FI116583B - Method of calibrating an apparatus for measuring moisture content in wood - Google Patents

Description

116583

Method for calibrating a device for measuring moisture in timber

TECHNICAL FIELD The present invention relates to the measurement of the moisture content of wood by the thermalization of fast neutrons dampened by the hydrogen content of the wood. The calibration data can then be used to calculate the moisture content of the timber from the number of thermalized neutrons per unit time and can be used to adjust the drying parameters during the drying process.

State of the art

About 30-35 years ago the timber was dried in the open air. Today, almost all timber is dried in drying chambers or ducts. In order to create suitable drying geometry for such a drying chamber, the boards are arranged in stacks, with spacings of the board layers of about 22 mm. In between, wooden moldings are used. In the space between the board layers, warm air circulates as a carrier of energy and outflow moisture. With regard to the transportation conditions, the drying arrangement is formed of 20 board bundles, each consisting of 100-200 boards> ': depending on the board size. The drying chamber may have: 16 bundles interconnected to form, for example, four columns. The large circulation fans cause air to flow between the bundles, and the heater combination delivers the 25 required heat. To obtain the desired climate in the drying process; During this period, some of the air can be vented through the vent and replaced with drier outdoor air, or if it is »» *; * too dry for use in a drying chamber, the following can be added: steam or water through nozzles (dehumidification / wetting).

30 In fresh timber, water is not bound and bound; '. · Mode. Unconsolidated water is in the capillary tubes and is easily removed by evaporation. However, the combined water forms part of the cellular structure of the wood and must be removed by diffusion. When the uncoupled water is removed, the moisture of wood 116583 2, defined as the wood's saturation point, is achieved. Any change in the moisture content below the saturation of the wood causes a change in the volume of the wood: as it dries it shrinks and absorbs water swells. From impregnation point 5 to absolute dry timber, shrinkage and swelling occur approximately in proportion to the change in moisture content. Changes in the moisture content above the saturation point do not cause volume changes.

In order to maintain the original quality of the wood during drying below 10 saturation points, the drying parameters must be adjusted so that the water transport from the inside of the wood towards the surface is balanced against the appropriate stress in the wood, as this stress results from water transport. This can be achieved by knowing the moisture content of the lumber at all times and adjusting the drying process parameters accordingly. In other words, the drying parameters must be synchronized with the moisture content of the timber, the desired drying rate and the desired final moisture content.

Today, the control of timber drying is usually based on 20 drying charts, which indicate dry and wet temperature values throughout the drying process. Such drying conditions are based on empirical values and constantly try to produce ': "ί drying climate that causes minimal drying damage: to timber. Without knowing the moisture content of the timber, it is not possible to achieve normal regulation of the drying climate, since such regulation must be based on knowledge of humidity. all the time.

Continuous measurement of the moisture content of the timber during the drying process provides an opportunity to improve the control of the drying climate so as to reduce the damage caused by drying.

, ···, It is also possible to terminate the drying process when wood has been found to have the desired final moisture content, and thus predetermined definitions of moisture content can be easily and reproducibly implemented. . In addition, there is no need to follow the material, saving both time and drying energy. There are other uses of 116583 3 where it is useful to measure moisture content, such as coarse sorting and quality control.

Various methods are known for measuring the moisture content of timber. The most common is the drying and weighing method, and one where handheld instruments are used to measure timber resistance or capacitance. Reference is made to the publication of Literature Reference 1 at the end of the specification. These methods can be used to measure the moisture content during timber drying.

10 To continuously measure the moisture content of the timber, the moisture level can be measured with electrodes drilled into the wood to measure electrical resistance. However, in this way the moisture content can only be measured on a small portion of the timber, and it may therefore be difficult to obtain an impression of the average moisture content of the drying chamber as a whole. Also, this measuring method is not suitable for measuring moisture contents of more than 25-30% and therefore it is not possible to control the drying course above the fiber saturation point by using such a measuring device. In practice, most of the humidity meters that are commercially available have very limited accuracy due to the lack of suitable temperature compensation, and / or because they cannot continuously measure the moisture content in the timber furnace.

In such measurements, the measurement accuracy is very uncertain due to the complex interaction of the resistor, temperature. and moisture (see publications according to references 2, 3 and 4).

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: Measurement of the moisture content of wood by nuclear radiation has been previously studied by, for example, Loos (see publication reference 5). At that time, we came to the conclusion,, ··. that two different measurements had to be used to determine the moisture content. Nuclear radiation technology has also been used to measure the moisture content of wood chips, but then such measurement had to be combined with another measurement method for density measurement (see publication 6).

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However, without knowing the density of the wood, it is still possible, according to the present invention, to measure the moisture content by measuring the moisture content in such a way that it can distinguish between water-bound hydrogen and wood-bound hydrogen, respectively. Measurements carried out at the Institutt for Energiteknikk show that the nuclear humidity meter developed according to the invention is well suited for continuous measurement of wood moisture without further measurement, since it has become apparent that the rapid neutron thermalization process in moist wood 10 is highly dependent on hydrogen binding to water or wood.

Accordingly, it is an object of the present invention to provide a measuring device for measuring the moisture content of timber, and a method for calibrating such a device such that the moisture content can be measured with satisfactory accuracy regardless of the density of the wood.

Another object of the invention is to define a method for controlling the drying of timber controlled by such a calibrated measuring device.

; 2 0 Description of the Invention »? »

The invention relates to a method for calibrating a device for measuring the moisture content of timber with respect to the variable dry density of various types of wood to be measured, whereby the wood is irradiated with fast neutrons from a source of neutrons, and the number of thermal neutrons hydrogen atoms per unit of time have been suppressed, • at least one detector is used to record the moisture content of the wood * and indicated by a measuring device, the method being characterized by the following steps: »· (a) the timber to be measured is recorded plotting the measurement as a function of the device's known moisture content at maximum and minimum dry densities, respectively, and determining the mean curve between the maximum curve and the minimum curve, 116583 5 b) determining the first change in the device reading the mean curve and maximum and c) determining a second change as a change in the device reading per percent moisture content measured from the mean curve; and d) dividing the first change by a second variation of the device, 10 as a defined measuring range between the maximum and minimum dry density curves, respectively.

The invention also relates to an apparatus for measuring the moisture content of a wood, comprising a neutron source for irradiating the wood with fast neutrons and at least one detector 15 for counting the number of thermal neutrons generated by the hydrogen atoms The device of the invention is characterized in that: the neutron source and the detector are arranged side by side on a wall, floor or ceiling next to the timber, and that the device comprises a data register for storing drying parameters and, ·,: calibrated hand of neutron measurement data obtained from the detector! 'To make them visible to the measuring device.

· 25 Preferably, the measuring device comprises a computer display for plotting the moisture content of the wood as a function of time.

The invention further relates to a method for drying wet timber by means of a controlled compressed air flow and a controlled air: flow wet and dry temperature using the measuring apparatus and calibration described above.

: Then, the characteristic features of the method consist of the following steps: 116583 6 a) reading the type of wood and the dimensions of the timber to be measured, the registered outdoor temperature, the initial average moisture content of the timber and its desired final moisture content; calculating empirically appropriate drying parameters (i.e., wet and dry temperatures, and fan operation program) based on the data recorded in step a), 10 c) plotting the drying process on the computer screen of the measuring device in the form of curves showing the moisture content and (d) during drying, the moisture content of the timber is measured by means of 15 measuring devices and any variation in the drying parameters corrects any difference between the measured drying value and the intended and visible between drying.

Preferably, the moisture content of the timber is measured continuously with a measuring device and the average !!! 20 the moisture content at a selected time, such as the last three hours, and displayed on the computer screen as the actual control value, and if this actual value at the time of display differs from the corresponding nominal value on the drying curve; '·· Make a quick correction to the drying climate by changing the drying parameters ν': 25. The calculated moisture content can then be displayed on the computer screen in fixed intervals, such as once an hour oven.

* 1 · · · * · '; 1 The measurement principle of the invention is based on the ability of hydrogen to convert fast neutrons into slow, so-called thermalized neutrons. Therefore, this neutron thermalization can be used to measure the moisture content of a mixture of water and dry matter, recording the ratio of the number of fast neutrons transferred to the measured medium to the number of reflected thermalized neutrons. The number of neutrals that are thermalized and detected are determined, inter alia, by the composition of the materials, the dry density, the water content and the dimensions of the object being measured (measurement geometry).

5 When the dry matter of the object to be measured consists mainly of molecules containing "heavy" atoms, in practice only hydrogen in water affects thermalization, and the number of thermal neutrons detected is then a measure of the moisture content of the object to be measured. This allows the device 10 to be calibrated on the basis of moisture samples taken from the medium to be measured.

If the dry matter of the object being measured is an organic substance, such as wood, the dry matter also contributes to some degree of thermalization. The number of thermal neutrons detected then is a function not only of the density and geometry of the water, but also of the dry density. In order to allow a single measurement based solely on the rapid neutron thermalization of two unknown quantities, i.e. solids and water, the effect of the solids on thermalization must be mapped to a calibration 20-bar. The dry matter of the wood contains about 6.2% hydrogen (H), 50% carbon (C) and about 43% oxygen (0), and the concentrations vary from wood to wood. In addition, wood species exhibit large differences in dry density.

, 1 'The mass of the hydrogen atom and the neutron is approximately the same (1 amu).

, 25 The collision rate of hydrogen atoms and neutrons varies from skid, '·' 'to almost no loss of neutron energy, to collision, whereby the neutron loses almost everything • *; energy. On average, about 18 collisions with hydrogen atoms are required to slow the fast neutron from source to thermal 30, which means that it is thermally equilibrated with the medium being measured. By way of comparison, on average thermal neutronization with carbon atoms is required; 114 collisions, and 150 collisions, respectively, if the thermalization t *: is performed by oxygen atoms. Therefore, the sensitivity of neutrons to hydrogen atoms is substantially higher than to other atoms in the tree.

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It is known that the effective range of hydrogen in water is greater than that of hydrogen in paraffin. No known values for the effective range of hydrogen in wood are available. However, it was sought to determine whether the latter differed so much from the effective range of hydrogen in water that it was possible to measure the extent to which the hydrogen atoms were bound to water or wood. If the effective fields of action were substantially different, the number of thermalized neutrons would be variable even though the number of hydrogen atoms is constant.

BRIEF DESCRIPTION OF THE DRAWINGS Against this background, the invention will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a drying chamber; reading curves on a kos-20 meter for a completely dry wood with a different wattage density and a wood with a constant dry density and variable moisture content, respectively, as a function of density, Figure 4 is a graph showing the curves for two different drying - ** ' for comparing the event with one and the same wood material with different initial moisture content, Figure 5 shows a calibration curve showing device readings as a function of 50mm spruce moisture content, Figure 6 is a diagram of the system order using the core teem meters for adjusting the wood furnace,

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H '*: 30 Figure 7 is a graph showing device readings as a function of moisture concentration in timber with a variable dry density that can be used to calculate measurement accuracy, and Figure 8 is a calibration graph showing device readings as a function of low, average mouth-5 rele density, respectively.

The best way to carry out the invention

Initial tests were performed with a nuclear humidity meter developed by Noratom / Norcontrol ram. for measuring the water content of coke. The primary purpose was to see if this moisture-10 meter could be used to determine the moisture content of my board trees. First, the device was used as usual with the measurement geometry being undefined and unspecified, respectively. It turned out that an existing device could be used to determine the moisture content of the unspecified measurement geometry, if the dry density was constant. The device was then operated in a different way, that is, with only one neutron channel. The results of these experiments were so interesting that additional work was needed.

Therefore, a nuclear humidity meter was developed that was suitable for use ;;; 20 for drying wood in a wood burning stove. The measuring device · · · · · · consisted of the following main components: - a tank comprising a source 7, a detector 4, a reflector 3: *. and a lifting mechanism 5, 8, 10 neutron for a source, collectively referred to as a sensor, shown in Figure 2, * · · 25 - a junction box with pipe connections, · · · · · · · · · · · · · · · · · · · · - unit and control cabinet, - hydraulic cylinder and pressure regulator, and - unit for calculating the moisture content of the timber.

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The neutron source 7 used in the measuring sensor consists of 30 americium-beryllium sources. The americium emits α-particles which react with the beryllium nuclei, which then emit neutrons. Detector 4 comprises two 3He proportionality counters. The reflector is a water tank 3 covered with a cadmium plate 18.

The probe was placed in a hole in the floor below the board-5. At the Institut for Energy Technician (IFE), the calibration of the measuring device was performed using substances of known moisture content and variable density. These experiments confirm the assumption that the effective areas of action of water and wood bound hydrogen, respectively, are so different that it is possible to measure the moisture content with a measuring device developed in timber. As shown by the solid line in Fig. 3, measurements on a completely dry wood of variable density resulted in a straight line having a rising gradient α of 15 reading the device, indicating the number of thermal neutrons per unit time as a function of density. Measurements with substances of constant density and variable moisture content produced a straight line having a rising gradient b in the same coordinate system, as indicated by the kat-20 dashed line in Figure 3. The rising gradient b is apparently several times greater than the gradient a.

(i »: after the laboratory tests at IFE, the device *;« *; was examined for one year in a log furnace at Rcparn Fossum: Bruk, Oslo. It can be clearly deduced from the measurements made that the thermalization process is dependent on;. i) hydrogen bound to water or *? * wood.

»* • | ; The measuring device was further developed to include, inter alia, a counting circuit installed on a computer unit (PC) as a counting unit. The device was then installed in a chamber dryer

Frognerseteren in Bruk, Oslo. Here, the purpose of the experiments was to * provide reference quality of sufficient quality to convert the read pulse rate to humidity with sufficient accuracy; percent using the experimentally determined calibration curve.

35 In addition, tests were carried out to evaluate, inter alia, the reproducibility of the measured values of 116583 11 and to check for any deviations in the measuring equipment.

Measurements on a single stack of timber confirmed acceptable reproducibility for one and the same species of wood, 5 but with a clear difference between spruce and pine. The operation of the device was controlled by placing an aluminum container of known volume above the measuring device. Initially, it was found that the device deviations were too large, but this was corrected by replacing the amplifier and 10 high voltage sources. In this case, no deviation of the device in the measuring device could be detected.

Then the measuring device was installed in the oven with the Ormstad Sag og Ηφν-ler and some further improvements were made. In Ormstad the measuring device had a very good reproducibility of the measurement results. This can most clearly be seen by comparing the saturation point of the fibers at various drying events. In Figure 4, the humidity, dry temperature, and temperature difference between dry and wet temperatures are shown for two drying events A and B. Roughly the temperature difference is the same for two drying events, and the difference in dry temperature is relatively small. In the drying events A and B, the initial moisture content was 63% and 46%, respectively. Initially, the moisture content increased (the heating phase) and the wood began to dry when the moisture content had dropped. When; the temperature difference is constant, the drying rate is also constant. At drying A, the temperature difference after 60 hours at drying increased, and this was also reflected in the drying rate, which increases significantly. Although the temperature difference is constant for 120 hours of drying,. afterwards, however, the drying rate decreases, as evidenced by the "break * * *. * ta" of the moisture curve of drying event A. "Break" confirms 30 that the fiber saturation point has been reached and that it is now silent! : it is more important to force water out because it is more bound to the cellular structure of the wood. In the drying process B, the moisture content of the wood passes through the impregnation point of the fiber after drying for about 80 hours, and here also the temperature difference is constant. In both drying processes, the "break point" occurs at the same moisture content of the wood, namely 27%.

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Based on the measurement data obtained, a cataracting formula is computed on a computer, and the associated calibration curve is shown in Figure 5. Currently, experiments are performed to find calibration data for some additional dimensions in both spruce and pine.

One of the practical benefits of a measuring device is that it does not interfere with the process or cause additional work when loading timber into and out of the furnace. The design and placement of the measuring device makes it physically secure. It is very important that a well-defined measurement geometry be measured, and this is usually accomplished by placing the measurement sensor below the innermost board stack.

In practice, the measuring device comprises a detector unit which reads the moisture content in one or two board stacks during the drying process, a computer arrangement (control module) with input devices for entering information on wood species, wood dimensions, outdoor temperature and desired moisture content, see Figure 6. Several detector units may be connected to the same PC as shown in the figure.

Once the timber is stacked in the drying chamber, the drying process; the operator gives the message to the control module on the keypad of his computer. with regard to the type of wood, the dimensions of the board and the final moisture content. At the same time, the average moisture content and outdoor temperature of the 25 bar are measured.

V 'Once this information has been obtained and stored, the correct drying parameters, based on experience, will be calculated to be * t ;, · · used for the complete drying process (i.e., wet 1h i dry temperatures and fan operation program). Then the drying process can be started.

The intended drying process is plotted on the PC screen as a plot of moisture content on the y axis and drying time on the x axis. (ij This curve then determines the nominal value of the drying event. To control the drying, the moisture content of the timber is continuously measured, and the average moisture content over the last 116583 13 hours is displayed as an hourly value. If there is a difference between actual and nominal value , repairs to the drying environment can be made quickly by changing the drying parameters to maximize follow-up of the 5 optimal drying events, which can be accomplished automatically or manually.

Here, the measuring device comprises two detectors 4, one neutron source 7, one pulse amplifier and a high voltage unit (not shown). Normally, the unit is mounted in 100 x 120 mm 10 acid-proof ducts underneath the inner plank post transversely to the direction of the plank. The same channel is intended to serve as one or more supports for the column. The power supply and registers "R", which are updated according to the measurement data, are placed in a box outside the drying chamber. The PC-15 unit takes data every 60 seconds and stores it in a database. The average of the deposits made every hour during the last three hours is calculated as a percentage of humidity. The result is displayed on a computer screen and stored in a database. The reason for such a long integration time is that the neutrons recording pulses from the detector 20, which represent the moisture content, are statistically distributed. Therefore, _ · *, a large number of pulses is needed to produce accurate measurements. Instead of using a large source current, time is used. This is * i t advantageous for both economic and technical reasons.

: The detector unit can also be located between two board stacks. This can be done especially when doing experimental work and research.

When a measuring instrument is calibrated, the thermal: f; The number of neutrons as a function of the total hydrogen density ", the hydrogen density in water, pH_ve3i, and the hydrogen density in wood, pH_wood, as variables. Because the neutron ranges of o. P and n are different, the curves shown in Figure 7 are obtained.

• From these curves, the meter can be calibrated and the resolution of the instrument can be determined.

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The dry density of the timber varies between dry (max) and dry (min). The calibration process determines the average dry density dry (min). To determine the accuracy of the measuring device for these changes in dry density, the change in the number of thermal neutrons as a function of the maximum humidity as a function of the maximum difference (change 1) must be compared to the number of thermonized neutrons with the percentage change in change 2. The measurement accuracy of the device is then obtained by subtracting the change from 1 to 2 as the difference between the actual humidity and + or -.

Accordingly, the measurement accuracy can be determined by comparing variations in the number of thermal neutrons: 1. as a function of the maximum change in dry density (at constant moisture content).

2. as a function of the change in percent moisture (at average dry density).

The measurement accuracy, + or - is then obtained from the following equation. or " • " *

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'··· * 20 Xl (max / min density) - X2 (average density)' 'X3 (humidity B) - X4 (humidity B +/- 1%) • t f »

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*. where: • t * '·' * Xl = the number of thermalized neutrons in a tree with max

the minimum or minimum dry density at the moisture value A

·, · J 25 (as shown in Figure 7), * '·' X2 = the number of thermalized neutrons with the average density and humidity of A, X3 = the number of thermalized neutrons with the moisture value B ( as shown in Figure 7) and the average density, and 116583 15 X4 = the number of thermalized neutrons at wood humidity of B +/- 1% and average density.

Figure 8 gives three calibration curves a, b, and c for low, average, and high dry density, respectively, plotted against the number of thermal neutrons per unit time as a function of the total hydrogen density pH. Each of these calibration curves shows three calibration points with different humidity values. The dots marked with the same symbol represent the same moisture content. If line 10 rises through the same symbolic sign from the y-axis toward the right side of the image, there will be an increasing undercompensation for the decreasing dry density in the calibration. If the slope of this line is reversed, there is overcompensation for the falling pew. When the lines through the marked points of the same moisture are horizontal, the device completely compensates for the changes in the dry density.

The measurement accuracy is within the accuracy of the reference values used in the calibration process, and in practice is approximately +1.5.

t t t I 1 * <t · »! ·

Claims (2)

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