and calculating the humidity from the value of resistance.

EFFECT: enhanced accuracy.

1 cl, 6 dwg, 1 tbl

Description

The invention relates to measuring technique, in particular to measuring the moisture content of wood.

There is a method of measuring wood moisture [see Prince Lapshin A.A. Electric moisture meters. - M .: Gosenergoizdat, 1960. - P.15-20], where as the parameter by which humidity is determined, the differential electrical resistance of the material sample is used. The method consists in determining the electrical resistance of a sample of material at a direct current at one fixed voltage.

The disadvantages of this method are the low accuracy of the measurements due to the dependence of the electrical resistance of the sample material on the applied voltage, high voltage to go to a linear steeper section of the characteristic and the narrowness of the measurement range due to voltage fixation.

The known method [see Prince Berliner M.A. Electrical measurements, automatic control and regulation of humidity. - M.-L.: Energy, 1965. - P.56], which consists in measuring the electrical resistance at subbands, on the basis of which a conductometric measuring instrument for moisture content of wood was developed.

The disadvantages of this method are the low accuracy of the measurements, caused by the non-consideration of the non-linearity of the I – V characteristics, high voltages, divergence of moisture readings on overlapping subranges.

Common disadvantages of the methods are the low accuracy of measuring the electrical resistance and, as a consequence, the moisture content of the capillary-porous material caused by the non-linearity of the current-voltage characteristic (CVC) of the sample, as well as the use of high voltage, which requires the use of additional measures of protection against electric shock.

The prototype adopted a method for determining the moisture content of capillary-porous materials [see Patent No. 2187098 (RF), G 01 N 27/04, 2002. Bull. No. 22], which consists in measuring the diffusion conductivity by the current-voltage characteristic (CVC). They apply voltage to the measuring cell and measure the current, change the voltage in a multiple of two from the original and measure the second current, find the diffusion conductivity of the sample from two voltages and currents, which determine the humidity.

The disadvantages of the prototype are low accuracy due to the limitation of the control range by the multiplicity of voltages U ₂ / U ₁ = 2 supplying the measuring cell, and the lack of optimal measurement modes.

The technical task of the method is to increase the accuracy and expand the control range for a given metrological characteristics.

The technical task is achieved in that:

1. In the method of determining the moisture content of wood, contact with the sample is carried out using two electrodes located along a line perpendicular to the sample fibers, at a fixed distance from each other, apply a voltage to the measuring cell, consisting of series-connected wet material and a reference resistance, set this minimum current at which it is possible to measure the voltage drop across the sample, and determine the humidity, in contrast to the prototype, change the current according to a linear law with respect to initial and the second voltage is measured, the diffusion resistance of the sample is determined by two currents and voltages, by which moisture is determined.

2. In the method according to claim 1, the diffusion resistance is determined by the optimal operating parameters on the reference materials, which are found by comparing the experimental and simulated current-voltage characteristics at various current ratios for which the error is minimal.

The essence of the proposed method is illustrated in figure 1, 2, 3. The proposed method includes 2 stages:

1) measurement of diffusion resistance of the test sample;

2) the determination of diffusion resistance is carried out according to the optimal operating parameters on the reference materials.

1. The moisture content of wood is determined by measuring the diffusion resistance of the test sample. To do this, contact with the sample using two electrodes located along a line perpendicular to the fibers of the sample at a fixed distance from each other. Apply voltage to the measuring cell (figure 1), consisting of series-connected wet material and a reference resistance, set the minimum current I ₁ at which it is possible to measure the voltage drop across the sample U ₁ (figure 2), change the current according to the linear law I ₂ = n · I ₁ relative to the original and measure the second voltage U ₂ (I ₂ ), two currents and voltages find the diffusion resistance R _{d of the} sample.

The CVC of the sample has a nonlinear character, and the current changes exponentially (Fig. 2)

where I is the current value of the current through the sample of material, U is the applied voltage at the sample of material, I _d is the current due to diffusion of ions through the cell membranes (diffusion current), U _d is the voltage drop across the sample of the material caused by diffusion of ions through the cell membrane ( emf corresponding to diffusion current).

A unique property is that, regardless of the conditions of the experiment, the diffusion resistance R _d (or conductivity Y _d = 1 / R _d ), which is an informative parameter of material moisture, has a constant reflecting the nature of the current – voltage dependence

To expand the range of calculation of diffusion resistance, the current I _{1 is} increased proportionally by n times

.Each i, j value from the data obtained in the experiment can be used to calculate according to the dependencies below, where i ≠ j,

.

The diffusion resistance R _d can be determined by dependence (1) from the system of equations for currents I _i , I _j :

We bring system (4) to a form convenient for logarithm:

and dividing one equation of system (5) by another, we obtain the expression

, i≠ j - порядковые номера режима измерения.Where

, i ≠ j - serial numbers of the measurement mode.

Prologarithm the equation:

and express U _d :

To find I _d, we express it from the first equation of system (4)

and substitute the expression (6) in the formula (7):

The obtained solution (8) with respect to the current I _{d is} obtained in an implicit form and has a solution only for numerical simulation according to the iterative algorithm, so we rewrite it for iterative calculation in the form:

Using relations (9) and (6), we calculate the value of current I _d and voltage U _d .

с предыдущим значением

(

- число итераций):To calculate the diffusion current, implicitly given (9), we introduce a criterion for assessing the adequacy of the subsequent

with previous value

(

- number of iterations):

where ε ₀ = 0.001 is the permissible error.

The algorithm for calculating formula (9) is as follows:

находим по формуле (9) при

=0 (k=l);- initial value

we find by formula (9) for

= 0 (k = l);

получим из выражения (9) при использовании

(

);- subsequent value

we obtain from expression (9) when using

(

);

- according to iterative criterion (10), we evaluate the accuracy of calculating ε _k with measure ε ₀ ;

принимают за действительное значение диффузионного тока

пробы;- if relation (10) is satisfied, then the calculation of I _d by formula (9) is terminated, and the last result of the iteration

take the actual value of the diffusion current

samples

- if relation (10) is not satisfied, then the next (k + 1) iteration is performed.

According to the found value of the diffusion current, I _d is determined by formulas (6 and 2) U _d and resistance R _d , and also the I – V characteristics are constructed according to model (1). In this case, the voltage on the measuring cell during the experiment will change according to the logarithmic law

U _i = U ₀ ln (i + 1).

The diffusion resistance R _d according to the proposed method is an informative parameter on humidity and does not depend on the voltage applied to the sample material (see figure 3), while the differential resistance R _i decreases exponentially (see efficiency by accuracy, item 1).

2. The diffusion resistance is determined by the optimal operating parameters on the reference materials, which are found by comparing the experimental and simulated I – V characteristics of various current ratios with a minimum error.

To determine the optimal operating parameters, the process of normalizing the values of the scales of the experimental CVC is carried out (Fig. 2), and the subsequent analysis is carried out using the algorithm for calculating the informative parameters described above.

In the process of searching for the optimal operating parameters of the proposed method, a series of current – voltage characteristics are constructed based on a search of the current values, where the number of the selected current value with respect to which the search is performed is constant to the last value (i = 1; j = i + 1, ..., p) . Similarly, the next value is selected, with respect to which the currents are also enumerated (i = 2; j = i + l, ..., p). This process is carried out until the penultimate current number turns out to be a constant value (i = n-1; j = p, where n = p).

The adequacy error of the simulated I – V characteristics with experiment is calculated by the geometric mean deviation (SGE) α:

Where

- относительная погрешность s-ого шага оценки

.

is the relative error of the s-th step of the estimate

.

Figure 2 shows the optimal approximation of the curves in the operating conditions i = 5, j = 7 out of 50 pairwise samples of the modes, where the SSS is not more than 2%.

The connection of moisture with electrophysical characteristics is carried out by analogy with weight analysis

voltage divider model (see figure 4):

- вес воды; М_m - вес сухого материала.where Y _d = 1 / R _d , Y _m = 1 / R _m - diffusion conductivity of moisture and dry material;

- weight of water; M _m - weight of dry material.

Let us prove the effectiveness of the proposed method relative to the prototype.

1) The effectiveness of the reliability of the measurements will be evaluated through the ratio η _{N of the} flexibility N _U and N _{I of the} compared methods.

и n≥ 2, тогда для j=n-1 находим соотношение между максимальной электрической величиной U_n и минимальной U₁ мерой разбиения U_n=2^n-1· U₁ (см. фиг.5а).The flexibility of measurements of the prototype is determined by the number N _U combinations of dividing the I – V characteristic range into subranges according to binary code. Given the binary voltage multiplicity, for any several points of the controlled measurement range (see Fig. 5), the dependence U _{j + 1} = 2 · U _{j is} valid, connecting analytically the entire controlled range of n measurements with the algorithm U _n = 2 ^j · U _nj , where

and n≥ 2, then for j = n-1 we find the ratio between the maximum electric quantity U _n and the minimum U ₁ partition measure U _n = 2 ^n-1 · U ₁ (see Fig. 5a).

The number N _{U of} combinations of possible subbands is determined by the binary logarithm of the relation

Let us evaluate the measurement flexibility of the proposed method according to the code N _{I of} measurement combinations characterizing the flexibility of controlling the controlled range according to a linear law.

If, to increase the flexibility of the range for calculating diffusion resistance, the current I _{1 is} increased proportionally n times according to the linear law (see Fig. 5b), then the number of combinations N _I for j subbands has the form

,

,

.where n≥ 2 is the number of intervals

.

The sum of the line series characterizes the flexibility of the controlled range.

Then the efficiency η _N in the reliability of the measurement range of the proposed method relative to the prototype is proportional to half the number of subbands, because

Thus, by increasing the number of combinations N _I in n / 2 times increases the flexibility of choosing the optimal modes, therefore, the reliability of the measurements in the proposed method with increasing breakpoints n = 4-20 increases η _N = 2-10 times relative to the prototype, increasing measurement accuracy.

2) Let us evaluate the measurement ranges of the compared methods.

For equal in length subbands Δ, the controlled range D _I is determined for the proposed method of multiple currents by the number n of subbands D _I = Δ _I · n.

In the prototype, due to the limited measurement range, the multiplicity of voltages U ₂ / U ₁ = 2 supplying the measuring cell reduces the number of subbands, since in each subsequent subband the distance between the extreme points is two times the distance of the previous one, which leads to a decrease in the range, and therefore, the reliability of the measurements (see figa). Therefore, the controlled range D _{U of the} prototype is described by the dependence of the form D _U = Δ _U · n / 2.

Then, at equal intervals Δ _I = Δ _U = Δ, the efficiency of expanding the measurement range of the proposed method is defined as the ratio of the ranges of the compared methods

или

or

From relation (13) it follows that the use of the linear law of control of measurement modes in the proposed method leads to an increase in the measurement range by 2 times relative to the prototype with regulated errors Δ. Therefore, with a fixed measurement range D _I = D _{U, the} accuracy of the selection of the I – V characteristics is doubled.

3) We will evaluate the efficiency in the accuracy of measuring the I – V characteristics in the process of modeling the experimental I – V characteristics through SGO (11).

Due to the limitation of the controlled range by the voltage multiplicity in the prototype, the simulation of the experimental I – V characteristic is carried out in several subbands for the modes {i, j} = {1-2, 2-3}. As a result, we obtained two simulated curves (I ₁₂ (U ₁₂ ), I ₂₃ (U ₂₃ )), the average value of α _{U of} which is from the experimental I – V characteristic (I _e (U _e )) (see Fig. 6a) is α _U = 6.3%

In the proposed method due to the use of the linear law of transformation, the modeling of experimental curves is performed according to the optimal operating conditions of measurements i = 5, j = 7 (see fig.6b), where the SGA is not more than 2%.

The efficiency η α in accuracy is determined by the ratio of the SGO of the compared methods

and therefore, 3 times higher than the proposed method relative to the prototype.

Thus, due to the linear law of control of measurement modes in the proposed method, in contrast to the prototype, the number of combinations N _I increases, which increases the flexibility of choosing optimal modes and the reliability of measurements (p. 1), and therefore, to expand the range (p .2) and improving the accuracy of measurements (Clause 3).

4) Efficiency in accuracy of humidity measurement.

In the analysis of the compared methods on the accuracy of determining moisture, it was found that the linear law of control of the measurement modes in the proposed method corresponds to the stabilization of the current change during humidity drift, and in the prototype stabilization of the current voltage due to its multiplicity U ₂ / U ₁ = 2 is observed.

When stabilizing the current from (1), an expression of the form

Then the sensitivity of the current voltage to humidity fluctuations has the form

The measurement instability η _I is determined by the ratio of the sensitivities of the current voltage Δ U / Δ W and diffusion Δ U _d / Δ W relative to humidity and changes with increasing stabilization current according to the logarithm

To stabilize the current voltage, an expression of the form (1) is valid. Then, by analogy with mathematical transformations during current stabilization, the instability η _U in the sensitivity of the current current Δ I _i / Δ W relative to the diffusion Δ I _d / Δ W increases exponentially

Then the efficiency η _W by the accuracy of measuring humidity η _W when stabilizing the current is determined by

To analyze the effectiveness of the calculation results are summarized in the table:

The analysis revealed that the accuracy of measuring humidity during stabilization of the current current of the proposed method relative to the stabilization of the current voltage of the prototype increases 2.2-11.3 times, i.e. 7 times on average - almost an order of magnitude.

Therefore, due to the linear selection of multiple currents in the proposed method, they are stabilized, which leads to a decrease in the instability of the modes and parameters, while the humidity is proportional to the logarithmic function (14). In the prototype, when stabilizing stresses due to the binary selection of multiple voltages, the instability of the modes and parameters from humidity increases in proportion to the exponential dependence of (15) on stresses.

Implementation of the proposed method is based on a microprocessor wood moisture analyzer "TEMP-284", built on the basis of a personal computer PC.

The results of experiments conducted on the basis of the microprocessor moisture analyzer “TEMP-284”, built on the basis of a personal computer PC, are presented in figures 2, 3, 6. From the graphs it can be seen that the proposed method allows to determine the desired value of diffusion resistance and humidity in a given range with regulated accuracy of control.

Thus, the proposed method, due to the linear law of controlling optimal measurement modes, unlike the prototype, allows the flexibility to carry out moisture measurements while stabilizing the current, which increases the accuracy of humidity measurements by almost an order of magnitude and extends the control range by at least two times for given metrological characteristics .

Application of the proposed method for determining the moisture content of wood will lead to an increase in raw materials and economic indicators in the woodworking industry; grade of wood during its drying and quality control during storage, automation and expressness of the drying process of wood.

Claims (2)

1. The method of determining the moisture content of wood, which consists in the fact that they make contact with the sample using two electrodes located along a line perpendicular to the fibers of the sample at a fixed distance from each other, apply a voltage to the measuring cell, consisting of series-connected wet material and a reference resistance, set such a minimum current at which it is possible to measure the voltage drop on the reference resistance, and determine the humidity, characterized in that they change t to linearly relative to the initial and the second voltage measured at two currents and voltages are the diffusion resistance of the sample which determine the humidity. 2. The method according to claim 1, characterized in that the diffusion resistance is determined by the optimal operating parameters on the reference materials, which are found by comparing the experimental and simulated current-voltage characteristics of various current ratios for which the error is minimal.

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