WO2007088215A1 - Apparatus and method for determining the moisture content in porous media using optical spectroscopy - Google Patents

Apparatus and method for determining the moisture content in porous media using optical spectroscopy Download PDF

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Publication number
WO2007088215A1
WO2007088215A1 PCT/EP2007/051088 EP2007051088W WO2007088215A1 WO 2007088215 A1 WO2007088215 A1 WO 2007088215A1 EP 2007051088 W EP2007051088 W EP 2007051088W WO 2007088215 A1 WO2007088215 A1 WO 2007088215A1
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water
signal
absorption
oxygen
wood
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PCT/EP2007/051088
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French (fr)
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Sune Svanberg
Mats Andersson
Linda Persson
Mikael Sjöholm
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Gasporox Ab
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3554—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for determining moisture content
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
    • G01J3/433—Modulation spectrometry; Derivative spectrometry
    • G01J3/4338—Frequency modulated spectrometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3563—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing solids; Preparation of samples therefor
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/359—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using near infrared light
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/47—Scattering, i.e. diffuse reflection
    • G01N21/49—Scattering, i.e. diffuse reflection within a body or fluid
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N2021/3185—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry typically monochromatic or band-limited
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
    • G01N2021/396—Type of laser source
    • G01N2021/399—Diode laser
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3554—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for determining moisture content
    • G01N21/3559—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for determining moisture content in sheets, e.g. in paper

Definitions

  • This invention pertains to the field of apparatuses and methods for determining the moisture content in porous media, and in particular by using optical spectroscopy.
  • the resistance type of moisture meter measures the electric resistance in wood between two pins that are con- nected at the wood surface or inserted into the wood, with typical distances of centimeters between the pins. If the wood is dry it acts as an isolator and the resistance is in the order of 105 M ⁇ . However, since water in wood contains ions, the resistance in wood decreases down to about 0.5 M ⁇ when the moisture content is close to the saturation point.
  • the dielectric type moisture meter uses a non- intrusive measurement technique that measures the dielectric constant of the combined wood and water material. Since the dielectric constant is much higher for water com- pared to wood (a factor of 25) , the moisture content may be estimated. Otherwise, each type of wood has its specific dielectric constant. Specific wood calibration data are stored in the device and controlled by a micro processor.
  • the dielectric moisture meter is known to have poor per- formance compared to the resistance type. However, it is commonly used for relative measurements.
  • the surface electrodes are sensitive for other materials close to the surface and density fluctuations of the wood. The measurement range varies from 5 to about 25% MC.
  • US 4,785,185 discloses a dielectric material moisture detection apparatus that uses a sub-millimeter laser beam, which is repeatedly swept over a pre-programmed path onto a dielectric sheet having several detectors for measurements of water or filler content of sheets and bands of dielectric material.
  • the laser radiation is scanned across the face of the material and the detectors, placed on the side of the material opposite the laser, measure the material transmittance . Based on this transmittance information the moisture or filler content of the dielectric material sheet is determined.
  • this measurement technique requires that the laser radiation source has such a high output that transmittance through the sheet of dielectric material is provided. This is a drawback in many aspects, for instance from a energy consumption view and limited area of use to rather thin material sheets, such as plywood sheets or wood veneer slices, or materials having high transparency.
  • WO 01/65237 discloses wood fiber flow properties determination, involves measuring spectral values from fiber sample reflectance spectrum to determine properties based on specific relation between spectral and reference values. More precisely, a method for continuous determination of the properties of a wood fiber flow in fiberboard fabrication is disclosed. A sample of the fiber flow is illuminated with light, and one or more of the properties fiber length distribution, resin content, fiber moisture and optical properties for the sample, are determined from the measured reflectance spectrum, based on predetermined relations between measured spectral values and absolute magnitudes for the properties. However, the predetermined rela- tions between measured spectral values and reference values for resin content, fiber moisture and optical properties have to be determined for each type of fiber analyzed at an occasion.
  • the reference value for fiber moisture is disclosed as having to be determined by drying and weighing calibration samples of the fibers to be measured.
  • the method disclosed in WO 01/65237 has a number of drawbacks, e.g. when the humidity contents in unknown porous materials has to be determined, or when it is not possible to acquire samples in order to perform the required reference measurement, e.g. on antique furniture.
  • Atmospheric monitoring of oxygen and water is known by Gagliardi, G. et. al "Trace-gas analysis using diode lasers in the near-IR and long-path techniques", Optics and lasers in engineering, 8 March 2002, Vol. 37, issue 5, May 2002, p. 509-520.
  • the measurement principle disclosed therein is not suited for porous scattering media. The measurement is only possible in free atmospheric gas.
  • the measurement principle disclosed in the paper of Gagliardi et. al is not suited for porous material, where scattering is immense.
  • Quantitative absorption spectroscopy is hampered by the strong scattering in inhomogeneous materials, making the Beer-Lambert law not directly applicable.
  • an improved apparatus and method for determining the moisture content in porous media would be advantageous and in particular allowing for increased flexibility, cost-effectiveness, and/or reliability would be advantageous .
  • embodiments seek to mitigate, alleviate or eliminate one or more deficiencies, disadvantages or issues in the art, such as the above-identified, singly or in any combination by providing an apparatus, a method and a computer program according to the appended patent claims.
  • an appara- tus for monitoring a moisture content and/or a progress of a drying process of a material.
  • the apparatus is devised to make measurements by optical spectroscopy, wherein the material is a natural or man-made porous, scattering, and partly translucent material.
  • the apparatus is configured to measure free molecular oxygen and molecular water vapor in the material by using the optical spectros- copy for providing a measure of the moisture content in the material and/or said progress of said drying process of the material .
  • optical spectroscopy may be differential absorption optical spectroscopy operating in transmission mode.
  • optical spectroscopy may be differential absorption optical spectroscopy operating in backscattering mode.
  • the optical spectroscopy may be nar- row-band optical spectroscopy.
  • the optical spectroscopy may in use of the apparatus be performed in the near infrared region of 700 - 1400 nm.
  • the apparatus may be configured to monitor molecules of said free water vapor by narrow-band diode laser spectroscopy.
  • the narrow-band diode laser spectroscopy may be made in the 930 - 1000 nm region, when the apparatus is used.
  • the monitoring of free molecular oxygen may be performed by narrow-band oxygen diode laser absorption spectroscopy, to achieve a signal increasing as the material dries and produces pores that become filled with oxygen-containing air.
  • the narrow-band oxygen diode laser absorption spectroscopy may be made in the A-band around 760 nm.
  • the apparatus may further comprise a unit for formation of a ratio of the molecular water vapor and said molecular oxygen as normalized absorption signals from said narrow-band diode laser spectroscopy and said narrow-band oxygen diode laser absorption spectroscopy, providing a dimensionless signal useful for an instant measurement of the state of drying of the material.
  • the apparatus may perform an addi- tional optical absorption measurement of total transmitted light in the material sampling a broadband liquid water absorption, at an on-water-resonance wavelength, such as a liquid water absorption peak wavelength at about 980 nm, and an off-water-resonance wavelength reference measurement, such as at about 760 nm, whereby either individual signal levels of the two measurements at the on-water-resonance wavelength and the off-water-resonance wavelength are measured to follow the progress of the drying process of said material, or a ratio of the individual signal levels of the two measurements is formed providing a dimensionless signal useful for providing an instant measurement of the state of drying of said material.
  • an on-water-resonance wavelength such as a liquid water absorption peak wavelength at about 980 nm
  • an off-water-resonance wavelength reference measurement such as at about 760 nm
  • the apparatus may further comprise a video camera sampling the broadband light transmitted from narrow band laser light sources of said optical spectroscopy.
  • additional broadband light-emitting diodes may be arranged as light sources for the on-water- resonance wave-length and the off-water-resonance wave- length for measuring the broadband liquid water absorption.
  • additional broadband thermal radiation source and optical filters may provide isolating of appropriate bands for performing a differential absorption measurement .
  • a video camera may be provided for the recording of a light intensity emerging from the material at the on-water-resonance wavelength and the off- water-resonance wave-length, respectively, whereby the spatial extent of the recorded light provides additional in- formation on the drying process according to well-known principles for light absorption and scattering in turbid media .
  • the apparatus may comprise a unit for generation of a process intervention signal when the recorded ratio value of molecular water vapor and molecular oxygen normalized absorption signals from the narrow-band diode laser spectroscopy passes a pre-selected threshold value .
  • the apparatus may comprise a unit for generation of a process intervention signal when the recorded ratio value of the two measurements at the on- water-resonance wavelength and the off-water-resonance wavelength passes a pre-selected threshold value.
  • the apparatus may be provided in a compact, handheld configuration, adapted to be applied to the outside of the material for measuring the moisture contents in the material, or for monitoring said progress of said drying process of said material.
  • said natural or man-made porous, scattering, and partly translucent material may be a wood material; a construction material; an agricultural product; a food product, such as cereals; a cosmetic product; or a pharmaceutical preparation.
  • a method for monitoring of moisture content and/or the progress of a drying process in a natural or man-made porous, scattering, and partly translucent material comprises using an apparatus according the above mentioned aspect of the invention, providing diagnostic in- formation on the progression of natural or forced drying processes from using optical spectroscopy measurements, wherein free molecular oxygen and free molecular water vapor in said material are measured by said optical spectroscopy for providing a measure of said moisture content in said material and/or the progress of said drying process of said material.
  • the method may comprise providing said diagnostic information by simultaneously measuring said free molecular oxygen and free molecular water vapor in the material.
  • the method may be made by Gas in
  • GASMAS Scattering Media Absorption Spectroscopy
  • wavelengths that are used for the free water vapor and free oxygen gas monitoring may be approximately 980 and 760 nm, and also on and off-resonant for a broad liquid water absorption feature, and thus in- formation of the bulk water in said material is obtained additionally from the same apparatus.
  • the time when all the free water vapor has evaporated from the material may be identified by a strong fall-off in the water vapor signal accompanied by the reaching of a high-level plateau in the free molecular oxygen signal.
  • a ratio between the free water vapor signal and the free molecular oxygen signal being largely independent of scattering may show the obtainment of a fully dry sample by a sharp increase, which may also be identified in the differential optical absorption signal for liquid water, with a sharp increase of an order of a magnitude in the ratio of broadband signal intensities at approximately 980 nm and 760 nm.
  • the photon history in the material for the two wavelengths may provide independent information on the scattering of the material and the liquid water contents in the material.
  • additional measurements of the pho- ton history may be performed using white light, capturing the full spectrum, or employing a number of pulsed diode lasers, selected at appropriate wavelengths.
  • a computer-readable medium having embodied thereon a computer program for monitoring of moisture content and/or the progress of a drying process in a natural or man-made porous, scattering, and partly translucent material, using an apparatus according to the above described aspect of the invention.
  • the computer program is for processing by a computer, and comprises a code segment for providing diagnostic information on the progression of natural or forced drying processes from using optical spectroscopy measurements, wherein free molecular oxygen and free molecular water vapor in said material are measured by said optical spectroscopy for providing a measure of said moisture content in said material and/or the progress of said drying process of said material.
  • Some embodiments of the invention provide for non- intrusive measurement of humidity contents in a porous me- dium.
  • Some embodiments of the invention also provide for non-intrusive monitoring of an entire drying process.
  • FIG. 1 are microscopic pictures of hardwood (balsa) and softwood (pine) ;
  • Fig. 2 illustrates the wood drying process, wherein at first the cells are filled with water but in the end no free water exists and the absorbed water in cell walls is dried out until an equilibrium state with the surrounding environment is reached;
  • Fig. 3A is a schematic drawing of the spectroscopic setup of similar diode laser spectrometers for oxygen (left) and water vapor (right) ;
  • Fig. 3B are graphs illustrating typical readings of WMS signals for oxygen and water vapor in wood, wherein the half widths of the signals are of the order of a few GHz in both cases;
  • Fig. 4. is a graph illustrating measured moisture contents during the drying process of a sample, wherein the continuous line indicates the moisture contents measured by logging the weight of the sample, the dashed line when measured by using a resistance moisture meter (Protimeter Timbermaster) , and the dash-dotted line when measured by using a dielectric moisture meter (MC-300W, Exotek) .
  • Fig. 5A is a graph illustrating measured temperature at the surface of the sample and measured weight during the drying process
  • Fig. 5B is a graph illustrating vapor pressure as a function of the temperature, see for instance C. Nordling and J. Osterman, "Physics handbook,” 4th edition, (Stu- dentlitteratur 1987);
  • Fig. 6A is a graph illustrating the equivalent mean path length for water vapor and oxygen during the drying process of balsa
  • Fig. 6B is a graph illustrating The ratio between de- tected equivalent mean path length for water vapor and oxygen
  • Fig. 6C is a graph illustrating direct signals for water vapor and oxygen during the drying process of balsa;
  • Fig. 6D is a graph illustrating the ratio between the detected direct signal for water vapor and oxygen
  • Fig. 7A is a graph illustrating the absorption spectrum of 1 cm pure water reproduced from the measurements by Matcher et al, see S. Matcher, M Cope, and D. Delpy, "Use of the water absorption spectrum to quantify tissue chromo- phore concentration changes in near infrared spectroscopy, " Phys. Med. Biol. Vol. 39, p.177-196 (1994), which is incorporated herein by reference in its entirety; and
  • Fig.7B are images of the sample on the detection side for oxygen (760 nm) and water-vapor (980 nm) during its drying process.
  • the mate- rial in which moisture contents or humidity is measured or the monitoring of a drying process is performed upon is a natural or man-made porous, scattering, and partly translucent material, such as a wood material, such as described below; a construction material, such as plaster, cement or gypsum boards or plates; an agricultural product, such as straw; a food product, such as cereals; a cosmetic product; or a pharmaceutical preparation, e.g. in form of a tablet.
  • the scattering environment provided by the material is a challenge for measuring free gas in the material by optical spectroscopy.
  • GASMAS Gas in Scattering Media Absorption Spectroscopy
  • Wood is a porous material and has a complicated architecture, described in detailed, e.g., in the publication of J. M. Dinwoodie, cited above.
  • the wood structure supports the treetop, stores nutritious substances, and transfers minerals and water, which have been absorbed by the root system.
  • it needs to be strong, hydro- philic, and flexible since a tree grows and lives in a changing environment.
  • hardwood deciduous
  • softwood coniferous
  • Fig. 1 are microscopic pictures of hardwood (above in Fig. 1 at A, here balsa) and softwood (below in Fig. 1 at B, here pine), given for illustrative purposes.
  • Hardwood is the most complicated of the two and comprise wood such as balsa, white oak, ash, mahogany etc.
  • the density of hardwood may vary from about 0.1 to 1.2 g/cm 3 , depending on type of wood.
  • Hardwood is characterized by a combination of complicated cell types orientated both vertically, tangen- tially, and radially.
  • Softwood on the other hand, is distinguished by a much simpler combination of cells that runs mostly vertically in a tree. Typical examples of wood that belong to the softwood group are spruce, pine, and larch.
  • the weight of softwood may vary from 0.3 to 0.7 g/cm 3 . From a microscopic point of view wood consists of cellular structures that handle tasks carried out in a tree.
  • Water and minerals are stored and transported vertically via cells, forming vessels, and horizontally via cell structures called rays.
  • total wood cells can be classi- fied in four different types; parenchyma (storage of nutritious) , tracheids (support and conduction) , fibers (support) , and vessel cells (conduction) .
  • Fibers and vessels are responsible for support and conduction of water and minerals. Fibers are usually 1-2 mm long and 10-20 ⁇ m wide and their only function is support. However, vessel cells are about the same length as the fibers, but up to 0.5 mm wide. Since the ends of the vessels are situated on top of each other they can make up a long tube. Thus, vessels act as an efficient water tube since the ends of the vessels are more or less dissolved.
  • parenchyma In softwood only two cell types are present, namely parenchyma and tracheids. Tracheids are the most common cell type (about 90% of the softwood) and its main function is support and conduction of minerals and water. The size of the cell is about 2-4 mm long and about 30-50 ⁇ m wide. It is situated vertically in the tree and conducts water and minerals via small pits located on the cell surface. Parenchyma cells are small, 200x30 ⁇ m 2 in size, taking care of the storage of nutritious. Wood drying processes
  • the density of wood elements is about 1.5 g/cm 3 but since wood is based on a cell structure, with different size for different type of wood, the density of wood may vary from 0.1 to 1.2 g/cm 3 if the content of water is kept low. However, since the wood cell structure is hydrophilic and full of air, wood is heavily affected by water and moisture.
  • the moisture content (MC) in wood is defined as the ratio between the weight of water in a piece of wood and the weight of the wood when no water is present. This means that the moisture content is higher than 100% in a living tree. During this state water is stored in cells and ves- sels, i.e.
  • FSP fiber saturation point
  • the present embodiment deals with measuring the drying degree of wood using high-resolution near-IR laser ab- sorption spectroscopy.
  • Tuneable diode lasers operating close to 980 nm are used to monitor gaseous water.
  • Liquid water also exhibits a broad absorption peak in the same wavelength region.
  • atmospheric oxygen gas in the pores is also monitored in its A band close to 760 nm. This wavelength also provides a convenient off-resonance wavelength in the liquid water spectrum for assessment of liquid water contents.
  • Absorption spectroscopy is a method for measuring concentrations of substances utilizing the Beer-Lambert law and a suitable calibration procedure (See, e.g. D. A. Skoog and M. D. West, “Fundamentals of Analytical Chemistry,” 7th edition, (Saunders, Philadelphia 1995).
  • the application of this method is not straight-forward for natural materials such as wood, since the very fact that they are inhomogeneous and porous also means that they are highly scattering, making the optical pathlength through the sample undefined. This situation is common in medical optics, where scattering and absorption are intertwined (see G. Muller, B. Chance, R. Alfano, S. Arridge, J. Beuthan, E. Gratton, M.
  • GASMAS Scattering Media Absorption Spectroscopy
  • M. Sj ⁇ holm G. Somesfalean, J. Alnis, S. Andersson-Engels, and S. Svanberg, "Analysis of gas dispersed in scattering media," Opt. Lett. Vol. 26, p. 16-18 (2001), which is incorporated herein by reference in its entirety.
  • Normally a modest resolution is used in solid state spectroscopy because of the broad absorbing struc- tures.
  • single-mode diode laser spectroscopy using sensitive modulation techniques is employed, narrower structures due to free gas appear.
  • Concentrations could be determined by combining GASMAS measurements with time-resolved measurement revealing the time history of the photons inside the sample, see for instance G. Somesfalean, M. Sj ⁇ holm, J. Alnis, C. af Klinteberg, S. Andersson-Engels, and S. Svanberg, "Concentration measurement of gas imbedded in scattering media employing time and spatially resolved techniques," Appl . Opt. Vol. 41, p.3538- 3544 (2002), which is incorporated herein by reference in its entirety.
  • the GASMAS technique was also very recently applied to the monitoring of human sinus cavities, see L. Persson, K. Svanberg, and S.
  • the apparatus may be provided in a compact, handheld configuration, adapted to be applied to the outside of a porous medium for measuring the humidity contents in the porous medium, such as wood.
  • a porous medium for measuring the humidity contents in the porous medium, such as wood.
  • the oxygen measurement setup is explained in more detail in the publication "Diode laser absorption spectroscopy for studies of gas exchange in fruits" of L. Persson et al, cited above.
  • the photo multiplier tube (PMT) sensor is replaced by a silicon de- tector (Photovoltaic PIN-IODP from UDT) with an active surface of approximately 100 mm .
  • the major difference between the oxygen and the water-vapor setup is the wavelength of the diode lasers used.
  • a diode laser 1 such as a Sharp LT031MDO with an output power of 7 mW, is used in an em- bodiment to scan across the R7R7 oxygen line at 761.003 nm (vacuum wavelength) .
  • a diode laser such as a near-IR Fabry-Perot diode laser, such as Specdilas F760 with a nominal output of 200 mW at 980 nm
  • a diode laser such as a near-IR Fabry-Perot diode laser, such as Specdilas F760 with a nominal output of 200 mW at 980 nm
  • the measurement is based on that the wavelength of the laser light is scanned across the absorption lines by sweeping the operating current of the lasers by the use of a 4 Hz saw tooth ramp.
  • the operating current is also modulated by a sinusoidal wave at 9 kHz.
  • the laser light is focused into a fiber guiding the light to the sample.
  • FIG. 3A A schematic diagram of the gas detection setup is shown.
  • a near-IR single mode diode laser 1 namely a Sharp LT031MDO with a nominal output power of 7 mW, was used as the spectroscopic light source.
  • the diode laser 1 was temperature tuned across the R7R7 molecular oxygen line, situated at 761.003 nm (vacuum wavelength) .
  • a 9 kHz sine-wave was superimposed onto the current ramp to produce a wavelength modulation of the light, thereby providing sensitive wavelength modulation spectroscopy (WMS) .
  • WMS wavelength modulation spectroscopy
  • An optical fiber 2 e.g. with a 600 ⁇ m core diameter, is used to guide the light to the sample 5.
  • a small right-angle prism may in an embodiment of the appara- tus be positioned in front of the distal end of the fiber 2 and centrally located of the detector 4, and may be used to provide total internal reflection in order to launch the light into the sample 5.
  • an aperture e.g. an annular aperture with an inner and an outer aperture diameter of 10 and 21 mm, respectively, may be used to collect the back- scattered photons from the sample 5.
  • the fiber was positioned above the porous medium sample 5 and transmitted to detector 4, as shown in Fig. 3A.
  • the absorption signal is detected by splitting up the signal from the photodetector, such as the silicon detector (Photovoltaic PIN-IODP from UDT), into two parts.
  • One part referred to as the direct signal
  • the other part referred to as the WMS signal
  • a lock-in amplifier 9 such as an EG&G Princeton Applied Research 5209, providing phase-sensitive detection at twice the modulation frequency, before being sent to another channel on the os- cilloscope 8, as illustrated in Fig. 3A.
  • Wavelength modulation spectroscopy with lock-in detection is often referred to as derivative spectroscopy, since the signal looks like the derivative of the absorption profile. In this case, when detection at twice the modulation frequency is done, the lock-in signal looks like the second derivative of the absorption profile.
  • the amplitude of the WMS signal is determined by the absolute size of the narrow gas absorption feature, i.e., the fractional absorption due to the gas and the amount of light reaching the detector.
  • the absorption due to the gas of interest which may for instance be water vapor and oxygen, is estimated.
  • the WMS signal is proportional to the absorbance and thus to the product of the gas concentration and the path length traveled by the light.
  • the water vapor setup uses the same type of silicon detector as the one used for oxygen.
  • the output from each detector is split into two signal branches, as mentioned above.
  • One branch is directly connected to oscilloscope 8, referred to as the direct signal, while the second part goes via the lock-in amplifier before being connected to another channel of the oscilloscope, and is referred to as the WMS signal.
  • a broad-band thermal radiation source and optical filters may provide isolating of the appropriate bands for performing the differential absorption measurement .
  • the data are stored and analyzed using a computer. Typical readings for oxygen and water vapor are shown in Fig. 3B, where graphs illustrate WMS signals for oxygen and water vapor in wood, wherein the half widths of the signals are of the order of a few GHz in both cases.
  • the peak-to-peak value of the WMS signal is measured and normalized by dividing with the direct signal value at the corresponding wavelength in order to determine the fractional absorption during the drying process.
  • a measurement device may be configured to perform said optical absorption spectroscopy in scattering media in transmission, as explained with reference to Fig. 3A, or in backscattering according to another embodiment.
  • the light emitter and light receiver may be arranged in a transmission geometry or in a backscat- tering geometry.
  • a backscattering geometry the humidity and oxygen contents are determined in a layer extending downward into the porous medium from the surface thereof.
  • a drying process of porous media samples having a larger volume may be determined as in a transmis- sion geometry.
  • the analysis of a surface region allows de- termination of the drying process of the whole material, for instance by correlation to previous measurements.
  • a method called standard addition is used in order to calibrate a measured normal- ized WMS signal and transfer it to a more meaningful quantity; a so-called equivalent mean path, L eq . It is determined by adding free air path lengths in the collimated laser beam before it enters the wood.
  • Precautions were taken when the water-vapor WMS signal was calibrated. The room temperature and relative humidity was measured by a hygrometer (Testo 608-H1) sensor and the partial pressure for the water vapor was estimated.
  • an analog lab scales (Libror EB-280, Shimadzu) is used to measure the weight of the wood in order to calculate the average moisture content during the drying process.
  • a commercial convection oven was used to dry the wood at 110 0 C, in order to determine the truly dry weight.
  • balsa wood pieces of 10 cm width, 30 cm length and 0.8 cm thickness derived from the same batch were used in the measurements.
  • the figures of balsa wood shown in Fig. 1 are from the wood samples studied. The wood was kept under water for typically three days and was studied directly after being exposed to the ambient laboratory air. The drying could be followed by reading off the analog scales, onto which the piece of wood was attached. The results of conventional measurements of the moisture contents during the drying process of a sample are shown in Fig.
  • the continuous line indicates the moisture contents measured by logging the weight of the sample, the dashed line when measured by using a resistance moisture meter (Protimeter Timbermaster) , and the dash- dotted line when measured by using a dielectric moisture meter (MC-300W, Exotek) .
  • the instrument is applied in a straight forward manner.
  • the weight reading from the scale expressed as moisture con- tent, is plotted together with data from the resistance and the dielectric moisture meters. It can clearly be seen, that the results from the meters poorly describe the real drying process monitored by the scale for moisture content above the FSP. Obviously there is a need for improved meas- urement techniques for moisture.
  • thermocouples The temperature of the wood surface was also monitored with thermocouples. The measurements were preformed on a different sample. In this case the sample had been soaked in water for a longer time than the case when re- cording the data in Fig. 4. Typical results are shown in
  • Fig. 5A where decreasing curve 51 shows a weight loss to a stable value of about 33 percent of the original wet weight, the dry material plateau being reached after about 1200 minutes.
  • the drying process is, as expected, accompa- nied by a lowering of the temperature.
  • the temperature as illustrated with curve 52 in Fig. 5A, was stable at 9°C below the ambient value (25°C), which was gradually reached after 1200 minutes.
  • the signal is proportional to the oxygen concentration times the effective distance traveled through the gas-filled pores, the latter factor being strongly sensitive to the degree of light scattering in the material.
  • the signal is plotted as an equivalent path length in normal air containing 21 percent oxygen, as previous discussed. It should be noted that the effective path length through the material, useful in a standard Beer-Lambert law view, can be expected to change during the drying process.
  • One aspect is that while liquid water is an index-matching fluid reducing the scattering when the pores are water filled, the scattering should increase when the air-filled inhomogeneities develop during the drying process.
  • the oxygen signal increases from a non-zero value (there are air-filled pores even in the very wet wood) by a factor of about 8 to then stay constant at the high value, reached at about 1300 minutes when all the liquid water is driven out and all the pores are instead gas-filled.
  • the simultaneously measured gaseous water signal is included in Fig. 6A as curve 62.
  • the curve 62 starts at a non-zero level, corresponding to the presence of gas- filled pores with saturated water vapor also in the very wet wood.
  • the signal increases to a maximum at about 1200 minutes, reached slightly before the oxygen signal maximum.
  • the water vapor signal gradually falls off to a steady value of about 25 percent of the maximum value. This could be inter- preted in the following way: Around 1200 minutes the pores are almost void of water, as indicated by the dashed line in Fig. 5A, but there is still a sufficient amount of water to sustain the full saturated water vapor partial pressure.
  • the high initial value in the ratio curve may be related to the time constant for oxygen diffusion through the material also discussed with regard to the different times for the two gases to reach the maximum WMS signal. It should be noted that in the drying up of a pore, the walls of the cavity will be uniformly covered by liquid water due to the surface tension. This may impair the transport of oxygen through the film while the water vapor can freely build up in the central micro-bubble of the pore, see Fig. 2. Effects of this kind may also influence the balance between the two gases in the cavities during the drying process. As noted, when everything is dry, conditions are static with regard to scattering and the behavior of both curves is clearly understood as discussed above.
  • the 760 nm signal starts at a high level and acceleratingly reaches a steady value about 5 times lower when the wood is dry.
  • the ratio of detected direct signal for water and oxygen is shown in Fig. 6D at curve 66.
  • An important aspect in ex- plaining these phenomena is to note, that liquid water has a quite strong broadband absorption around 980 nm as detailed in Fig. 7A and already noted above. We note that this absorption influences the on- and off-resonance frequencies for the narrow-band water-vapor signals alike, and does not influence the fractional absorption determining the water-vapor concentration. However, the laser beam is clearly attenuated by the water contents.
  • the initial flat response of the 980 nm curve for the drying wood can be interpreted as that the reduction of water during the initial drying is compensated by the increased light path length due to the developing gas-filled pores. Finally, the absorbing liquid water disappears and the detected light settles on a level determined by the attenuation of the light- scattering dry wood.
  • the 760 nm light is off-resonance from the broad liquid water absorption as shown in Fig. 7A, and bulk absorption due to the water is small, in strong contrast to the case for 980 nm.
  • the high initial light level can be seen as an effect of the index-matching effect of the water in the pores, reducing the lateral scattering and allowing more light to reach the detector.
  • the dynamics of the transmitted light levels at the two wavelengths could also be studied using near-IR imaging of the transmitted light using standard web-cameras (Q-TEC 100) .
  • the intensity in the centre of the blobs (where the detector is situated) is related to the intensities given in Fig. 6C; the 760 nm signal is strongly reduced, the 980 nm is increased. From recordings, for instance made by a video camera, such as the ones shown in Fig. 7B, information of the spatial distribution may also be extracted and compared with theory.
  • the wavelengths used for the free gas monitoring, 980 and 760 nm, are also on and off-resonant for a broad liquid water ab- sorption feature, and thus information of the bulk water may also be obtained.
  • the signal intensities observed are mostly to be interpreted as due to the interplay between specific absorption and scattering, both changing during the drying process.
  • the time when all the free water has evaporated from the wood can be readily identified by a strong fall-off in the water vapor signal accompanied by the reaching of a high-level plateau in the molecular oxygen signal.
  • the ratio between these signals, being dimensionless and largely independent of scattering, shows in particular the obtaining of a fully dry sample.
  • the measurement apparatus may be provided as a compact, handheld configuration, adapted to be applied to the outside of the material for measuring the moisture contents in the material, or for monitoring said progress of said drying process of said ma- terial.
  • the method may be performed in software, for instance stored on a computer-readable medium.
  • the software may be implemented in the apparatus according to embodiments.
  • the computer program may comprise a code segment for providing diagnostic information on the progression of natural or forced drying processes from using optical spectroscopy measurements, wherein free molecular oxygen and free molecular water vapor in a material are measured by said optical spectroscopy for providing a measure of a moisture content in the material and/or the progress of a drying process of the material.
  • time-resolved measurements of the photon history in the wood for the two wavelength provides independent information on the scatter- ing of the material and the liquid water contents.
  • Such measurements may be performed using white light, capturing the full spectrum, or employing a number of pulsed diode lasers, selected at appropriate wavelengths, see T. Svens- son, Johannes Swartling, Paola Taroni, Alessandro Tor- ricelli, Pia Lindblom, Christian Ingvar, and Stefan Anders- son-Engels, "Characterization of normal breast tissue heterogeneity using time-resolved near-infrared spectroscopy, " Phys. Med. Biol. Vol.
  • the time when all the free water has evaporated from the wood is readily identified by a strong fall-off in the water vapor signal accompanied by the reaching of a high-level plateau in the molecular oxygen signal. Furthermore, the same point is identified in the differential optical absorption signal for liquid wa- ter, with a sharp increase by an order of magnitude in the ratio of the signal intensities at 980 nm and 760 nm.
  • connection or “coupled” as used herein may include wirelessly connected or coupled.
  • the term “and/or” includes any and all combinations of one or more of the associated listed items. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
  • the present invention may be embodied as device, system, method or computer program product. Accordingly, the present invention may take the form of an entirely hardware embodiment, a software embodiment or an embodiment combining software and hardware aspects all generally referred to herein as a "circuit" or "module.” Furthermore, the present invention may take the form of a computer program product on a computer-usable storage medium having computer-usable program code embodied in the medium. Any suitable computer readable medium may be utilized including hard disks, CD-ROMs, optical storage devices, a transmis- sion media such as those supporting the Internet or an intranet, or magnetic storage devices.

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Abstract

By the use of wavelength-modulation diode laser spectroscopy, water vapor and oxygen molecules are detected in scattering, porous media non-intrusively, at 980 and 760 nm, respectively. Free gases have extremely sharp absorption structures compared to the broad features from the bulk-material. In an embodiment, water vapor and oxygen measurements during the drying process of wood are disclosed. The technique gives complementary information about the drying process of wood compared to the commercially available moisture meters. In particular, the time when all the free water has evaporated from the wood is readily identified by a strong fall-off in the water vapor signal (62) accompanied by the reaching of a high-level plateau in the molecular oxygen signal (61). Furthermore, the same point is identified in the differential optical absorption signal for liquid water, with a sharp increase by an order of magnitude in the ratio of the signal intensities at 980 nm and 760 nm.

Description

APPARATUS AND METHOD FOR DETERMINING THE MOISTURE CONTENT IN POROUS MEDIA USING OPTICAL SPECTROSCOPY
Field of the Invention
5 This invention pertains to the field of apparatuses and methods for determining the moisture content in porous media, and in particular by using optical spectroscopy.
Background of the Invention 0 Drying processes, i.e. the removal of moisture from materials, are of outmost importance in many industrial and every-day experience contexts. For instance, wood needs to be dried from its natural moisture before its use as a fuel or as a construction material, as for instance is described 5 in J. M. Dinwoodie, "Timber: Its nature and behaviour," (E & FN Spon, 2000) . An important aspect is that moisture in building materials induces mould, especially if the ventilation is insufficient, which is of major concern in the building sector, which is disclosed in B. Berglund, B. 0 Brunekreef, H. Knδppel, T. Lindvall, M. Maroni, L. Mølhave, and P. Skov, "Effects of indoor air pollution on human health," Indoor Air, Vol. 2, 2-25 (1992). Grain and cerials need drying for storage and for further processing in the food industry, as described in S. Joseph Cohen and T CS 5 Yang, "Progress in food dehydration," Trends in Food Science & Technology, Vol. 6, (1995). Paper processing includes important drying steps as is the case also in many further industrial processes.
Many natural materials are porous and hydrophilic. 0 The pores can be filled with air or other gases, but may also be partially or fully filled with water; a frequently undesired situation. Capillary action is an important passive process when porous materials take up water. Materials frequently swell, i.e. increase in volume, when wet. In 5 non-porous media the process of osmosis is in action. If the material has some rigid scaffolding structure, such as wood and other building materials, the swelling is minor. Then the pores originally filled with air instead become water-filled. By increasing the vapor pressure of water by heating in combination with securing an environment with reduced relative humidity, forced drying can be achieved. Commercially available moisture meters, such as handheld electric moisture meters are commonly used to measure the moisture content in wood. The measurement technique applied prior to this disclosure was developed in the late 1930' s and today there exists a number of manufactures that develop and sell handheld moisture meters for measuring, e.g., moisture in wood and concrete. Mainly, two different measurement principles exist; resistance and dielectric type moisture meters, which for instance are described in L. James, "Electric Moisture Meters for Wood," Gen, Tech. Rep. FPL-GTR-6. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, 1988; H. Forsen and V. Tarvainen, "Accuracy and functionality of hand held wood moisture content meters," SBUF, VTT Publications 420 (2000); or P. J. Wilson, "Accuracy of a capaci- tance-type and three resistance-type pin meters for measuring wood moisture content," Forest Products, Journal, (1999) .
The resistance type of moisture meter measures the electric resistance in wood between two pins that are con- nected at the wood surface or inserted into the wood, with typical distances of centimeters between the pins. If the wood is dry it acts as an isolator and the resistance is in the order of 105 MΩ. However, since water in wood contains ions, the resistance in wood decreases down to about 0.5 MΩ when the moisture content is close to the saturation point.
In order to measure the moisture content using the resistance principle, one has to know the type of wood and from what region it comes. It turns out that all species of wood have different resistance curves even for one type of wood that has grown at two different locations. Today, a number of resistance curves for many species of wood are stored in the device and controlled by a micro processor. Although the resistance type moisture meter is easy to produce and operate, precautions should be considered. At first this measurement method is intrusive. Secondly, the resistance curve for wood is affected by temperature. Thus, a temperature sensor is included in some resistive- type moisture meters. Other disadvantages are the limited measurement range and that the result depends on if the pins are inserted in parallel or perpendicular to the wood fibers. The upper limit is set by the fact that above the fiber saturation point (about 25-30% moisture content, MC) , the resistance measurement data are not reliable. The lower limit (about 7% MC) depends on the difficulties to measure resistance in the order of 105 MΩ and above.
The dielectric type moisture meter uses a non- intrusive measurement technique that measures the dielectric constant of the combined wood and water material. Since the dielectric constant is much higher for water com- pared to wood (a factor of 25) , the moisture content may be estimated. Anyway, each type of wood has its specific dielectric constant. Specific wood calibration data are stored in the device and controlled by a micro processor. The dielectric moisture meter is known to have poor per- formance compared to the resistance type. However, it is commonly used for relative measurements. The surface electrodes are sensitive for other materials close to the surface and density fluctuations of the wood. The measurement range varies from 5 to about 25% MC. US 4,785,185 discloses a dielectric material moisture detection apparatus that uses a sub-millimeter laser beam, which is repeatedly swept over a pre-programmed path onto a dielectric sheet having several detectors for measurements of water or filler content of sheets and bands of dielectric material. The laser radiation is scanned across the face of the material and the detectors, placed on the side of the material opposite the laser, measure the material transmittance . Based on this transmittance information the moisture or filler content of the dielectric material sheet is determined. However, this measurement technique requires that the laser radiation source has such a high output that transmittance through the sheet of dielectric material is provided. This is a drawback in many aspects, for instance from a energy consumption view and limited area of use to rather thin material sheets, such as plywood sheets or wood veneer slices, or materials having high transparency.
WO 01/65237 discloses wood fiber flow properties determination, involves measuring spectral values from fiber sample reflectance spectrum to determine properties based on specific relation between spectral and reference values. More precisely, a method for continuous determination of the properties of a wood fiber flow in fiberboard fabrication is disclosed. A sample of the fiber flow is illuminated with light, and one or more of the properties fiber length distribution, resin content, fiber moisture and optical properties for the sample, are determined from the measured reflectance spectrum, based on predetermined relations between measured spectral values and absolute magnitudes for the properties. However, the predetermined rela- tions between measured spectral values and reference values for resin content, fiber moisture and optical properties have to be determined for each type of fiber analyzed at an occasion. Specifically, the reference value for fiber moisture is disclosed as having to be determined by drying and weighing calibration samples of the fibers to be measured. Hence, the method disclosed in WO 01/65237 has a number of drawbacks, e.g. when the humidity contents in unknown porous materials has to be determined, or when it is not possible to acquire samples in order to perform the required reference measurement, e.g. on antique furniture. Atmospheric monitoring of oxygen and water is known by Gagliardi, G. et. al "Trace-gas analysis using diode lasers in the near-IR and long-path techniques", Optics and lasers in engineering, 8 March 2002, Vol. 37, issue 5, May 2002, p. 509-520. However, the measurement principle disclosed therein is not suited for porous scattering media. The measurement is only possible in free atmospheric gas. However, the measurement principle disclosed in the paper of Gagliardi et. al is not suited for porous material, where scattering is immense.
Several papers of the applicants of the present application disclose measurements of free gas in the free atmosphere, e.g. atmospheric spectroscopy. However, these methods are not suited for measurement of free gas in a po- rous or scattering material, where totally different conditions make use of the known methods impossible for providing measurement result concerning free gas molecules in the porous, scattering material.
Quantitative absorption spectroscopy is hampered by the strong scattering in inhomogeneous materials, making the Beer-Lambert law not directly applicable.
Hence, an improved apparatus and method for determining the moisture content in porous media would be advantageous and in particular allowing for increased flexibility, cost-effectiveness, and/or reliability would be advantageous .
Summary of the Invention
Accordingly, embodiments seek to mitigate, alleviate or eliminate one or more deficiencies, disadvantages or issues in the art, such as the above-identified, singly or in any combination by providing an apparatus, a method and a computer program according to the appended patent claims.
According to one aspect of the invention, an appara- tus is provided for monitoring a moisture content and/or a progress of a drying process of a material. The apparatus is devised to make measurements by optical spectroscopy, wherein the material is a natural or man-made porous, scattering, and partly translucent material. The apparatus is configured to measure free molecular oxygen and molecular water vapor in the material by using the optical spectros- copy for providing a measure of the moisture content in the material and/or said progress of said drying process of the material .
In an embodiment the optical spectroscopy may be differential absorption optical spectroscopy operating in transmission mode.
In an embodiment the optical spectroscopy may be differential absorption optical spectroscopy operating in backscattering mode.
In an embodiment the optical spectroscopy may be nar- row-band optical spectroscopy.
In an embodiment the optical spectroscopy may in use of the apparatus be performed in the near infrared region of 700 - 1400 nm.
In an embodiment the apparatus may be configured to monitor molecules of said free water vapor by narrow-band diode laser spectroscopy.
In an embodiment the narrow-band diode laser spectroscopy may be made in the 930 - 1000 nm region, when the apparatus is used. In an embodiment the monitoring of free molecular oxygen may be performed by narrow-band oxygen diode laser absorption spectroscopy, to achieve a signal increasing as the material dries and produces pores that become filled with oxygen-containing air. In an embodiment the narrow-band oxygen diode laser absorption spectroscopy may be made in the A-band around 760 nm.
In an embodiment the apparatus may further comprise a unit for formation of a ratio of the molecular water vapor and said molecular oxygen as normalized absorption signals from said narrow-band diode laser spectroscopy and said narrow-band oxygen diode laser absorption spectroscopy, providing a dimensionless signal useful for an instant measurement of the state of drying of the material.
In an embodiment the apparatus may perform an addi- tional optical absorption measurement of total transmitted light in the material sampling a broadband liquid water absorption, at an on-water-resonance wavelength, such as a liquid water absorption peak wavelength at about 980 nm, and an off-water-resonance wavelength reference measurement, such as at about 760 nm, whereby either individual signal levels of the two measurements at the on-water-resonance wavelength and the off-water-resonance wavelength are measured to follow the progress of the drying process of said material, or a ratio of the individual signal levels of the two measurements is formed providing a dimensionless signal useful for providing an instant measurement of the state of drying of said material.
In an embodiment the apparatus may further comprise a video camera sampling the broadband light transmitted from narrow band laser light sources of said optical spectroscopy.
In an embodiment additional broadband light-emitting diodes may be arranged as light sources for the on-water- resonance wave-length and the off-water-resonance wave- length for measuring the broadband liquid water absorption.
In an embodiment additional broadband thermal radiation source and optical filters may provide isolating of appropriate bands for performing a differential absorption measurement . In an embodiment a video camera may be provided for the recording of a light intensity emerging from the material at the on-water-resonance wavelength and the off- water-resonance wave-length, respectively, whereby the spatial extent of the recorded light provides additional in- formation on the drying process according to well-known principles for light absorption and scattering in turbid media .
In an embodiment the apparatus may comprise a unit for generation of a process intervention signal when the recorded ratio value of molecular water vapor and molecular oxygen normalized absorption signals from the narrow-band diode laser spectroscopy passes a pre-selected threshold value .
In an embodiment the apparatus may comprise a unit for generation of a process intervention signal when the recorded ratio value of the two measurements at the on- water-resonance wavelength and the off-water-resonance wavelength passes a pre-selected threshold value.
In an embodiment the apparatus may be provided in a compact, handheld configuration, adapted to be applied to the outside of the material for measuring the moisture contents in the material, or for monitoring said progress of said drying process of said material.
In an embodiment said natural or man-made porous, scattering, and partly translucent material may be a wood material; a construction material; an agricultural product; a food product, such as cereals; a cosmetic product; or a pharmaceutical preparation.
According to another aspect of the invention, a method for monitoring of moisture content and/or the progress of a drying process in a natural or man-made porous, scattering, and partly translucent material, is provided. The method comprises using an apparatus according the above mentioned aspect of the invention, providing diagnostic in- formation on the progression of natural or forced drying processes from using optical spectroscopy measurements, wherein free molecular oxygen and free molecular water vapor in said material are measured by said optical spectroscopy for providing a measure of said moisture content in said material and/or the progress of said drying process of said material. In an embodiment the method may comprise providing said diagnostic information by simultaneously measuring said free molecular oxygen and free molecular water vapor in the material. In an embodiment the method may be made by Gas in
Scattering Media Absorption Spectroscopy (GASMAS) technique applied in transmission or backscattering for the monitoring of said free molecular water vapor in said material, using said free molecular oxygen gas measurements as a ref- erence.
In an embodiment wavelengths that are used for the free water vapor and free oxygen gas monitoring may be approximately 980 and 760 nm, and also on and off-resonant for a broad liquid water absorption feature, and thus in- formation of the bulk water in said material is obtained additionally from the same apparatus.
In an embodiment during a drying process of the material the time when all the free water vapor has evaporated from the material may be identified by a strong fall-off in the water vapor signal accompanied by the reaching of a high-level plateau in the free molecular oxygen signal.
In an embodiment a ratio between the free water vapor signal and the free molecular oxygen signal being largely independent of scattering may show the obtainment of a fully dry sample by a sharp increase, which may also be identified in the differential optical absorption signal for liquid water, with a sharp increase of an order of a magnitude in the ratio of broadband signal intensities at approximately 980 nm and 760 nm. In an embodiment the photon history in the material for the two wavelengths may provide independent information on the scattering of the material and the liquid water contents in the material.
In an embodiment additional measurements of the pho- ton history may be performed using white light, capturing the full spectrum, or employing a number of pulsed diode lasers, selected at appropriate wavelengths.
According to a further aspect of the invention, a computer-readable medium having embodied thereon a computer program for monitoring of moisture content and/or the progress of a drying process in a natural or man-made porous, scattering, and partly translucent material, using an apparatus according to the above described aspect of the invention, is provided. The computer program is for processing by a computer, and comprises a code segment for providing diagnostic information on the progression of natural or forced drying processes from using optical spectroscopy measurements, wherein free molecular oxygen and free molecular water vapor in said material are measured by said optical spectroscopy for providing a measure of said moisture content in said material and/or the progress of said drying process of said material.
Some embodiments of the invention provide for non- intrusive measurement of humidity contents in a porous me- dium.
Some embodiments of the invention also provide for non-intrusive monitoring of an entire drying process.
It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
Brief Description of the Drawings
These and other aspects, features and advantages of which certain embodiments are capable of will be apparent and elucidated from the following description of embodiments, reference being made to the accompanying drawings, in which Fig. 1 are microscopic pictures of hardwood (balsa) and softwood (pine) ;
Fig. 2 illustrates the wood drying process, wherein at first the cells are filled with water but in the end no free water exists and the absorbed water in cell walls is dried out until an equilibrium state with the surrounding environment is reached;
Fig. 3A is a schematic drawing of the spectroscopic setup of similar diode laser spectrometers for oxygen (left) and water vapor (right) ;
Fig. 3B are graphs illustrating typical readings of WMS signals for oxygen and water vapor in wood, wherein the half widths of the signals are of the order of a few GHz in both cases; Fig. 4. is a graph illustrating measured moisture contents during the drying process of a sample, wherein the continuous line indicates the moisture contents measured by logging the weight of the sample, the dashed line when measured by using a resistance moisture meter (Protimeter Timbermaster) , and the dash-dotted line when measured by using a dielectric moisture meter (MC-300W, Exotek) . The fiber saturation point (FSP) is indicated in the figure (moisture content=30%) ;
Fig. 5A is a graph illustrating measured temperature at the surface of the sample and measured weight during the drying process;
Fig. 5B is a graph illustrating vapor pressure as a function of the temperature, see for instance C. Nordling and J. Osterman, "Physics handbook," 4th edition, (Stu- dentlitteratur 1987);
Fig. 6A is a graph illustrating the equivalent mean path length for water vapor and oxygen during the drying process of balsa;
Fig. 6B is a graph illustrating The ratio between de- tected equivalent mean path length for water vapor and oxygen; Fig. 6C is a graph illustrating direct signals for water vapor and oxygen during the drying process of balsa;
Fig. 6D is a graph illustrating the ratio between the detected direct signal for water vapor and oxygen; Fig. 7A is a graph illustrating the absorption spectrum of 1 cm pure water reproduced from the measurements by Matcher et al, see S. Matcher, M Cope, and D. Delpy, "Use of the water absorption spectrum to quantify tissue chromo- phore concentration changes in near infrared spectroscopy, " Phys. Med. Biol. Vol. 39, p.177-196 (1994), which is incorporated herein by reference in its entirety; and
Fig.7B are images of the sample on the detection side for oxygen (760 nm) and water-vapor (980 nm) during its drying process.
Description of embodiments
Specific embodiments will now be described with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The terminology used in the detailed description of the embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention. In the drawings, like numbers refer to like elements.
The following description describes amongst others an embodiment applicable to measure the humidity content in wood and in particular in balsa wood. However, it will be appreciated that the invention is not limited to this application but may be applied to many other fields where porous media are present, including for example industrial processing; handling of construction materials; preparation of agricultural products; handling of foodstuff, such as cereals; or handling of pharmaceutical products. The mate- rial in which moisture contents or humidity is measured or the monitoring of a drying process is performed upon is a natural or man-made porous, scattering, and partly translucent material, such as a wood material, such as described below; a construction material, such as plaster, cement or gypsum boards or plates; an agricultural product, such as straw; a food product, such as cereals; a cosmetic product; or a pharmaceutical preparation, e.g. in form of a tablet. The scattering environment provided by the material is a challenge for measuring free gas in the material by optical spectroscopy.
An embodiment focuses on spectroscopic monitoring of the drying of wood. Moreover, the Gas in Scattering Media Absorption Spectroscopy (GASMAS) technique is in the pre- sent context extended to water-vapor monitoring, which has not been disclosed before. The experimental arrangements used are presented further below. Measurements and results are also described. The results are then discussed and conclusions are presented.
Firstly, the structure of wood and the basics of wood drying processes are presented.
Wood is a porous material and has a complicated architecture, described in detailed, e.g., in the publication of J. M. Dinwoodie, cited above. In a tree the wood structure supports the treetop, stores nutritious substances, and transfers minerals and water, which have been absorbed by the root system. Thus, it needs to be strong, hydro- philic, and flexible since a tree grows and lives in a changing environment. Even though there exist hundreds of different types of wood materials, they can be divided into two groups, hardwood (deciduous) and softwood (coniferous) . Fig. 1 are microscopic pictures of hardwood (above in Fig. 1 at A, here balsa) and softwood (below in Fig. 1 at B, here pine), given for illustrative purposes. Hardwood is the most complicated of the two and comprise wood such as balsa, white oak, ash, mahogany etc. The density of hardwood may vary from about 0.1 to 1.2 g/cm3, depending on type of wood. Hardwood is characterized by a combination of complicated cell types orientated both vertically, tangen- tially, and radially. Softwood, on the other hand, is distinguished by a much simpler combination of cells that runs mostly vertically in a tree. Typical examples of wood that belong to the softwood group are spruce, pine, and larch. The weight of softwood may vary from 0.3 to 0.7 g/cm3. From a microscopic point of view wood consists of cellular structures that handle tasks carried out in a tree. Water and minerals are stored and transported vertically via cells, forming vessels, and horizontally via cell structures called rays. In total wood cells can be classi- fied in four different types; parenchyma (storage of nutritious) , tracheids (support and conduction) , fibers (support) , and vessel cells (conduction) .
In hardwood all four cell types are present, but tracheids are rarely common. Instead fibers and vessels are responsible for support and conduction of water and minerals. Fibers are usually 1-2 mm long and 10-20 μm wide and their only function is support. However, vessel cells are about the same length as the fibers, but up to 0.5 mm wide. Since the ends of the vessels are situated on top of each other they can make up a long tube. Thus, vessels act as an efficient water tube since the ends of the vessels are more or less dissolved.
In softwood only two cell types are present, namely parenchyma and tracheids. Tracheids are the most common cell type (about 90% of the softwood) and its main function is support and conduction of minerals and water. The size of the cell is about 2-4 mm long and about 30-50 μm wide. It is situated vertically in the tree and conducts water and minerals via small pits located on the cell surface. Parenchyma cells are small, 200x30 μm2 in size, taking care of the storage of nutritious. Wood drying processes
All types of wood consist of cellulose, hemicellu- loses, lignin, and extractives. The density of wood elements is about 1.5 g/cm3 but since wood is based on a cell structure, with different size for different type of wood, the density of wood may vary from 0.1 to 1.2 g/cm3 if the content of water is kept low. However, since the wood cell structure is hydrophilic and full of air, wood is heavily affected by water and moisture. The moisture content (MC) in wood is defined as the ratio between the weight of water in a piece of wood and the weight of the wood when no water is present. This means that the moisture content is higher than 100% in a living tree. During this state water is stored in cells and ves- sels, i.e. as free water, but also in cell walls, i.e. as bound water, which have expanded due to absorbed water. If a tree is cut into pieces the moisture content starts to decrease immediately, as is illustrated in Fig. 2. At first free water is moved to the wood surface by cap- illary forces where it is evaporated into the atmosphere. Due to the evaporation process the surface temperature is decreased and heat must be transferred from the environment in order to maintain the drying of the wood.
When all free water has been evaporated, the bound water starts to evaporate as well. This state of the wood is known as the fiber saturation point (FSP) and it corresponds to a moisture content of about 25-30% depending on the type of wood. Since bound water is situated inside the cell walls, more energy is needed to evaporate it. Thus, the drying rate decreases and the wood shrinks. In the end of the drying process, the wood reaches an equilibrium state with its environment. The moisture content inside the wood depends on the temperature and humidity level of the environment. A typical value of the moisture content for dried wood is 12-15%, see for instance L. Sandra, B.
Roderick, and M. L. Roderick, "Plant-water relations and the fibre saturation point," New Phytologist Vol. 168, p.25-37 (2005); P. Perre, "The Role of Wood Anatomy in the Drying of Wood: Great Oaks from Little Acorns Grow," 8th Int. IU- FRO Wood Drying Conference, 2003; M. Goyeneche, D. Lasseux, and D. Bruneau, "A Film-Flow Model to Describe Free Water Transport during Drying of a Hydroscopic Capillary Porous Medium," Transport in Porous Media Vol. 48, p.125-158 (2002) .
As mentioned above, the status of drying materials is often studied by handheld moisture meters that measure the water content by an indirect method relying on the fact that the electrical properties of the material are dependent on the water content. However, these instruments are not able to monitor the whole drying process and are in some cases intrusive.
By the recent developments in diode laser-based spectroscopy an interesting alternative approach has been iden- tified to monitor the water vapor as well as the liquid water by a substantially direct spectroscopic approach non- intrusively .
The present embodiment deals with measuring the drying degree of wood using high-resolution near-IR laser ab- sorption spectroscopy. Tuneable diode lasers operating close to 980 nm are used to monitor gaseous water. Liquid water also exhibits a broad absorption peak in the same wavelength region. For reference, atmospheric oxygen gas in the pores is also monitored in its A band close to 760 nm. This wavelength also provides a convenient off-resonance wavelength in the liquid water spectrum for assessment of liquid water contents.
Absorption spectroscopy is a method for measuring concentrations of substances utilizing the Beer-Lambert law and a suitable calibration procedure (See, e.g. D. A. Skoog and M. D. West, "Fundamentals of Analytical Chemistry," 7th edition, (Saunders, Philadelphia 1995). However, the application of this method is not straight-forward for natural materials such as wood, since the very fact that they are inhomogeneous and porous also means that they are highly scattering, making the optical pathlength through the sample undefined. This situation is common in medical optics, where scattering and absorption are intertwined (see G. Muller, B. Chance, R. Alfano, S. Arridge, J. Beuthan, E. Gratton, M. Kaschke, B. Masters, S. Svanberg, P. van der Zee (eds.): "Medical optical tomography, functional imaging and monitoring," SPIE Institute Series, Vol. 11, (SPIE BeI- lingham 1993)), which is incorporated herein by reference in its entirety. This is also the case in analytical spectroscopy of pharmaceutical preparations, as for instance described in C. Abrahamsson, J. Johansson, S. Andersson- Engels, S. Svanberg, and S. Folestad, "Time-Resolved NIR Spectroscopy for Quantitative Analysis of Intact Pharmaceutical Tablets," Anal. Chem. Vol. 77, p. 1055-1059 (2005), which is incorporated herein by reference in its entirety. Different techniques for handling multiple scattering of light have been developed for assessing the concentration of liquid and solid absorbing constituents. Spatially separated measurements (see for instance T. J. Farrell, M. S. Pattersson, and B. Wilson, "A diffusion theory model of spatially resolved, steady-state diffuse reflectance for noninvasive determination of tissue optical properties in vivo," Med. Phys . Vol. 19, p. 879-888 (1992), which is incorporated herein by reference in its entirety) and time- resolved techniques (see for instance M. S. Pattersson, B. Chance, and B. C. Wilson, "Time-resolved reflectance and transmittance for the noninvasive measurement of tissue optical properties," Appl . Opt. Vol. 28, p. 2331-2336 (1989), which is incorporated herein by reference in its entirety) constitute two main approaches for enabling concentration determinations. A recent example on the determination of tumor sensitizer concentration in vivo is given in C. af Klinteberg, A. Pifferi, S. Andersson-Engels, R. Cubeddu, and S. Svanberg, "In vivo Absorption Spectroscopy of Tumor Sensitizers using Femtosecond White Light," Appl . Opt. Vol. 44, p. 2213-2220 (2005), which is incorporated herein by reference in its entirety.
Such techniques have been extended to free gases present in porous media through the introduction of the GASMAS (Gas in Scattering Media Absorption Spectroscopy) technique, which is described in M. Sjδholm, G. Somesfalean, J. Alnis, S. Andersson-Engels, and S. Svanberg, "Analysis of gas dispersed in scattering media," Opt. Lett. Vol. 26, p. 16-18 (2001), which is incorporated herein by reference in its entirety. Normally a modest resolution is used in solid state spectroscopy because of the broad absorbing struc- tures. However, if single-mode diode laser spectroscopy using sensitive modulation techniques is employed, narrower structures due to free gas appear. These structures are the typically in the range of 10,000 times narrower than for bound gas. That means that the wavelength window specific for a free gas is approximately 1/10000 of that of bound molecules. So far the GASMAS technique was applied to the monitoring of free molecular oxygen in materials as diverse as wood, polystyrene foam and fruits, as for instance described in J. Alnis, B. Anderson, M. Sjδholm, G. Somes- falean, and S. Svanberg, "Laser spectroscopy on free molecular oxygen dispersed in wood materials," Appl. Phys . B Vol. 77, p. 691-695 (2003); L. Persson, H. Gao, M. Sjδholm, and S. Svanberg, "Diode laser absorption spectroscopy for studies of gas exchange in fruits," Opt. Las. Eng., In press (2006); L. Persson, B. Anderson, M. Andersson, M. Sjόholm, and S. Svanberg, "Studies of Gas Exchange in Fruits using Laser Spectroscopic Techniques," Proc. Fruitic05, Montpieller, September 2005; which all are incorporated herein by reference in their entirety. In particular, gas transport through the media was studied by first subjecting the material to a pure nitrogen atmosphere and then observing the time constant for the re- invasion of normal ambient (oxygen-containing) air. Concentrations could be determined by combining GASMAS measurements with time-resolved measurement revealing the time history of the photons inside the sample, see for instance G. Somesfalean, M. Sjδholm, J. Alnis, C. af Klinteberg, S. Andersson-Engels, and S. Svanberg, "Concentration measurement of gas imbedded in scattering media employing time and spatially resolved techniques," Appl . Opt. Vol. 41, p.3538- 3544 (2002), which is incorporated herein by reference in its entirety. The GASMAS technique was also very recently applied to the monitoring of human sinus cavities, see L. Persson, K. Svanberg, and S. Svanberg, "On the potential of human sinus cavity diagnostics using diode laser gas spec- troscopy," Appl. Phys . B Vol. 82, p.313-317 (2006), which is incorporated herein by reference in its entirety. Setup for diode laser spectroscopy on wood The spectroscopic setup for moisture studies will now be described. It is based on two almost identical arrange- ments that are run simultaneously. One setup is used for oxygen measurement while the second is used to measure water vapor (moisture) contents in the same piece of wood. In Fig. 3A a schematic drawing of the spectroscopic setup of similar diode laser spectrometers for oxygen (left) and wa- ter vapor (right) is shown. According to an embodiment the apparatus may be provided in a compact, handheld configuration, adapted to be applied to the outside of a porous medium for measuring the humidity contents in the porous medium, such as wood. The oxygen measurement setup is explained in more detail in the publication "Diode laser absorption spectroscopy for studies of gas exchange in fruits" of L. Persson et al, cited above. However, in the present setup the photo multiplier tube (PMT) sensor is replaced by a silicon de- tector (Photovoltaic PIN-IODP from UDT) with an active surface of approximately 100 mm . The major difference between the oxygen and the water-vapor setup is the wavelength of the diode lasers used. For the oxygen detection a diode laser 1, such as a Sharp LT031MDO with an output power of 7 mW, is used in an em- bodiment to scan across the R7R7 oxygen line at 761.003 nm (vacuum wavelength) . Correspondingly, a diode laser, such as a near-IR Fabry-Perot diode laser, such as Specdilas F760 with a nominal output of 200 mW at 980 nm, is used in an embodiment as a spectroscopic light source to detect the water-vapor line (vibration; (000)→ (121), rotation; J"=5→ J' =4, Ka"=0→ Ka' =0, Kc"=5→ Kc' =4) at 978.509 nm (vacuum wavelength) .
The measurement is based on that the wavelength of the laser light is scanned across the absorption lines by sweeping the operating current of the lasers by the use of a 4 Hz saw tooth ramp. To achieve wavelength modulation spectroscopy (WMS) with lock-in detection, the operating current is also modulated by a sinusoidal wave at 9 kHz. The laser light is focused into a fiber guiding the light to the sample.
In more detail, an embodiment of an apparatus 110 is described with reference to Fig. 3A. A schematic diagram of the gas detection setup is shown. A near-IR single mode diode laser 1, namely a Sharp LT031MDO with a nominal output power of 7 mW, was used as the spectroscopic light source. By applying a ramp at a repetition rate of 4 Hz to the driving current, the diode laser 1 was temperature tuned across the R7R7 molecular oxygen line, situated at 761.003 nm (vacuum wavelength) . As can be seen on the left of Fig. 3A, a 9 kHz sine-wave was superimposed onto the current ramp to produce a wavelength modulation of the light, thereby providing sensitive wavelength modulation spectroscopy (WMS) . An optical fiber 2, e.g. with a 600 μm core diameter, is used to guide the light to the sample 5. For backscattering measurements (not illustrated) , a small right-angle prism may in an embodiment of the appara- tus be positioned in front of the distal end of the fiber 2 and centrally located of the detector 4, and may be used to provide total internal reflection in order to launch the light into the sample 5. Also, an aperture, e.g. an annular aperture with an inner and an outer aperture diameter of 10 and 21 mm, respectively, may be used to collect the back- scattered photons from the sample 5.
For a transmission geometry, as shown in Fig. 3A, the fiber was positioned above the porous medium sample 5 and transmitted to detector 4, as shown in Fig. 3A.
The absorption signal is detected by splitting up the signal from the photodetector, such as the silicon detector (Photovoltaic PIN-IODP from UDT), into two parts. One part, referred to as the direct signal, is directly sent to a computer controlled digital oscilloscope 8. The other part, referred to as the WMS signal, is sent to a lock-in amplifier 9, such as an EG&G Princeton Applied Research 5209, providing phase-sensitive detection at twice the modulation frequency, before being sent to another channel on the os- cilloscope 8, as illustrated in Fig. 3A. Wavelength modulation spectroscopy with lock-in detection is often referred to as derivative spectroscopy, since the signal looks like the derivative of the absorption profile. In this case, when detection at twice the modulation frequency is done, the lock-in signal looks like the second derivative of the absorption profile.
The amplitude of the WMS signal is determined by the absolute size of the narrow gas absorption feature, i.e., the fractional absorption due to the gas and the amount of light reaching the detector. By measuring the peak-to-peak value of the absorption signature in the WMS signal and normalize it with respect to the amount of light reaching the detector (the direct signal) , the absorption due to the gas of interest, which may for instance be water vapor and oxygen, is estimated. For small absorptions, the WMS signal is proportional to the absorbance and thus to the product of the gas concentration and the path length traveled by the light.
The water vapor setup uses the same type of silicon detector as the one used for oxygen. The output from each detector is split into two signal branches, as mentioned above. One branch is directly connected to oscilloscope 8, referred to as the direct signal, while the second part goes via the lock-in amplifier before being connected to another channel of the oscilloscope, and is referred to as the WMS signal.
In an embodiment a broad-band thermal radiation source and optical filters may provide isolating of the appropriate bands for performing the differential absorption measurement . The data are stored and analyzed using a computer. Typical readings for oxygen and water vapor are shown in Fig. 3B, where graphs illustrate WMS signals for oxygen and water vapor in wood, wherein the half widths of the signals are of the order of a few GHz in both cases. The peak-to-peak value of the WMS signal is measured and normalized by dividing with the direct signal value at the corresponding wavelength in order to determine the fractional absorption during the drying process.
According to embodiments, a measurement device may be configured to perform said optical absorption spectroscopy in scattering media in transmission, as explained with reference to Fig. 3A, or in backscattering according to another embodiment. The light emitter and light receiver may be arranged in a transmission geometry or in a backscat- tering geometry. By using a backscattering geometry, the humidity and oxygen contents are determined in a layer extending downward into the porous medium from the surface thereof. Hence, a drying process of porous media samples having a larger volume may be determined as in a transmis- sion geometry. The analysis of a surface region allows de- termination of the drying process of the whole material, for instance by correlation to previous measurements.
In the present embodiment, a method called standard addition is used in order to calibrate a measured normal- ized WMS signal and transfer it to a more meaningful quantity; a so-called equivalent mean path, Leq. It is determined by adding free air path lengths in the collimated laser beam before it enters the wood. The equivalent mean path length corresponds to the distance that the light has to travel in ambient air in order to obtain the same signal as in the sample (Leq • cair = [Lsampie] • csampie, where cair corresponds to the gas concentration in air, [Lsampie] to the mean path length in the sample, and csampie to the gas concentration in the sample) . Precautions were taken when the water-vapor WMS signal was calibrated. The room temperature and relative humidity was measured by a hygrometer (Testo 608-H1) sensor and the partial pressure for the water vapor was estimated.
For reference and verification purposes, an analog lab scales (Libror EB-280, Shimadzu) is used to measure the weight of the wood in order to calculate the average moisture content during the drying process. In the end of the drying process a commercial convection oven was used to dry the wood at 1100C, in order to determine the truly dry weight.
Measurements and results
Uniform balsa wood pieces of 10 cm width, 30 cm length and 0.8 cm thickness derived from the same batch were used in the measurements. The figures of balsa wood shown in Fig. 1 are from the wood samples studied. The wood was kept under water for typically three days and was studied directly after being exposed to the ambient laboratory air. The drying could be followed by reading off the analog scales, onto which the piece of wood was attached. The results of conventional measurements of the moisture contents during the drying process of a sample are shown in Fig. 4, wherein the continuous line indicates the moisture contents measured by logging the weight of the sample, the dashed line when measured by using a resistance moisture meter (Protimeter Timbermaster) , and the dash- dotted line when measured by using a dielectric moisture meter (MC-300W, Exotek) . The fiber saturation point (FSP) is indicated in the figure (moisture content=30%) . The instrument is applied in a straight forward manner. The weight reading from the scale, expressed as moisture con- tent, is plotted together with data from the resistance and the dielectric moisture meters. It can clearly be seen, that the results from the meters poorly describe the real drying process monitored by the scale for moisture content above the FSP. Obviously there is a need for improved meas- urement techniques for moisture.
The temperature of the wood surface was also monitored with thermocouples. The measurements were preformed on a different sample. In this case the sample had been soaked in water for a longer time than the case when re- cording the data in Fig. 4. Typical results are shown in
Fig. 5A, where decreasing curve 51 shows a weight loss to a stable value of about 33 percent of the original wet weight, the dry material plateau being reached after about 1200 minutes. The drying process is, as expected, accompa- nied by a lowering of the temperature. During the first 600 minutes the temperature, as illustrated with curve 52 in Fig. 5A, was stable at 9°C below the ambient value (25°C), which was gradually reached after 1200 minutes.
Since gaseous water (water vapor) is measured in our study it is interesting to consider the vapor pressure of water as a function of the temperature. The corresponding curve 53 is given in Fig. 5B. It shows the partial pressure of the vapor in a closed volume containing liquid water at the given temperature. We note, that the vapor pressure is increasing by a factor 1.8 when the temperature rises from 16°C to 25°C. We also note that water vapor at ambient tern- perature only accounts for few percent of the mass in normal air. Frequently ambient air does not feature the full water vapor pressure corresponding to the ambient temperature. If there is no air movement and large amounts of dis- tributed liquid water the relative humidity would be 100 percent, like in a sealed off volume. Due to the exchange of dryer air, the effective humidity becomes less. Readings of 20-40 percent relative humidity (percentage of the fully saturated value at the given temperature) are common in in- door environments at wintertime in Sweden.
Now data from our near-IR laser spectroscopic measurements is presented. Simultaneous measurements of (molecular) water vapor and molecular oxygen measurements were performed at points separated by about 10 cm on the piece of wood, assuming the material to be laterally uniform.
This data were recorded simultaneously as the temperature and weight data in Fig. 5A. A small air space was left between the sample and the detectors to allow water to ho- mogenously diffuse out of the sample. The setup is shown in Fig. 3. Oxygen does not have any liquid phase in the temperature range studied (it liquifies at 90K) . In contrast, water has a liquid as well as a gaseous phase, and the vapor pressure is, as just discussed, temperature dependent. In understanding the dynamics of wood drying it is useful to first consider the behavior of the molecular oxygen Leq during the drying, which as mentioned before is proportional to the normalized WMS signal. In Fig. 6A the fractional oxygen absorption is plotted as curve 61. The signal is proportional to the oxygen concentration times the effective distance traveled through the gas-filled pores, the latter factor being strongly sensitive to the degree of light scattering in the material. The signal is plotted as an equivalent path length in normal air containing 21 percent oxygen, as previous discussed. It should be noted that the effective path length through the material, useful in a standard Beer-Lambert law view, can be expected to change during the drying process. One aspect is that while liquid water is an index-matching fluid reducing the scattering when the pores are water filled, the scattering should increase when the air-filled inhomogeneities develop during the drying process. We notice, that the oxygen signal increases from a non-zero value (there are air-filled pores even in the very wet wood) by a factor of about 8 to then stay constant at the high value, reached at about 1300 minutes when all the liquid water is driven out and all the pores are instead gas-filled.
The simultaneously measured gaseous water signal is included in Fig. 6A as curve 62. Again, the curve 62 starts at a non-zero level, corresponding to the presence of gas- filled pores with saturated water vapor also in the very wet wood. The signal increases to a maximum at about 1200 minutes, reached slightly before the oxygen signal maximum. At the same time as the oxygen signal stays constant the water vapor signal gradually falls off to a steady value of about 25 percent of the maximum value. This could be inter- preted in the following way: Around 1200 minutes the pores are almost void of water, as indicated by the dashed line in Fig. 5A, but there is still a sufficient amount of water to sustain the full saturated water vapor partial pressure. Shortly thereafter this is no longer the case and the pores with constant volumes and situated in a material with constant scattering properties gradually looses the saturated vapor pressure and gradually attains the relative humidity value of the laboratory, which for the measurement case with an ambient temperature of 25°C was separately measured to 24 percent with a hygrometer. This is in good agreement to the measured signal fall off to about 25 percent of the value with full saturation. The displacement of about 100 minutes between the reaching of the maximum for oxygen and water vapor is compatible with the typical times for air diffusion in balsa wood measured separately. While the above observations are readily interpreted, the detailed behavior of the water vapor signal during the drying is due to many different influences. Let us first state, that if two permanent gases, such as nitrogen and oxygen had been studied, the same type of behavior (apart from small effects of different diffusivity) , i.e. a constant ratio between the signals during the drying phase, would have been expected. This statement has as a prerequisite that the two gases absorb at close lying wavelengths so that the scattering properties are similar. In contrast to this, the ratio between the water vapor signal and the oxygen signal is strongly varying during the drying process as can be seen in Fig. 6B at curve 65. The temperature increase in the time span 400- 1200 minutes (see Fig. 5, curve 52) leads to an acceleration in the water-vapor signal level beyond that for oxygen, for which the concentration is marginally reduced (a few percent) by the temperature increase. This may explain the tendency of increase of the ratio curve towards the end of the drying period. The high initial value in the ratio curve may be related to the time constant for oxygen diffusion through the material also discussed with regard to the different times for the two gases to reach the maximum WMS signal. It should be noted that in the drying up of a pore, the walls of the cavity will be uniformly covered by liquid water due to the surface tension. This may impair the transport of oxygen through the film while the water vapor can freely build up in the central micro-bubble of the pore, see Fig. 2. Effects of this kind may also influence the balance between the two gases in the cavities during the drying process. As noted, when everything is dry, conditions are static with regard to scattering and the behavior of both curves is clearly understood as discussed above.
So far the fractional absorptive imprint of the gas (intensity on the absorption line compared to the intensity off the line) , which is decisive for concentration measure- ments based on the Beer-Lambert law, has been discussed. The dynamics of the total light (off the narrow absorptive features) at the two wavelengths used in the embodiment, i.e. 980 and 760 nm, respectively, is also considered. Such measurements were performed and the results are shown in Fig. 6C-6D and in Figs. 7A and 7B. The curves 63, 64 in Fig. 6C show a very different behavior with curve 64 corresponding to the water signal increasing in the final phase of drying from an almost constant initial level to a final value about 2.5 times higher. On the contrary, the 760 nm signal starts at a high level and acceleratingly reaches a steady value about 5 times lower when the wood is dry. The ratio of detected direct signal for water and oxygen is shown in Fig. 6D at curve 66. An important aspect in ex- plaining these phenomena is to note, that liquid water has a quite strong broadband absorption around 980 nm as detailed in Fig. 7A and already noted above. We note that this absorption influences the on- and off-resonance frequencies for the narrow-band water-vapor signals alike, and does not influence the fractional absorption determining the water-vapor concentration. However, the laser beam is clearly attenuated by the water contents. The initial flat response of the 980 nm curve for the drying wood can be interpreted as that the reduction of water during the initial drying is compensated by the increased light path length due to the developing gas-filled pores. Finally, the absorbing liquid water disappears and the detected light settles on a level determined by the attenuation of the light- scattering dry wood. The 760 nm light is off-resonance from the broad liquid water absorption as shown in Fig. 7A, and bulk absorption due to the water is small, in strong contrast to the case for 980 nm. The high initial light level can be seen as an effect of the index-matching effect of the water in the pores, reducing the lateral scattering and allowing more light to reach the detector. As the wood dries the scattering increases and less and less light reaches the detector. Steady-state conditions then prevail in the dry wood. The strong change in the light levels as well as the normalized WMS signals for both wavelengths when the wood is almost dry, but not quite, can be interpreted as the final dry-up of the finest compartments long after the tube structure containing most of the water has dried up. The scattering increases a lot due to the appearance of small pores of size comparable with the wavelength. This reduces the detected 760 nm light level. Also the 980 nm light level changes due to the continued loss of bulk water absorption .
The dynamics of the transmitted light levels at the two wavelengths could also be studied using near-IR imaging of the transmitted light using standard web-cameras (Q-TEC 100) . The transmitted light blobs at 980 and 760 nm are shown in Fig. 7B for fully wet and fully dry (waiting 20 h = 1200 minutes) conditions. The intensity in the centre of the blobs (where the detector is situated) is related to the intensities given in Fig. 6C; the 760 nm signal is strongly reduced, the 980 nm is increased. From recordings, for instance made by a video camera, such as the ones shown in Fig. 7B, information of the spatial distribution may also be extracted and compared with theory. Removing moisture from materials is of major importance in fields as diverse as industrial processing, handling of construction materials, and preparation of agricultural products, food products, cosmetic or pharmaceutical preparations. The origin of excess water is frequently connected to material porosity. Optical measurement techniques are attractive since they are non-intrusive and frequently deliver data in real time. However, quantitative absorption spectroscopy is hampered by the strong scattering in inhomogeneous materials, making the Beer-Lambert law not directly applicable. The application of the GASMAS technique applied in transmission for the monitoring of wa- ter vapor in wood has been demonstrated, using corresponding molecular oxygen measurements as a reference. The wavelengths used for the free gas monitoring, 980 and 760 nm, are also on and off-resonant for a broad liquid water ab- sorption feature, and thus information of the bulk water may also be obtained. The signal intensities observed are mostly to be interpreted as due to the interplay between specific absorption and scattering, both changing during the drying process. In particular, the time when all the free water has evaporated from the wood can be readily identified by a strong fall-off in the water vapor signal accompanied by the reaching of a high-level plateau in the molecular oxygen signal. The ratio between these signals, being dimensionless and largely independent of scattering, shows in particular the obtaining of a fully dry sample.
This situation is also identified in the differential optical absorption signal for liquid water, with a sharp increase of an order of a magnitude in the ratio of the broadband signal intensities at 980 nm and 760 nm. According to an embodiment the measurement apparatus may be provided as a compact, handheld configuration, adapted to be applied to the outside of the material for measuring the moisture contents in the material, or for monitoring said progress of said drying process of said ma- terial.
The method may be performed in software, for instance stored on a computer-readable medium. The software may be implemented in the apparatus according to embodiments. The computer program may comprise a code segment for providing diagnostic information on the progression of natural or forced drying processes from using optical spectroscopy measurements, wherein free molecular oxygen and free molecular water vapor in a material are measured by said optical spectroscopy for providing a measure of a moisture content in the material and/or the progress of a drying process of the material. It is thus clear, that optical spectroscopy, is a valuable tool for the practical monitoring of drying processes, at the same time as it can yield additional information on specifics of drying. For the practical application of the method, measurements in back-scattering geometry are of great interest. In order to further elucidate the interplay between scattering and absorption, time-resolved measurements of the photon history in the wood for the two wavelength provides independent information on the scatter- ing of the material and the liquid water contents. Such measurements may be performed using white light, capturing the full spectrum, or employing a number of pulsed diode lasers, selected at appropriate wavelengths, see T. Svens- son, Johannes Swartling, Paola Taroni, Alessandro Tor- ricelli, Pia Lindblom, Christian Ingvar, and Stefan Anders- son-Engels, "Characterization of normal breast tissue heterogeneity using time-resolved near-infrared spectroscopy, " Phys. Med. Biol. Vol. 50, p.2559-2571 (2005), which is incorporated herein by reference in its entirety. In summary, the use of wavelength-modulation diode laser spectroscopy, water vapor and oxygen are detected in scattering media non-intrusively, at 980 and 760 nm, respectively. The technique demonstrated is based on the fact that free gases have extremely sharp absorption structures compared to the broad features from the bulk-material. Water vapor and oxygen measurements during the drying process of wood have been performed. Complementary information about the drying process of a porous material, such as wood, are obtained, compared to commercially available moisture meters. In particular, the time when all the free water has evaporated from the wood is readily identified by a strong fall-off in the water vapor signal accompanied by the reaching of a high-level plateau in the molecular oxygen signal. Furthermore, the same point is identified in the differential optical absorption signal for liquid wa- ter, with a sharp increase by an order of magnitude in the ratio of the signal intensities at 980 nm and 760 nm.
As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms "includes," "comprises," "including" and/or "comprising, " when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not pre- elude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It will be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements may be present. Furthermore, "connected" or "coupled" as used herein may include wirelessly connected or coupled. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
As will be appreciated by one of skill in the art, the present invention may be embodied as device, system, method or computer program product. Accordingly, the present invention may take the form of an entirely hardware embodiment, a software embodiment or an embodiment combining software and hardware aspects all generally referred to herein as a "circuit" or "module." Furthermore, the present invention may take the form of a computer program product on a computer-usable storage medium having computer-usable program code embodied in the medium. Any suitable computer readable medium may be utilized including hard disks, CD-ROMs, optical storage devices, a transmis- sion media such as those supporting the Internet or an intranet, or magnetic storage devices.
The present invention has been described above with reference to specific embodiments. However, other embodiments than the above described are equally possible within the scope of the invention. Different method steps than those described above, performing the method by hardware or software, may be provided within the scope of the invention. The different features and steps of the invention may be combined in other combinations than those described. The scope of the invention is only limited by the appended patent claims.

Claims

1. An apparatus for monitoring a moisture content and/or a progress of a drying process of a material, wherein said apparatus is devised to make measurements by optical spectroscopy, said material is a natural or man-made porous, scattering, and partly translucent material, and said apparatus is configured to measure free molecular oxygen and molecular water vapor in said material by said optical spectroscopy for providing a measure of said moisture content in said material and/or said progress of said drying process of said material.
2. The apparatus according to claim 1, wherein said optical spectroscopy is differential absorption optical spectroscopy operating in transmission mode.
3. The apparatus according to claim 1, wherein said optical spectroscopy is differential absorption optical spectroscopy operating in backscattering mode.
4. The apparatus according to any of claims 1 to 3, wherein said optical spectroscopy is narrow-band optical spectroscopy.
5. The apparatus according to any of claims 1 to 4, wherein said optical spectroscopy in use of said apparatus is performed in the near infrared region of 700 - 1400 nm.
6. The apparatus according to any of claims 1 to 5, wherein said apparatus is configured to monitor molecules of said free water vapor by narrow-band diode laser spec- troscopy.
7. The apparatus according to claim 6, wherein said narrow-band diode laser spectroscopy is made in the 930 - 1000 nm region, when said apparatus is used.
8. The apparatus according to any preceding claim, wherein monitoring of said free molecular oxygen is performed by narrow-band oxygen diode laser absorption spectroscopy, to achieve a signal increasing as the material dries and produces pores that become filled with oxygen- containing air.
9. The apparatus according to claim 8, wherein said narrow-band oxygen diode laser absorption spectroscopy is made in the A-band around 760 nm.
10. The apparatus according to claims 6 and 8, further comprising a unit for formation of a ratio of said molecular water vapor and said molecular oxygen as normalized absorption signals from said narrow-band diode laser spec- troscopy and said narrow-band oxygen diode laser absorption spectroscopy, providing a dimensionless signal useful for an instant measurement of the state of drying of said material .
11. The apparatus according to any of claims 1 to 10, further configured to provide an additional optical absorption measurement of total transmitted light in the material sampling a broadband liquid water absorption, at an on-water-resonance wavelength, such as a liquid water absorption peak wavelength at about 980 nm, and an off-water-resonance wavelength reference measurement, such as at about 760 nm, whereby either individual signal levels of the two measurements at the on-water-resonance wavelength and the off-water-resonance wavelength are measured to follow the progress of the drying process of said material, or a ratio of the individual signal levels of the two measurements is formed providing a dimensionless signal useful for providing an instant measurement of the state of drying of said material.
12. The apparatus according to claim 11, further comprising a video camera sampling the broadband light transmitted from narrow band laser light sources of said optical spectroscopy.
13. The apparatus according to claim 11 or 12, wherein additional broadband light-emitting diodes are arranged as light sources for the on-water-resonance wave- length and the off-water-resonance wavelength for measuring the broadband liquid water absorption.
14. The apparatus according to claim 11 or 12, wherein an additional broadband thermal radiation source and optical filters provide isolating of appropriate bands for performing a differential absorption measurement.
15. An apparatus according to Claim 13 or 14, wherein a video camera is provided for the recording of a light in- tensity emerging from the material at the on-water- resonance wavelength and the off-water-resonance wavelength, respectively, whereby the spatial extent of the recorded light provides additional information on the drying process according to well-known principles for light ab- sorption and scattering in turbid media.
16. The apparatus according to claim 10, comprising a unit for generation of a process intervention signal when the recorded ratio value of molecular water vapor and mo- lecular oxygen normalized absorption signals from the nar- row-band diode laser spectroscopy passes a pre-selected threshold value.
17. The apparatus according to claim 11, comprising a unit for generation of a process intervention signal when the recorded ratio value of the two measurements at the on- water-resonance wavelength and the off-water-resonance wavelength passes a pre-selected threshold value.
18. The apparatus according to any preceding claim, provided in a compact, handheld configuration, adapted to be applied to the outside of the material for measuring the moisture contents in the material, or for monitoring said progress of said drying process of said material.
19. The apparatus according to any preceding claim, wherein said natural or man-made porous, scattering, and partly translucent material is a wood material; a construction material; an agricultural product; a food product, such as cereals; a cosmetic product; or a pharmaceutical preparation .
20. A method for monitoring of moisture content and/or the progress of a drying process in a natural or man-made porous, scattering, and partly translucent material, comprising using an apparatus according to any of the preceding claims, providing diagnostic information on the progression of natural or forced drying processes from using optical spectroscopy measurements, wherein free molecular oxygen and free molecular water vapor in said material are measured by said optical spectroscopy for providing a measure of said moisture content in said material and/or the progress of said drying process of said material .
21. The method according to claim 20, comprising providing said diagnostic information by simultaneously measuring said free molecular oxygen and free molecular water vapor in the material.
22. The method according to claim 21, wherein said measurements are made by Gas in Scattering Media Absorption Spectroscopy (GASMAS) technique applied in transmission or backscattering for the monitoring of said free molecular water vapor in said material, using said free molecular oxygen gas measurements as a reference.
23. The method according to claim 22, wherein wavelengths that are used for the free water vapor and free oxygen gas monitoring are approximately 980 and 760 nm, are also on and off-resonant for a broad liquid water absorption feature, and thus information of the bulk water in said material is obtained additionally from the same apparatus .
24. The method according to claim 23, wherein during a drying process of the material the time when all the free water vapor has evaporated from the material is identified by a strong fall-off in the water vapor signal accompanied by the reaching of a high-level plateau in the free molecular oxygen signal.
25. The method according to claim 22 or 23, wherein a ratio between the free water vapor signal and the free mo- lecular oxygen signal being largely independent of scattering shows the obtainment of a fully dry sample by a sharp increase, which is also identified in the differential optical absorption signal for liquid water, with a sharp increase of an order of a magnitude in the ratio of broadband signal intensities at approximately 980 nm and 760 nm.
26. The method according to any of claims 23 to 25, wherein the photon history in the material for the two wavelengths provides independent information on the scattering of the material and the liquid water contents in the material.
27. The method according to claim 26, wherein additional measurements of the photon history are performed using white light, capturing the full spectrum, or employing a number of pulsed diode lasers, selected at appropriate wavelengths .
28. The method according to any of claims 20 to 27, wherein said scattering, porous, and partly translucent ma- terial is a wood material; a construction material; an agricultural product; a food product, such as cereals; a cosmetic product; or a pharmaceutical preparation.
29. A computer-readable medium having embodied thereon a computer program for monitoring of moisture content and/or the progress of a drying process in a natural or man-made porous, scattering, and partly translucent material, using an apparatus according to any of the claims 1-14, said computer program for processing by a computer, the computer program comprising a code segment for providing diagnostic information on the progression of natural or forced drying processes from using optical spectroscopy measurements, wherein free molecular oxygen and free molecular water vapor in said ma- terial are measured by said optical spectroscopy for providing a measure of said moisture content in said material and/or the progress of said drying process of said material .
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