Method and apparatus for non-contact, rapid and continuous moisture measurements

Abstract: A method and apparatus for non-contact determination of the moisture content in moving or stationary mass of fibrous products such as wood is provided. The method comprises irradiation of a substance with infra-red radiation which is intense enough to introduce microstructural modifications of the substance surface. The moisture content of the substance is determined by analyzing density of these surface modifications. It has been found, that an optical beam may be used in order to quantify the density of the surface modifications. The surface is scanned with the optical beam two (2) times. The first scanning is done prior to the infra-red irradiation; the second scanning is done following the infra-red irradiation. The invented method allows for non-contact, rapid and continuous in-line measurements of the moisture content.

Description: FIELD AND BACKGROUND OF THE INVENTION This invention concerns a new method and apparatus for in-line measurements of moisture contents in wood pieces, logs, lumber, veneer and the like. The common method of moisture measurements is based on determining of the difference in the weight of the substance before and after heating it at the temperature above the boiling point of water (usually at 105°C). However, such method requires long periods of time (24 hours or longer). That method is limited mostly to stationary measurements and it is not practical for in-line applications where virtually instantaneous water content measurements are required. Another method of moisture determination is based on spectral selective optical absorption which can be measured in scattered and (or) transmitted beams. Sets of light sources with precisely selected wavelengths are used in that method. One set of the wavelength(s) is utilized as a reference which is relatively unaffected by the characteristic (e.g. moisture content) being measured. The other set of the wavelength(s) is tuned to the water absorption peak. Several versions of this approach have been disclosed, in which either conventional infra-red optical sources (e.g. U.S. Patents # 4,266,878, # 4,634,856, # 4,788,853, # 3,851,175, # 3,965,356, # 4,052,615, # 4,300,049, # 4,465,929, # 4,577,104, # 4,733,078, # 4,823,008, # 4,840,706, #4,879,471, # 4,928,013) or a submillimeter laser (U.S. Patent # 4,755,678) were applied. These methods were disclosed for moisture measurements either in soil or in paper products. Moisture measurements based on absorption of fast neutron rays and gamma radiation were disclosed in U.S. Patents # 4,766,319 and # 3,748,473. Electrical meters based on measurements of an electrical property of a substance are used for the moisture content measurements where a relationship between the electrical property and the moisture content of a substance can be established. The moisture content of a substance may be estimated from electrical properties such as resistance, capacitance and conductivity. However, these type of meters are limited to lower end of the moisture content [according to The Standard Test Methods (ASTM: D 4444 - 84) the moisture content range for the electrical meters is from 6% to 27%]. The electrical meters can not be used efficiently outside that range of the moisture content because higher moisture levels cause no substantial change in electrical properties. A method of non-contact moisture measurements is disclosed in U.S. Patent # 4,612,802 (EP-A-0 155 187) issued September 23, 1986 to Forintek Canada Corporation. In that method, an area of one surface of a substance is subjected to a predetermined intensity of a heat source for a selected period of time. The moisture content is determined from measurements of the temperature rise of the heated surface of the substance. GB-A-2 062 219 discloses a method and apparatus for moisture measurements on wood in which infra-red radiation which is diffusely reflected from a sample surface is measured at a measurement and reference wavelength. SUMMARY OF INVENTION In accordance with this invention, a method and apparatus for non-contact and rapid measurements of the moisture content of a substance have been devised. The method and apparatus are derived from the fact that infra-red radiation is absorbed by water which is present in a substance. The surplus of this infra-red radiation causes modifications of a substance surface by changing its microstructure. It has been experimentally demonstrated, that a relationship exists between the moisture content of a substance and the density of surface modifications caused by intense infra-red radiation. Simply, it has been found that a substance with a lower water content will be affected stronger by the intense infra-red radiation than a substance with higher water content exposed to the same amount of the infra-red radiation. Furthermore, it has been determined, that the density of surface modifications can be translated into a quantitative measure by using an optical beam. An optical beam is scanned along an analyzed portion of a substance surface. The surface is scanned two (2) times: prior to - and after - the surface modifications. The difference in intensities of reflected optical radiation is recorded by optical detector(s). It has been experimentally determined, that the moisture content is inversely proportional to a normalized intensity of the reflected optical radiation. Furthermore, it has been determined, that an analysis of a portion of a substance surface is sufficient for acceptably accurate approximation of the average moisture content of a substance. It has been found, that the relationship between the moisture content and intensity of scattered optical radiation can be approximated by the following equation: (I in - I f ) / I in = A - B M c where: M c - is the moisture content; I in - is the intensity of reflected optical radiation from the analyzed portion of a substance surface prior to irradiation with infra-red radiation; I f - is the intensity of reflected optical radiation from the analyzed portion of a substance surface after it was irradiated with infra-red radiation; A and B - are calibration constants. The constants A and B are determined experimentally for a given substance. The values of I f and I in are measured by the optical detector(s). The equation [1] is the 1-st order approximation of the relationship. This approximation may be extended by including higher order components in order to improve precision of the measurements. The invented method allows for non-contact measurements of the average moisture content of a stationary as well as a fast moving substance. The main advantage of the method is that it provides means for the measurements over the entire range of the moisture content. The additional advantage of the method is that the apparatus can be placed at a distance from the analyzed substance because all the measurements may be carried out by low-divergence beams. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying Figures 1 and 2 illustrate schematic views of two versions of the apparatus. Figures 3, 4, 5, 6 and 7 illustrate data obtained by using exemplary apparatus for measuring the moisture content in accordance with this invention. Each point on Figures 3, 4, 5, 6 and 7 represents the mean value of 50 experimental measurements. FIGURE 1 illustrates a schematic view of apparatus capable of measuring the moisture content in a stationary sample of a substance. The apparatus includes a single source of infra-red radiation, a single source of optical radiation, and an optical detector. FIGURE 2 illustrates a schematic view of apparatus capable of measuring the moisture content in a moving sample of a substance. The apparatus includes a single source of infra-red radiation, two sources of optical radiation, and two optical detectors. FIGURE 3 presents a graph plotting the percentage moisture content versus the normalized intensity of the scattered optical beam for spruce lumber. FIGURE 4 presents a graph plotting the percentage moisture content versus the normalized intensity of the scattered optical beam for balsam lumber. FIGURE 5 presents a graph plotting the percentage moisture content versus the normalized intensity of the scattered optical beam for pine lumber. FIGURE 6 illustrates the calibration curve for moisture content of wafers of aspen. FIGURE 7 illustrates results of the continuous measurements of the moisture content of wooden wafers transported by a conveyor at the speed of 30 ft/min. DESCRIPTION OF PREFERRED EMBODIMENTS The apparatus embodiment illustrated in Figure 1 is suitable for moisture measurements in a stationary sample of a substance. Referring to Figure 1, the apparatus, generally designated by reference numeral 10, comprises an optical light source 12 such as a HeNe laser and a low-divergence light beam source 14 providing intense infra-red radiation. The optical light emitted from optical light source 12 is represented by arrow 16. Flat mirror 18 directs optical light 16 to beam combiner 20. The intense infra-red radiation emitted by low-divergence light beam source 14 is represented by arrow 22. Curved mirror 24 reflects both optical light 16 and intense infra-red radiation 22 after their passage through beam combiner 20. Lens 26 is provided to converge reflected light 36 to optical detector 28. Amplifier 30 amplifies the signal provided from detector 28. The components of the system are controlled by computer 32 which is equipped with a data acquisition board which is used for data recording and analysis. The arrangement of optical axis of the detector and the radiation sources is such that they converge at the sample surface. When it is desired to measure the moisture level of stationary sample 34, a selected area 38 of sample 34 is first scanned with optical beam 16 from HeNe laser 12 which has been reflected through mirrors 18 and 24 and passed through beam combiner 20. The reflected optical radiation, which is identified by arrow 36, is collected by lens 26, forwarded to optical detector 28 through amplifier 30 and recorded by computer 32. Selected area 38 of sample 34 is then irradiated by intense infra-red radiation 22 from low-divergence light beam source 14 such as a CO₂ laser. The intense infra-red radiation 22 is combined to optical light 16 through beam combiner 20, preferably a ZnSe beam combiner. The intense infra-red radiation 33 is focused on selected area 38 of sample 34 through mirror 24. This scanning causes the microstructure of sample 34 to be permanently modified at selected area 38. Following irradiation, selected surface 38 of sample 34 is scanned again with optical light 16 and the reflected intensity 36 is recorded by the optical detector 28. The average moisture content of selected area 38 is approximated by comparing the measured intensities of the reflected optical radiation before and after irradiation with the calibrated data for a given substance. The apparatus embodiment illustrated in Figure 2 is used in situations where moisture measurements are taken on moving samples. Referring to Figure 2, the apparatus, generally designated by reference numeral 50, presents some similarities with the apparatus described in Figure 1. Hence, the apparatus 50 comprises a first optical light source 52, a flat mirror 54 to reflect optical light 56 to beam combiner 58 and a curved mirror 60 to focus optical light beam 56. Also included is a low-divergence light beam source 62 which provides intense infra-red radiation 64. Lens 66 provides reflected optical light 68 to detector 70. The detected signal 72 is amplified through amplifier 74 and analyzed by computer 76. The system 50 illustrated in Figure 2 also comprises a further optical light source 80 which is also preferably a HeNe laser, a second curved mirror 82 to focus optical light 84 and a second lens 86 to provide reflected optical light 88 to detector 90. The detected signal 92 is amplified by amplifier 94 and analyzed by computer 76. When it is desired to measure the moisture content of sample 100, at selected area 102, sample 100 is moved on conveyor 104 and selected area 102 is subjected to optical light 56. Reflected optical light 68 is analyzed through detector 70. The sample 100 which continuously moves on conveyor 104 is then irradiated at the same selected area 102 by intense infra-red radiation 64 which causes a permanent modification of the microstructure of sample 100 at selected area 102. Sample 100 then continues its path in the direction of arrow 106 and is scanned again at selected area 102 by optical beam 84 from further light source 80. Reflected optical light 88 is then analyzed through second detector 90. The arrangement of optical axis of the radiation sources and detectors is such that they are in line with respect to the direction 106 in which sample 100 is being moved. Detector 70 records the intensity of reflected optical radiation 68 prior to its irradiation with infra-red beam 64. The optical radiation reflected from the modified surface is then recorded by detector 90. The average moisture content of sample 100 is then approximated by comparing the measured intensities of the reflected optical radiation with the calibration data for a given substance. In the following examples a CO₂ laser has been used as the source of infra-red radiation (MPB's model IN-70: wavelength 10.6 micron, 60 Watt output power, 7 mm beam diameter, pulsed at 50 msec). HeNe lasers have been used as the sources of the optical beams (Melles Griot's model 05LHP 991: wavelength 0.6328 micron, 10 m Watt output power, 0.7 mm beam diameter). The curved mirror (1 meter radius of curvature, 1 inch diameter) was used to focus the laser beam on the samples. The distance between the sample surface and the mirror was approximately 0.5 meter. The optical detectors used were manufactured by EG&G (Model 30809). The computer was equipped with a data acquisition board (a 12 bit analog-to-digital converter). It has been determined, that there is a linear relationship between the optimum intensity of the CO₂ laser beam and the speed of the conveyor. For example, the 60 Watt output power corresponds to the optimum speed of the conveyor of 60.8 cm/s (120 feet per minute). Example 1 Pieces of two by four spruce lumber of different moisture contents were analyzed with the described apparatus. The samples were moving with the speed of 7.6 cm/s (15 feet/minute). A CO₂ laser was used as the source of the infra-red radiation. The normalized intensity of the scattered HeNe laser radiation versus the average moisture content of the samples are plotted on FIGURE 3. A close relationship between the intensity of the scattered optical radiation and the average moisture content is self evident. It is apparent, that the average moisture content can be determined with reasonable accuracy from the measurements of the normalized intensities of the scattered optical radiation. Example 2 In this example, pieces of two by four pine lumber of different moisture contents were analyzed with the described apparatus. The samples were moving with the speed of 7.6 cm/s (15 feet/minute). A CO₂ laser was used as the source of the infra-red radiation. The normalized intensity of the scattered HeNe laser beam versus the average moisture content are plotted on FIGURE 4. Example 3 In this example, pieces of two by four balsam lumber of different moisture contents were analyzed with the described apparatus. The sample was moving with the speed of 7.6 cm/s (15 feet/minute). A CO₂ laser was used as the source of the infra-red radiation. The normalized intensity of the scattered HeNe laser beam versus the average moisture content are plotted on FIGURE 5. Example 4 In this example, pieces of wooden (aspen) wafers of different moisture content were analyzed with the described apparatus (these wafers are being used for the production of pressed boards). The wafers were placed on a conveyor and they were moving with the speed of 15.2 cm/s (30 feet/minute). A CO₂ laser was used as the source of the infra-red radiation. The normalized intensity of the reflected HeNe laser radiation versus the average oven-dried moisture content of the samples is plotted on FIGURE 6. A close relationship between the intensity of the reflected optical radiation and the moisture content is self-evident. The relationship presented on FIGURE 6 was used to calibrate the moisture sensor. The calibrated sensor was used to monitor the moisture content of wooden wafers. A typical result of such measurements is illustrated on FIGURE 7.

Claims: A method for the non-contact determination of the moisture content of a sample, said method comprising: subjecting said sample to an optical light beam of predetermined energy density and measuring the intensity of said optical light reflected from said sample; irradiating an area of said sample with a low-divergence light beam having a density level sufficient to produce modifications of the microstructure of said sample; subjecting said irradiated area to an optical light beam of known intensity and measuring the intensity of said optical light reflected from said irradiated area after irradiation at said energy density level; and determining the moisture level of said sample by comparing the light reflected from said irradiated area and the light reflected from said sample prior to irradiation with calibration data for a given substance. An apparatus for measuring the moisture content of a wood product, said apparatus comprising in combination: a light source for subjecting a selected area of said wood product to optical light; a low-divergence light beam source for irradiating at least one selected area of said wood product at an energy density sufficient to permanently modify the microstructure of said wood product at said selected area; and a detector positioned relative to said light source to measure the intensity of light reflected by said selected area of said wood product when illuminated by said light source before and after said selected area has been irradiated with said low-divergence light beam source and means for determining the moisture level of said sample by comparing the light reflected from said irradiated area and the light reflected from said sample prior to irradiation with calibration data for a given substance. An apparatus according to claim 2, further comprising amplifier means for amplifying the signal resulting from the measure of the intensity of reflected light measured by said detector. An apparatus according to claim 3, further comprising means to calculate the moisture level of said wood product by comparing the intensity of optical light reflected by said selected area of said wood product before and after irradiation of said selected area with said low-divergence beam source. An apparatus according to claim 2, wherein said low-divergence light beam source is a laser beam source. An apparatus according to claim 5, wherein said laser beam is from a CO₂ laser. An apparatus for measuring the moisture content of a wood product, said apparatus comprising in combination: a first light source for subjecting a selected area of said wood product to an optical light beam of known intensity; a low-divergence light beam source for irradiating said selected area of said wood product at an energy density sufficient to permanently modify the microstructure of said wood product at said selected area; a further light source for subjecting said wood product to an optical light beam of known intensity at said selected area after said selected area has been irradiated by said low-divergence light beam source; and detector means positioned relative to said first and further light source to measure the intensity of light reflected by said wood product when illuminated by said first and further light source and means for determining the moisture level of said sample by comparing the light reflected from said irradiated area and the light reflected from said sample prior to irradiation with calibration data for a given substance. An apparatus according to claim 7, wherein said detector means comprising: a first detector associated with said first light source to measure the intensity of light reflected by said selected area when illuminated by said first light source before said selected area has benne irradiated with said low-divergence light beam source; and a second detector associated with said further light source to measure the intensity of light reflected by said selected area when illuminated by said further light source after selected area has been irradiated with said low-divergence light beam source. An apparatus according to claim 8, wherein said first and second detectors are coupled with amplifier means for amplifying the signal resulting from the measure of the intensity of reflected light measured by said first and second detector. An apparatus according to claim 9, further comprising means to calculate the moisture level of said wood product by comparing the intensity of optical light reflected by said selected area of said wood product before and after irradiation of said selected area with said low-divergence beam source. An apparatus according to claim 7, wherein said low-divergence light beam source is a laser beam source. An apparatus according to claim 11, wherein said laser beam is from a CO₂ laser. A method according to claim 1, wherein said low-divergence light beam source is a laser beam source. A method according to claim 13, wherein said laser beam is from a CO₂ laser. The method as claimed in claim 1, wherein said piece of wood is moving at a speed relative to radiant energy sources. A method according to claim 1, wherein said low-divergence light is infra-red light. The method as claimed in claim 16, wherein said piece of wood moves at a speed relative to raciant energy sources. A method for the non-contact determination of the moisture content of a wood product, said method comprising: subjecting said wood product to a first optical light beam of known intensity and measuring the intensity of said first optical light reflected from said wood product; irradiating an area of said wood product with a low-divergence light beam having a density level sufficient to produce modifications of the microstructure of said wood product; subjecting said irradiated area to a further optical light beam of known intensity and measuring the intensity of said further optical light reflected from said wood product after modification of microstructure of said area by said low-divergence beam irradiation; and determining the moisture level of said wood product by comparing said first and further optical light reflections to predetermined wood product optical reflection data. The method as claimed in claim 18, wherein said piece of wood moves at a speed relative to radiant energy sources. A method according to claim 18, wherein said low-divergence light beam source is a laser beam source. A method according to claim 19, wherein said laser beam is from a CO₂ laser. A method of determining relative variations in the approximate average moisture content of a wood product regardless of the thickness of said wood product at different locations therealong, comprising: (a) subjecting each one of a plurality of preselected areas on any surface of said wood product to optical radiation of known intensity and measuring the initial intensity of said optical radiation reflected from said preselected areas on said surface of said wood piece; (b) subjecting said preselected areas on said surface of said wood piece to modifications of their microstructure by irradiating said selected areas with infra-red radiant energy of predetermined energy density to yield structurally modified preselected areas; (c) subjecting said structurally modified preselected areas to optical radiation of known intensity and measuring the intensity of optical radiation reflected from said structurally modified preselected areas; and (d) comparing the intensity of optical radiation reflected from said structurally modified preselected areas of said wood piece and said initial intensity of optical radiation reflected from said preselected areas on the surface of said wood piece with calibration data for a given substance to determine the average moisture content of said wood piece. A method according to claim 22, wherein said piece of wood is moving at a speed relative to radiant energy sources. A method according to claim 22, wherein said low-divergence light beam source is a laser beam source. A method according to claim 24, wherein said laser beam is from a CO₂ laser.

Inventors: JAMROZ WES R, TREMBLAY JULIEN, WONG BRIAN, JAMROZ, WES R., TREMBLAY, JULIEN, WONG, BRIAN
Applicants: MPB TECHNOLOGIES INC, MPB TECHNOLOGIES INC.
Application year: 1993
Main classification: G01N21
IPC codes: G01N21, G01N33