WO2010046692A2

WO2010046692A2 - Non- destructive method for determining the moisture content in a hygroscopic material

Info

Publication number: WO2010046692A2
Application number: PCT/GB2009/051407
Authority: WO
Inventors: Daniel James Ridley-Ellis; John Robert Moore; Andrew Jamie Lyon
Original assignee: The Court Of Edinburgh Napier University
Priority date: 2008-10-22
Filing date: 2009-10-20
Publication date: 2010-04-29
Also published as: GB0819314D0; WO2010046692A3

Abstract

Method and apparatus for the determination of moisture content in a hygroscopic material. Measurements of parameters such as the modulus of elasticity and the time of flight, are recorded on the material using two methods of measurement. The two methods are each affected differently by moisture content. The moisture content is derivable by taking a ratio of the measurements with a factor to account for the species or type of hygroscopic material. A non-destructive method for determining the absolute moisture content in wood,which provides measurements above the fibre saturation point, is described.

Description

NON- DESTRUCTIVE METHOD FOR DETERMINING THE MOISTURE CONTENT IN A HYGROSCOPIC MATERIAL

The present invention relates to the determination of moisture content in a hygroscopic material and in particular, though not exclusively, to a non- destructive method for determining the absolute moisture content in wood.

Wood, also known as timber or lumber, is a basic commodity used in many building applications, the manufacture of domestic products and biomass for heat and energy. Wood is a hygroscopic material and its properties are affected by its moisture content (the ratio of the mass of water to the mass of dry wood). In the living tree the moisture content is very high with water in both the cell voids (lumens) and cell walls, but the moisture content can vary within the tree, from tree to tree and throughout the seasons (roughly ranging in value from 50 to 300 percent). For almost all uses of wood, this water is a problem and a portion of it must be removed through air (ambient temperature) or kiln (elevated temperature) drying.

The water in the cell voids evaporates first and once it has gone the mechanical properties and dimensions of the wood begin to change. This moisture content is known as the fibre saturation point (FSP), and is approximately 30-40 percent. In a typical indoor situation, wood will reach an equilibrium moisture content that is lower than the fibre saturation point (approximately 12 percent).

Knowledge of the moisture content is critical to a number of industrial processes in the wood chain. Green timber is often sold by weight, but much of this is water that must be removed at cost (especially so for softwoods). Thus, it is advantageous to know the moisture content (and hence dry mass) at point of sale. For wood used in construction, sawn planks are kiln dried with an aim to dry the plank to the equilibrium value, this being the moisture content it should reach in service. However, as all the planks in a kiln are unlikely to have the same moisture content, some planks may be over dried while others are under dried. A plank which is 'wet' following kiln drying will be treated as waste as it risks shrinkage when the wood dries out in service. An over dried plank will distort or crack. When the moisture content of a piece of wood decreases below FSP, shrinkage occurs. The amount of shrinkage that occurs in wood as it dries differs in the longitudinal, radial and tangential directions. When the amount of shrinkage in a particular direction within a piece of wood is not uniform, the wood can distort. Moisture related disorders of wood (spring, twist and bow) during kiln drying at the sawmill or in service in a building is one of the major problems for construction timber.

In addition to dimensional changes in wood with changing moisture content mechanical properties also change. Wood stiffens linearly with decreasing moisture content below FSP and its stiffness is approximately constant above this point.

A common measurement made on timber is the modulus of elasticity (MOE). The MOE provides a measurement of the stiffness of the timber. The strength of the timber is proportional to the stiffness of the timber and thus the MOE can be used to provide an indication of standard or quality of the timber.

Modulus of elasticity can be defined as the product of the density of the timber and the square of the velocity of an acoustic wave transmitted through the timber. Many devices have been proposed that measure or predict a modulus of elasticity for evaluation of timber properties at various stages. For example, one device uses an impact at one end of a cut log and a detector at the other end to measure the transmission of a longitudinal wave through the log. The acoustic velocity of the log is then determined from the detected wave, and the modulus of elasticity estimated by the above relationship.

A number of acoustic techniques are available for estimating MOE including both longitudinal and flexural resonant vibration, stress wave time of flight velocity and ultrasonic time of flight velocity. These techniques are becoming increasingly used by the forestry and wood processing sectors for segregating material on the basis of wood stiffness. However, the modulus of elasticity is known to vary with density and the density varies with moisture content.

Clearly any measurement of stiffness is limited by the true or absolute moisture content of the timber being measured.

The "true" moisture content of wood can be determined by the oven-dry method (i.e., gravimetrically), being generally the standard against which other methods of assessing moisture content are judged, since it is the most reliable in terms of precision and accuracy. This method uses samples cut from the log, board or plank. The samples are weighed to provide a "wet weight". The samples are then placed in an oven, typically at over 100⁰C and left for 18 to 72 hours (e.g. BS EN 13183-1 :2002) . Samples are weighed at several one-hour intervals to assure they have reached a constant dry weight. This dry weight is then measured. Moisture content is calculated using the following formula:

Moisture content (%) = (wet weight - dry weight) x 100

(dry weight)

While this method provides an absolute determination of water moisture, it has a number of disadvantages. A sample of the wood must be taken and thus the process is invasive, damaging the wood being tested. The test is also time consuming, typically requiring two to three days to get a result. This process is impractical for determining moisture content on planks in a sawmill, for instance, due to the time, damage and cost involved. It is therefore mainly limited to laboratory studies and is of limited application in a commercial environment.

In the commercial environment two types of non-invasive moisture meters exist. These are resistive and capacitive moisture meters. Both work on the basis that the electrical properties of wood vary with its moisture content.

DC-resistance meters operate by inserting a pair of pins into the wood that penetrate the wood to a depth of up to 7 centimeters and measure the resistance to the flow of a DC current. In this regard they do damage the wood and any given reading is only for the narrow band of wood between the tips of the pins. Additionally, the pins must be inserted along the gradient of the wood avoiding knots, pitch pockets and incipient decay which can affect the density or extractive content of the wood and alter the readings. Ions from corrosion of fasteners, or preservative treatment, can also distort readings.

The typical operating range for resistance moisture meters is from 7 to 30% moisture content, i.e., operating below the fibre saturation point. These meters are sensitive to the temperature of the wood and this can be input into the meter. Similarly they are wood species dependent and this must also be selected. The meters must therefore be regularly calibrated for species type and temperature.

Dielectric (capacitive) moisture meters have a sensor in the form of flat plate electrodes that are brought into direct contact with the wood being tested. They emit a radio frequency field that penetrates the wood and measure the response. Both wood density and moisture level affect the readings, and corrections are required to compensate for differences in density between the species. These meters are less affected by wood temperature, but species corrections are required. The typical operating range for these meters is from 7 to 30% moisture content. Again this is below the fibre saturation point.

Though dielectric meters are not affected so much by temperature, they are limited to only measuring at a top surface of the wood to the depth of penetration. It is therefore common that this type of meter will not function well for a plank or board that may be well dried through most of its cross- section but has been rewetted on the surface by recent exposure to moisture. In this instance, a dielectric meter will over-estimate the average moisture content. This may be a common problem at building sites where partially framed structures are exposed to rain or melting snow.

While these portable resistive and dielectric meters offer spot checks on wood they are of limited use in commercial units such as sawmills. In these environments, each piece of wood needs to be measured and in-line moisture meters have been developed for this. The meters are typically located on the timber chain after the kiln to assist in accepting or rejecting a plank. Resistance in-line moisture meters use wire brushes that resemble drum snares to contact the wood's surface and measure moisture content. This reduced contact makes the meters really only suitable for measuring veneers. More typical is the in-line dielectric meter with the plates located at various orientations to but never touching the wood as it passes.

In practice in a sawmill, timber that is intended for construction use is generally dried in a kiln, planed and then machine graded. This is sub- optimal as energy is expended and costs incurred in drying timber that is subsequently rejected on the basis of its modulus of elasticity, defects such as knots and wane or excessive distortion. In some situations this excessive distortion can be the result of some timber within the kiln being over-dried due to the wide range of moisture contents of the sawn timber. Timber can also be rejected due to low stiffness caused by high moisture content due to insufficient drying. To prevent under-or-over drying in a kiln it would be useful to first sort timber by its moisture content, prior to kiln drying. This cannot currently be achieved as the meters available are limited to an upper moisture content of around 30%.

Accordingly, the ability to measure moisture content above the fibre saturation point in a non-invasive real-time manner would provide great advantages to the timber industry. Additionally, while determining the modulus of elasticity provides the standard for stiffness determination in wood, it can only be a relative measurement in a commercial environment, due to it's variability with moisture content.

It is therefore an object of the present invention to provide a non-invasive method of determining the moisture content in a hygroscopic material which obviates or mitigates the disadvantages of the prior art measurement techniques.

It is an object of at least one embodiment of the present invention to provide a non-invasive method of determining the moisture content in wood which provides measurements above the fibre saturation point.

It is a further object of at least one embodiment of the present invention to provide apparatus for determining the moisture content in a hygroscopic material. According to a first aspect of the present invention there is provided a noninvasive method of determining the moisture content in a hygroscopic material, the method comprising the steps of:

(a) determining a first modulus of elasticity (MOE) of the hygroscopic material by a first method;

(b) determining a second modulus of elasticity (MOE) of the hygroscopic material by a second method, the second method being affected differently by the presence of water than the first method; and

(c) taking the ratio of the first modulus of elasticity and the second modulus of elasticity, acting upon it by a species factor, and thereby obtaining an absolute moisture content of the hygroscopic material.

In this way, two known methods of determining the modulus of elasticity can be used to arrive at the absolute value. By using methods each affected differently by moisture content, the moisture content becomes derivable using a single multiplication factor for the species or type of hygroscopic material.

Preferably the hygroscopic material is wood and the species factor relates to the species of tree from which the wood is cut.

Preferably the first method is an ultrasonic time of flight method. This comprises the steps of creating an ultrasonic wave in the material and measuring the transit time of an ultrasonic wave passed though the material. Advantageously the ultrasonic wave is passed longitudinally through the material. The first MOE may then be determined from the product of bulk density and velocity. Preferably the second method is an acoustic measurement technique. Such a technique is simple and inexpensive. Preferably, the second method is selected from a group of known techniques comprising, but not limited to: sonic wave time of flight; stress wave time of flight; flexural resonant frequency and longitudinal resonant frequency. The second

MOE may then be determined by the same technique as the first MOE or by calculations based on the frequency measurement.

Preferably the species factor is a multiplication factor, the factor being predetermined from measurements made on different types of the material. Alternatively the factor may comprise an added value and a multiplication factor.

According to a second aspect of the present invention there is provided a non-invasive method of determining the moisture content in wood, the method comprising the steps of:

(a) determining a first modulus of elasticity (MOE) of the wood by a first method;

(b) determining a second modulus of elasticity (MOE) of the wood by a second method, the second method being affected differently by the presence of water than the first method; and

(c) taking the ratio of the first modulus of elasticity and the second modulus of elasticity, acting upon the ratio by a species factor, and thereby obtaining an absolute moisture content of the wood.

Preferably the first and second methods are according to the first aspect. In this way a non-destructive, rapid in-situ measurement can be made. Preferably the wood is a sawn length of timber or log. More preferably the first and second methods are applied across the full length of the wood.

According to a third aspect of the present invention there is provided a non-invasive method of determining the moisture content in wood, the method comprising the steps of:

(a) determining a first time of flight of an ultrasonic wave through the wood;

(b) determining a second time of flight of a sonic wave through the wood; and

(c) taking the ratio of the square of the second time of flight to the square of the first time of flight, acting upon the ratio by a species factor, and thereby obtaining an absolute moisture content of the wood.

In this way, the MOE is not calculated and thus the density and the length of wood do not require to be determined.

According to a fourth aspect of the present invention there is provided a method of determining a species factor for use in a method of determining the moisture content in wood, comprising the steps:

(a) providing a plurality of wood samples from a desired species of tree;

(b) determining a first modulus of elasticity (MOE) of at least one wood sample by a first method on a plurality of dates;

(c) determining a second modulus of elasticity (MOE) of the at least one wood sample by a second method on each date, the second method being affected differently by the presence of water than the first method;

(d) recording a mass of at least one wood sample on each date; oven drying at least one wood sample to determine an oven-dry mass and from this calculating an absolute moisture content for each date using the mass recorded on each date;

(e) calculating a ratio of the first modulus of elasticity and the second modulus of elasticity for the at least one wood sample for each date;

(f) plotting the ratio against the absolute moisture content and determining an equation of a straight line passing through the plotted points; and

(g) defining a species factor by the equation.

Preferably the samples are randomly selected. In this way, typical defects found in sawn timber or logs are considered. Alternatively, the samples are selected to be clear specimens, having a straight grain and being free of knots and other defects.

Preferably the wood samples are dried between each date. The drying may be achieved by air drying or may be by kiln drying.

Preferably the first and second methods are according to the first aspect.

According to a fifth aspect of the present invention there is provided apparatus for determining the moisture content in a hygroscopic material, the apparatus comprising: a first evaluation means for making a first measurement of the material; a second evaluation means for making a second measurement of the material, the second measurement being affected differently by the presence of water than the first measurement; input means for a user to input physical parameters of the material; a processor for receiving first and second measurements, for receiving the physical parameters, for storing a species factor, calculating a modulus of elasticity for each evaluation means, determining a ratio of the moduli of elasticity and applying the species factor to provide a moisture content for the material; and display means to display the moisture content.

Preferably the first evaluation means is an ultrasonic device. More preferably the ultrasonic device is an ultrasonic time of flight recorder.

Preferably the second evaluation means is an acoustic device. The acoustic device may be a sonic time of flight recorder, a resonance recorder or a bending test apparatus. Other evaluation means as would be known to those skilled in the art may also be used.

Both the first and second evaluation means and the associated modulus of elasticity calculation based on them are as known in the art.

Preferably the processor includes a table of species factors for different types of material. More preferably a user can input the type of material and the processor will select and use the species factor relating to the type of material from the table.

Preferably the input means is a keypad. Preferably the display means is a liquid crystal display.

According to a sixth aspect of the present invention there is provided apparatus for determining the moisture content in wood, the apparatus comprising: a pulse generator; a time of flight recorder; control means to operate the apparatus to fire at least one pulse in to an end of the wood; a processor for receiving an ultrasonic time of flight measurement and a sonic time of flight measurement from the time of flight recorder, determining a ratio of the squares of the times of flight and applying a species factor to provide a moisture content for the wood; and display means to display the moisture content.

Preferably the processor includes a table of species factors for different species of tree which the wood may have come from. More preferably the apparatus includes input means for a user to select a species of tree from a list stored in the processor. More preferably the processor selects and uses the species factor relating to the tree species from the table.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a flow chart illustrating a method for determining the moisture content in wood according to an embodiment of the present invention;

Figure 2 is a schematic illustration of apparatus for determining the moisture content in wood according to an embodiment of the present invention;

Figure 3 is a schematic illustration of apparatus for determining the moisture content in wood according to an alternative embodiment of the present invention; and

Figure 4 is a graph of the ratio of stiffness against measured moisture content to determine a species factor.

Reference is initially made to Figure 1 of the drawings which illustrates a flow chart, generally indicated by reference numeral 10, showing the steps in a method for determining the moisture content in wood according to an embodiment of the present invention. The method will be described with reference to wood, but applies to any hygroscopic material which exhibits a measurable stiffness.

As presented hereinbefore, measuring the modulus of elasticity to provide an indication of the stiffness of a piece of wood is well known in the art. This modulus of elasticity is typically provided at a standard temperature and moisture content, or is given as a relative measurement from a point where the moisture content could be measured by other means e.g. dry oven or resistive/capacitive measurement. In the present invention it is these same methods of measuring the modulus of elasticity which are used. For ease of use in a sawmill and due to costs, we will limit ourselves to techniques such as the bending test and acoustic tests. It is accepted that techniques exist using gamma rays and other electromagnetic signals and while they could be applied here, they will not be described.

In measuring the modulus of elasticity, a direct measurement is usually taken on the wood 12,14. In acoustics this may be by measuring time of flight or recording the sound when the wood is caused to vibrate at a resonant frequency. In order to determine the modulus of elasticity it is common to provide a physical measurement from the wood 16,18. This may be the dimensions, for example. These parameters 16,18 will be dependent on the measurement technique 20,22 chosen for the modulus of elasticity.

In selecting which techniques 20,22 to use to measure the modulus of elasticity, the techniques must be distinct and non-identical. In this regard one technique must be affected by high moisture content i.e. those above the fibre saturation point while the second technique should show little effect with moisture variation above the saturation point.

For each technique 20,22 a modulus of elasticity is calculated. It will be appreciated by those skilled in the art that commercially available measurement meters can be used to obtain the modulus of elasticity. For example, a first technique 20 could use a PUNDIT (CNS Farnell, UK) which is an ultrasonic device incorporating 54kHz transducers. This measures 12 the transit time of an ultrasonic pulse to travel through the wood. Using physical parameters 16 to determine the density, the modulus of elasticity 24 can be calculated. For example, a second technique 22 could utilize a Ghndosonic instrument (J.W. Lemmens, Belgium) to determine the modulus of elasticity 26. In this technique the wood is supported at a distance L/5, where L is the wood length, from each end. The wood is tapped at its centre using a hammer to cause it to vibrate at its natural resonant frequency. The vibrations are picked up on a microphone placed above the top surface of the wood and the Ghndosonic instrument calculates the frequency. Using the physical parameters 18 of height, length and density of the wood, a modulus of elasticity 26 can be determined.

The values of the modulus of elasticity 24, 26 may then be processed to obtain a ratio 28. Advantageously, depending on the techniques selected, certain physical parameters may not be required. In this example, both calculations of the modulus of elasticity 24,26 require a multiplication by the density. It is obvious that this density will be identical for the two techniques 20,22 as it is a physical parameter 16,18 of the wood. Accordingly, there is no requirement to measure the density as the ratio 28 can be found by combining each modulus of elasticity 24,28 assuming the density will cancel on each side of the ratio. This means that the mass of the wood need not be measured or estimated. This cancellation is used extensively in a further embodiment of described later with reference to Figure 3.

In order to obtain the moisture content 30, a calibration factor is required. This is referred to as a species factor 32 as it is dependent upon the species or variety of tree from which the wood was felled. It will be necessary to know the species 34 to obtain the correct factor 32. The process to obtain species factors is detailed later with reference to Figure 4.

Typically the species factor 32 will take the form of a linear equation so that the moisture content 30 will be calculated as:

Moisture Content = a(Ratio) + b, where a and b are the species factor 32.

The moisture content can then be displayed 36 for a user or stored for future reference.

This method of determining the moisture content can provide measurements of moisture content below, above and at the fibre saturation point. Values as high as 283% are not uncommon and have been determined using this method.

Reference is now made to Figure 2 of the drawings which illustrates an apparatus, generally indicated by reference numeral 40, for determining a moisture content in wood 42 according to an embodiment of the present invention. Apparatus 40 is based on the example techniques 20,22 described with reference to Figure 1 , but is provided as a single handheld apparatus suitable for the non destructive testing of the wood in any environment. It will be obvious to those skilled in the art that while a handheld device is illustrated modifications could be made to adapt the apparatus for use in a timber chain as provided in a saw mill, where wood is conveyed passed the apparatus 40 and stopped for minimal time while both measurements 20,22 are made.

Apparatus 40 comprises a hand held control unit 44. Unit 44 incorporates a Ghndosonic instrument with a microphone 46 which can be placed against an upper surface 48 of the wood 42. Supports, not illustrated, locate under the wood 42, and a hammer 50 is used to create the natural vibration in the wood from which the frequency 14 can be determined. A processor in the unit 44 determines the modulus of elasticity 26 using a pre-programmed formula.

The unit 44 also controls an ultrasonic pulse generator 52 and a time of flight recorder 54. The generator 52 and recorder 54 are positioned on either end 56,58 of the wood 42 respectively. The unit 44 sends a signal to create the ultrasonic pulse and times its arrival 12 at the recorder 54. The processor is pre-programmed with the modulus of elasticity calculation 24. When each measurement 12, 14 has been made, the ratio 28 is calculated in the processor and the species factor 32 applied. The species factor 32 is also pre-programmed into the unit 44 in the form of a look-up table, giving the values a and b for each species of tree. A user will select the species from a menu on the display 60 either before the measurements 12,14 are made or once the ratio 28 is derived. The moisture content 30 is calculated in the processor and the result appears on the display 60. Other parameters such as a reference number for the wood 42 can be input to the unit 44 and the measurements 12,14 and / or the calculated values 30, 24, 26, 28 can be stored in a memory for later download and reference.

Apparatus 40 may also be considered as an improved stiffness meter.

With the parameters 16 or 18 supplied, the modulus of elasticity 24,26 by either technique can be displayed to provide an indication of stiffness at a known moisture content 30. This removes the need for a separate measurement of moisture content which was required to calibrate the prior art stiffness meters.

An alternative embodiment of an apparatus, generally indicated by reference numeral 70, for determining the moisture content in wood 72, is shown in Figure 3. Apparatus 70 comprises a hand held control unit 74 which operates an acoustic wave generator 76. Generator 76 is capable of producing both ultrasonic (greater than or equal to 2OkHz) and sonic (between 20 and 20,000 Hz) waves. The generator 76 is adapted to locate upon an end 80 of the wood 72. A receiver 78 is located at an opposing end 82. The wood 72 may be of any selected length, for example a sample of approximately 30cm or a plank of approximately 3 to 5 m.

Receiver 78 is an adapted time of flight recorder which can measure the time of arrival of consecutive pulses or wavelets.

As with the previous apparatus 40, a user selects the species 34 on the unit 74. The user then activates the generator and a sonic and an ultrasonic pulse are transmitted together. The receiver 78 records the time of flight for both 12,14 and transmits this to a processor in the unit 74. As the modulus of elasticity for both 24,26 will be the product of the density and the velocity, the density and length will cancel each other out when the ratio 28 is formed, leaving only an inverse ratio of squares of the times of flight as the remaining parameters. Thus in this embodiment no physical parameters 16,18 need to be determined. The species factor 32 is found from the look-up table stored in the unit 74 and the moisture content 30 is calculated and displayed 36 on panel 84 on the unit 74 as for the earlier embodiment.

Each embodiment of the invention requires a species factor 32. This is a pre-calibrated value, or more precisely, two values a and b which define a linear equation from which the moisture content can be evaluated. The species factor can be determined as follows. A species of tree was selected, for example a Sitka spruce, which was felled and a number, say 85, specimens where cut. For our example each specimen had dimensions of approximately 22 x 22 x 350 mm. All specimens had a straight grain and where free of any knots or other defects.

For each specimen its mass was recorded along with its dimensions. Each day, over a fourteen day period, the samples where measured and a ratio of stiffness 28 calculated as per the techniques 20,22 described with reference to Figure 1. The specimens were allowed to dry naturally in this period. After 14 days the specimens where oven-dried by the method described herein before with reference to the prior art. Drying was maintained for 3 days at 103⁰C in order to obtain an oven-dry mass for each specimen. Using the oven-dry mass, the moisture content was calculated for each specimen for each day using the equation provided in the prior art. A plot of the ratio 28 against the moisture content was drawn up and a linear fitting model applied to determine a straight line 92 having an equation y=ax+b. a and b are thus provided as the species factor 32. For our example, a plot 90 is illustrated in Figure 4 where the x axis 94 shows the ratio 28 with values from 1 to 3 and the y axis 96 shows the moisture content (%) with values from 0 to 300. The initial range of moisture contents in the specimens ranged from 65% to 283% representing both sapwood and heartwood. Individual points 98 are indicated and a straight line 92 is fitted through the points 98. It will be apparent that a is the gradient of the line 92 and b is the moisture content value where the line 92 crosses the y axis 96.

It will be appreciated that while the species factor is dependent on tree variety it may also be dependent on temperature. It will be apparent to those skilled in the art that a further variable i.e. temperature could be incorporated in the species factor 32, such that a user inputs a temperature as well as a tree species. Accordingly there would be a range of species factors for each species dependent on temperature.

The principal advantage of the present invention is that it provides a method and apparatus for determining the moisture content in a hygroscopic material which gives a non-destructive, rapid, in-situ measurement of the absolute moisture content. A further advantage of at least one embodiment of the present invention is that it provides a method and apparatus for determining the moisture content in wood across its full range and in particular for values over the fibre saturation point. This allows green timber and pre-dhed timber to be graded. It also offers a sawmill the opportunity to batch planks of similar moisture content together before kiln drying or remove planks which are particularly wet or dry prior to entering the kiln. This improves the efficiency of the drying process by not under or over drying planks. This would provide great cost savings by reducing the amount of waste in a sawmill.

A yet further advantage of at least one embodiment of the present invention is that it provides a method and apparatus for determining the moisture content in a wood which uses two techniques which are known and accepted in the industry. Indeed the modulus of elasticity calculated can be improved by knowing the absolute moisture content.

A still further advantage of at least one embodiment of the present invention is that it provides a method and apparatus for determining the moisture content in a hygroscopic material which is not dependent upon the size of the material being tested. For wood, this means that a uniform measurement can be achieved largely regardless of the presence and location of knots or other defects in the wood. This is in comparison to measurements made by resistivity which only determine a value between two closely spaced points.

A still further advantage of at least one embodiment of the present invention is that it provides a method and apparatus for determining the moisture content in a hygroscopic material using only time of flight measurements, thus removing the requirement to weigh or measure dimensions of the material. It will be appreciated by those skilled in the art that various modifications may be made to the invention herein described without departing from the scope thereof. For example, the hand-held unit may be replaced with a desk mounted unit which could be operated from a mouse, keyboard or another hand held unit. The processor may not directly calculate the modulus of elasticity but be pre-programmed with an equation representing the ratio into which the measured values and physical parameters are directly entered. The display may be a computer screen and the entire apparatus integrated with other apparatus in a working environment. Additionally the technique can be applied to any desired or available dimensions of timber. For example, the technique could be used on wood chips or on a section of timber split from a log. Equally the measurement does not have to be made over the full length of the wood, any length or dimension on the wood can be selected.

Claims

1. A non-invasive method of determining the moisture content in a hygroscopic material, the method comprising the steps of:

(a) making a first measurement of the hygroscopic material by a first method;

(b) making a second measurement of the hygroscopic material by a second method, the second method being affected differently by the presence of water than the first method; and

(c) taking the ratio of the first and the second measurements, acting upon it by a species factor, and thereby obtaining an absolute moisture content of the hygroscopic material.

2. The method according to claim 1 wherein the hygroscopic material is wood and the species factor relates to the species of tree from which the wood is cut.

3. The method according to claim 1 or claim 2 wherein the first measurement is a determination of a first modulus of elasticity (MOE) of the hygroscopic material by the first method, the second measurement is a determination of a second modulus of elasticity (MOE) of the hygroscopic material by the second method.

4. The method according to claim 1 or claim 2 wherein the first measurement is a determination of a first time of flight of a wave through the hygroscopic material by the first method, the second measurement is a determination of a second time of flight of a wave through the hygroscopic material by the second method and the ratio is taken on the square of the second time of flight to the square of the first time of flight.

5. The method according to any preceding claim wherein the first method is an ultrasonic time of flight method comprising the steps of creating an ultrasonic wave in the material and measuring the transit time of the ultrasonic wave passed through the material.

6. The method according to any preceding claim wherein the the second method is an acoustic measurement technique selected from a group of known techniques comprising, but not limited to: sonic wave time of flight; stress wave time of flight; flexural resonant frequency and longitudinal resonant frequency.

7. The method according to any one of claims 2 to 6 wherein the wood is a sawn length of timber or log.

8. The method according to any one of claims 2 to 7 wherein the first and second methods are applied across the full length of the wood.

9. Apparatus for determining the moisture content in a hygroscopic material, the apparatus comprising: a first evaluation means for making a first measurement of the material; a second evaluation means for making a second measurement of the material, the second measurement being affected differently by the presence of water than the first measurement; input means for a user to input physical parameters of the material; a processor for receiving first and second measurements, for receiving the physical parameters, for storing a species factor, calculating a ratio of the first and second measurements and applying the species factor to provide a moisture content for the material; and display means to display the moisture content.

10. Apparatus according to claim 9 wherein the first evaluation means is an ultrasonic device.

11.Apparatus according to claim 10 wherein ultrasonic device is an ultrasonic time of flight recorder.

12. Apparatus according to any one of claims 9 to 11 wherein the second evaluation means is an acoustic device.

13. Apparatus according to claim 12 wherein the acoustic device is selected from a group comprising, but not limited to: a sonic time of flight recorder, a resonance recorder or a bending test apparatus.

14. Apparatus according to any one of claims 9 to 13 wherein the processor includes a table of species factors for different types of material.

15. Apparatus according to claim 14 wherein the apparatus includes means for a user to input the type of material and wherein the processor selects and uses the species factor relating to the type of material from the table.