FI110640B - Method for measuring the moisture distribution in the depth of the surface layer of a hygroscopic material - Google Patents

Description

110640

METHOD FOR THE COATING OF HYGROSCOPIC MATERIAL

FOR MEASURING THE DEPTH MOISTURE DISTRIBUTION

This invention relates to a method for measuring the depth distribution of the surface layer of hygroscopic material according to the preamble of claim 1.

In the method, a variable electric field is generated on the measuring object through surface electrodes and the depth distribution of the material is determined by the influence of the electrical properties of the material under investigation on the α-dispersion frequency spectrum on the complex impedance spectrum.

The use of electronic methods for measuring moisture in hygroscopic materials is based on the fact that the effect of moisture on electrical properties is significant. For example, at low humidity, the electrical properties of wood change exponentially rather than linearly as a function of humidity. Two commonly used methods for measuring moisture in wood are measurement of electrical resistance of wood and measurement of dielectricity of wood. In resistance meters, the electrodes • ·, *. · Are inserted into the wood, causing the material to partially break I · · • *. Surface contact is usually sufficient for dielectric measurement methods, which is a completely non-destructive method. Surface Contact Based ···. the reliability of the meters is practically poor due to ···, in particular the method is very sensitive to surface moisture.

• ·] "· The maximum values of the wood moisture measuring range of resistance meters ·· * are at the point of saturation of the causes of wood. Dielectric-based methods have been able to measure the moisture content of the wood even at concentrations above 100% of the saturation point of the wood.

In theory, the dielectric constant of wood increases until the cellular cavities are completely filled with water. In practice, the accuracy of humidity meters measuring dielectricity 2 110640 decreases when the moisture exceeds the cause saturation point due to e.g. because the moisture distribution is then generally high and the single-frequency measurement method is sensitive to changes in wood surface moisture.

In commonly used surface moisture meters, the measurement is made at a single frequency, with the meters being sensitive to surface moisture. The sensitivity to surface humidity varies as a function of frequency but is significant regardless of frequency. If the sample to be measured has a moisture distribution, an accurate estimate of the deeper moisture distribution or average humidity of the entire sample cannot be made based on the surface humidity meter reading.

The object of the invention is to improve the aforementioned sensitivity to surface moisture and to provide a method which functions to evaluate the moisture content of both the surface and the deeper layer, whereby the moisture distribution under the surface and the average humidity of the material are determined much more accurately.

This purpose is achieved by measuring ··· according to the invention. electric impedance spectrum of sample from surface electrodes.

Spectral analysis can be used to determine the structure of the object * · and the interactions between the electrodes and the object. By physical and mathematical * ··· 'modeling, the object is transformed into an electrical model, the parameters of which represent the structure of the sample.

* · ·

The developed method is based on the fact that the material to be measured has a strong α-dispersion which depends on the moisture content of the material and especially on the depth distribution. In the method, an accurate estimation of the moisture content of the surface layer * »· '·' · is achieved by using parameters defined in the spectrum» · *: from different frequency ranges, i.e. by selecting the parameters based on their effective measurement depth.

The continuous function thus formed depicts humidity at various depths, thereby providing an estimate of the depth profile of the material under investigation 3 110640. Using information on the thickness of the material to be measured, moisture desorption and absorption models can be used to estimate the average moisture content of the material being measured.

More specifically, the method for measuring the moisture distribution of wood according to the invention is characterized by what is stated in the characterizing part of claim 1.

The invention will now be described in more detail with reference to the accompanying drawing (Fig. 1), which shows one measuring system according to the invention.

In the example case, the measuring probe uses 40 x 50 mm plate electrodes (3) spaced 5 mm apart. From the control unit (6), a sinusoidal voltage signal is applied to a measuring sensor, where the signal is applied to the electrodes. The sensor converts the voltage signal from the control unit into a current signal as well as a voltage signal, which compares the complex impedance for each frequency to be measured by measuring the amplitude ratio and phase difference between the signals. The apparatus is calibrated <«V. by performing open and closed loop measurement.

", In the example case, the absolute value of the impedance and the phase angle ... in the frequency range 10 kHz to 1 MHz are measured. For the parameters measured using straight damp samples,> moisture functions are obtained. Using calibration samples with different absorption and desorption distributions Effective measurement depth where the average error in the humidity function is minimized relative to the calibration data. Using a number of parameters with different effective measurement depths, and multivariate analysis, the functions for humidity are formed: with depths, minimizing the error in the humidity function .

4, 110640

At its simplest, the method is used by measuring two parameters having different effective measurement depths, for example, phase angle at 10 kHz and absolute value of impedance at 1 MHz. By measuring the calibration samples, a multivariate matrix is formed which contains, by column, the actual humidity (determined eg by weighing / drying method) of each calibration sample at different depths and the values of the impedance parameters determined. For example, using the Nelder-Mead algorithm and the Euclidean error rate, the error between the humidity function fit and the actual humidity is minimized. This yields a multivariate matrix containing the coefficients for both parameters and the constant factors. The depth humidity distribution is determined by measuring the impedance parameters of the sample and using humidity functions based on matrix coefficients.

Within the scope of the invention, solutions other than those described above may be contemplated. For example, other methods for measuring and determining complex impedance can be used.

The sensitivity to surface moisture can be reduced by placing:. * A dielectric film between the electrodes and the object to be measured.

• * · · ... · By changing the size or distance of the electrodes ν \ ·, the effective measuring depth can also be affected.

i · ·

The method can also improve surface moisture measurement accuracy because surface moisture measurement, ·. ; the effect of an effective deeper moisture layer can be measured ... ".

The method can also be used to measure the depth of a non-hygroscopic layered material in the direction of impedance: if there is an α-dispersion in the object.

* · ·

Claims (4)

5 Claims: 110040 A method for determining the depth-to-humidity distribution of a surface layer of a hygroscopic material, comprising generating an electric field on the surface layer of the material to be examined, characterized by measuring the complex impedance at at least two different frequencies and determining the Method according to Claim 1, characterized in that the complex impedance is measured in the frequency range of the α-dispersion of the material under investigation. Method according to claim 1, characterized in that the measured impedance parameters have different sensitivities to the surface moisture. Method according to Claim 1, characterized in that the determination of the humidity of the deeper layer under the surface is based on the correction for the moisture distribution of the surface layer, which is dependent on the shape and size of the distribution. 6 110640