CN112213346A - Method for measuring moisture content of wood - Google Patents
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- G01N24/00—Investigating or analyzing materials by the use of nuclear magnetic resonance, electron paramagnetic resonance or other spin effects
- G01N24/08—Investigating or analyzing materials by the use of nuclear magnetic resonance, electron paramagnetic resonance or other spin effects by using nuclear magnetic resonance
- G01N24/082—Measurement of solid, liquid or gas content
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Abstract
The invention provides a method for measuring the water content of wood. The method comprises the following steps: using free induction decay pulse sequence, at intervals of repeated sampling interval time t1Measuring the complete relaxation signal intensity of a plurality of pure water standard samples with different masses, and establishing a standard measurement curve according to the complete relaxation signal intensity of each pure water standard sample and the mass of the pure water standard sample; at interval of repeated sampling interval time t2Measuring the complete relaxation signal intensity of the sample to be measured, and obtaining the content of water in the sample to be measured according to a standard measurement curve; obtaining the water content of the sample to be detected based on the total mass of the sample to be detected and the water content in the sample to be detected; wherein the sampling interval time t is repeated1Is the same, the oversampling interval time t2Is the same, and the oversampling interval time t1And a oversampling interval time t2The same or different. The method has the advantages of rapid water content test, accurate result and large measurement range.
Description
Technical Field
The invention relates to a method for measuring the moisture content of wood, in particular to a method for measuring the moisture content of wood by using the response characteristic of water in the wood to nuclear magnetic resonance, belonging to the technical field of wood processing.
Background
The moisture content of wood is one of the most important parameters to be measured quickly and accurately in the drying process, because the control and adjustment of the drying environment conditions are greatly dependent on the moisture content of wood. The detection of the moisture content of the wood is of great significance in wood processing and utilization, and 75% of quality problems in wood products are caused by inaccurate control of the moisture content of raw materials used in wood processing. The moisture content of wood seriously affects the physical mechanical properties and the machining performance of the wood. Therefore, the measurement of the moisture content of wood is one of the most important methods for wood analysis. The moisture content of wood is an important characteristic parameter of wood and is closely related to almost all properties of wood. The moisture content of the wood is also a very critical factor in the wood processing and utilizing process, and the quality control of the wood product is directly influenced.
At present, the most common methods for measuring the moisture content of wood are a drying method and a resistance method. The drying method is a standard method for measuring the water content of the wood (GB/T1931-. The determination method is simple to operate; however, the time required for thoroughly drying the wood is long, so that the method is not suitable for the requirement of rapid detection, and the heating can influence the morphological structure of the wood, so that the method is not suitable for special samples with certain non-destructive requirements.
The resistance method is carried out according to the relation between the resistance characteristic and the water content of the wood, is a detection method commonly used in the industry at present, is convenient and quick to measure, but has a limited range of the measured water content, generally 5-30%, and has great influence on the measurement accuracy due to the material characteristic of the wood. The research also relates to an X-ray scanning method, the principle is carried out by utilizing the relation between the density and the water content of the wood, and the test is quick and nondestructive; but there is certain danger to the user, and the precision is influenced by the kind of material greatly, and needs the measurement sample size, increases human error, influences the precision. The research also relates to a near infrared spectrum method, and the test is quick, accurate and nondestructive; but generally only the moisture content of the surface of the wood can be detected.
Citation 3 discloses a method for measuring moisture content in a wood drying process and a method for drying wood, which includes: measuring the initial moisture content and the initial weight of the wood to be dried and stacking the wood in a drying kiln; measuring the temperature and the wind speed of the medium flowing through the two sides of the timber pile, obtaining the saturated vapor pressure of the wet air according to the dry bulb temperature, obtaining the relative humidity of the wet air according to the dry bulb temperature, finally obtaining the density and the moisture content of the wet air, and obtaining the moisture content of the timber to be dried at different moments; and drying the wood according to the measured moisture content of the wood and the wood drying standard to obtain high-quality dried wood. However, this method requires a large number of complicated calculation formulas, and the measurement method is highly required and difficult to operate.
Based on the reasons, the development of the wood moisture content detection method which is high in detection speed, accurate in measurement result, safe, feasible and nondestructive has very important significance.
Cited documents:
cited document 1: CN102072922A
Cited document 2: CN103822946A
Cited document 3: CN108802352A
Disclosure of Invention
Problems to be solved by the invention
The invention aims to provide a method for measuring the water content of wood, which is used for detecting the water content of the wood based on a time domain nuclear magnetic resonance technology, has no damage to a sample in the detection process, and has short test time and high accuracy.
Means for solving the problems
The invention firstly provides a method for measuring the moisture content of wood, which comprises the following steps:
using free induction decay pulse sequence, at intervals of repeated sampling interval time t1Measuring the complete relaxation signal intensity of a plurality of pure water standard samples with different masses, and establishing a standard measurement curve according to the complete relaxation signal intensity of each pure water standard sample and the mass of the pure water standard sample;
at interval of repeated sampling interval time t2Measuring the complete relaxation signal intensity of a sample to be measured, and obtaining the content of water in the sample to be measured according to a standard measurement curve;
obtaining the water content of the sample to be detected based on the total mass of the sample to be detected and the content of water in the sample to be detected;
wherein the sampling interval time t is repeated1Is the same, the oversampling interval time t2Is the same, and the oversampling interval time t1And a oversampling interval time t2The same or different.
According to the method for measuring the water content of the wood, the full relaxation signal intensity of the pure water standard sample is obtained through the following method:
by freedom of useA sequence of inductive decay pulses at an interval of repetitive sampling intervals of time tcMeasuring the nuclear magnetic resonance signal intensity of a pure water standard sample, establishing a relation curve of the nuclear magnetic resonance signal intensity of the pure water standard sample and the repeated sampling interval time, and obtaining a relaxation time constant of the pure water standard sample;
at interval of repeated sampling interval time t1Measuring the nuclear magnetic resonance signal intensity of a plurality of pure water standard samples with different masses, and combining the relaxation time constants of the pure water standard samples to obtain the complete relaxation signal intensity of each pure water standard sample;
wherein the sampling interval time t is repeatedcEach interval time in (a) is different.
The method for measuring the water content of the wood according to the invention, wherein the repeated sampling interval time tcIs sequentially increasing or sequentially decreasing.
The method for measuring the water content of the wood according to the invention, wherein the repeated sampling interval time tcThe step length in the process is 1000-3000 ms.
The method for measuring the water content of the wood according to the invention, wherein the repeated sampling interval time tcThe interval time in (1) is 500-25000 ms, preferably 1000-20000 ms, and more preferably 1000-17000 ms.
The method for measuring the water content of the wood according to the invention, wherein the repeated sampling interval time t1Or the oversampling interval time t2Each interval time in (1) is 1500-10000 ms.
The method for measuring the water content of the wood according to the invention, wherein the repeated sampling interval time t1Or the oversampling interval time t2Each interval time in the time interval is 2000-5000 ms.
According to the method for measuring the water content of the wood, the accumulated sampling frequency is more than 32 in each measurement.
According to the method for measuring the water content of the wood, the time domain nuclear magnetic resonance analyzer is used for measuring.
The method for measuring the water content of the wood further comprises the step of performing instrument calibration on the time domain nuclear magnetic resonance analyzer before measurement.
ADVANTAGEOUS EFFECTS OF INVENTION
The method for detecting the water content of the wood has at least one of the following effects:
1) establishing a standard curve by using pure water with a specific nuclear magnetic relaxation law as a standard sample without considering the relaxation speed of hydrogen protons in the standard sample; compared with other solutions as standards, there is no need to purchase solutes and make up solutions.
2) In the nuclear magnetic test process, the pulse signals can not damage the sample, and the method is a nondestructive test method.
3) The water content test is quick, the result is accurate, and the measurement range is large.
Drawings
FIG. 1 is a free decay plot of a pure water standard according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a fitted curve of the nuclear magnetic signal intensity and the resampling interval time of a pure water standard sample according to an embodiment of the invention;
FIG. 3 is a schematic diagram of a fitted standard measurement curve of the full relaxation NMR signal intensity and the moisture quality of a pure water standard sample according to an embodiment of the present invention.
Detailed Description
The present invention will be described in detail below. The technical features described below are explained based on typical embodiments and specific examples of the present invention, but the present invention is not limited to these embodiments and specific examples. It should be noted that:
in the present specification, the numerical range represented by "numerical value a to numerical value B" means a range including the end point numerical value A, B.
In the present specification, "plural" in "plural", and the like means a numerical value of 2 or more unless otherwise specified.
In this specification, the terms "substantially", "substantially" or "substantially" mean an error of 5% or less, or 3% or less or 1% or less, compared to the relevant perfect or theoretical standard.
In the present specification, "%" denotes mass% unless otherwise specified.
In the present specification, the meaning of "may" includes both the meaning of performing a certain process and the meaning of not performing a certain process.
In the present specification, when "normal temperature" or "room temperature" is used, the temperature may be 10 to 40 ℃.
In this specification, "optional" or "optionally" means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.
In the present specification, reference to "some particular/preferred embodiments," "other particular/preferred embodiments," "embodiments," and the like, means that a particular element (e.g., feature, structure, property, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and may or may not be present in other embodiments. In addition, it is to be understood that the described elements may be combined in any suitable manner in the various embodiments.
The invention provides a method for detecting the moisture content of wood, which comprises the following steps:
using free induction decay pulse sequence, at intervals of repeated sampling interval time t1Measuring the complete relaxation signal intensity of a plurality of pure water standard samples with different masses, and establishing a standard measurement curve according to the complete relaxation signal intensity of each pure water standard sample and the mass of the pure water standard sample;
at interval of repeated sampling interval time t2Measuring the complete relaxation signal intensity of a sample to be measured, and obtaining the content of water in the sample to be measured according to a standard measurement curve;
obtaining the water content of the sample to be detected based on the total mass of the sample to be detected and the content of water in the sample to be detected;
wherein the sampling interval time t is repeated1Is the same, the oversampling interval time t2Is the same, and the oversampling interval time t1And a oversampling interval time t2The same or different. Preferred oversampling interval time t1With a time t between repeated sampling2The same is true.
According to the invention, an accurate algorithm for obtaining the moisture quality of the unknown oven dry quality wood is obtained by reasonably establishing the proportional relation between the nuclear magnetic resonance signal intensity and the moisture quality, and the detection of the moisture content of the wood based on the time domain nuclear magnetic resonance technology is realized. Compared with the traditional test method, the method disclosed by the invention can realize measurement at normal temperature without heating the sample, is high in test speed and accurate in result, and provides a good technical means for measuring the water content of the wood. Specifically, the method comprises the following steps:
< nuclear magnetic resonance >
Nuclear Magnetic Resonance (NMR) is mainly caused by the spin motion of atomic nuclei. The nuclear magnetic resonance method comprises time domain nuclear magnetic resonance (TD-NMR) technology. The time domain nuclear magnetic resonance technology is an advanced characterization means, can quickly and accurately measure the dynamic changes of free water and bound water in the wood, and can also characterize the pore distribution of the wood based on moisture information, so the time domain nuclear magnetic resonance technology is widely applied to the field of scientific research of the wood.
Time domain nuclear magnetic resonance (TD-NMR) refers to a physical phenomenon in which when a nucleus in a static magnetic field is subjected to an electromagnetic wave of a frequency corresponding to another alternating electromagnetic field, a part of the nucleus in a low energy state absorbs energy and then transitions to a high energy state. After stopping the rf pulse, the nuclei that have transitioned to the high energy state release the absorbed rf energy back to the ground state, reaching boltzmann equilibrium, a process called relaxation, and the time required to complete this process called relaxation time. In the process of restoring the nuclear equilibrium state, a nuclear magnetic resonance signal is generated.
In testing wood, primarily to detect moisture therein1The H signal is dominant. Common nuclear magnetic resonance signals include Free Induction Decay (FID) and transverse relaxation time (T)2). The free induction decay signal is the simplest signal form in the nuclear magnetic signal and can qualitatively reflect the physical characteristics of the substance, and the free induction decay signal of the solid substance decays faster and the liquid substance decays slower. Transverse relaxation time is the process by which energy is exchanged between spinning nuclei during their return from an excited state to a ground state, and is therefore also referred to as spin-spin relaxation time. The fluidity or the degree of freedom of a substance is generally characterized by a transverse relaxation time, and the larger the transverse relaxation time, the better the fluidity of the substance and the larger the degree of freedom. Specifically, the invention uses the time domain nuclear magnetic resonance technology and adopts a free induction decay pulse sequence to carry out measurement.
Specifically, the water content of the wood can be measured by using a time domain nuclear magnetic resonance spectrometer, and further, the time domain nuclear magnetic resonance spectrometer can be a nuclear magnetic resonance nanopore analyzer.
In some embodiments, a calibration of the relevant instrument is required prior to taking the measurement. The specific method for calibrating the apparatus is not particularly limited in the present invention, and may be any detection method commonly used in the art. Specifically, in the present invention, the free induction decay pulse sequence can be used to measure the standard oil sample and correct the center frequency and pulse width of the nuclear magnetic resonance spectrometer. In addition, parameters such as the start sampling time, the number of repeated sampling, the repeated sampling waiting time, the analog gain, and the digital gain of the instrument are determined while the correction is performed.
< pure Water Standard sample >
The invention can use pure water standard sample to make standard curve. For pure water standards, the nuclear magnetic relaxation law of the invention needs to be satisfied (i.e. the repeated sampling interval time t is adopted)cSolving a relaxation time constant relation curve) as a pure water standard sample. Specifically, the specific choice of the pure water standard sample is not particularly limited, and may be ultrapure water, deionized water, purified water, double distilled water, etc. commonly used in the artAny feasible water. Compared with other solutions as standard samples, the invention can realize measurement only by using pure water standard samples without purchasing solutes and preparing solutions.
In particular, the invention employs a free induction decay pulse sequence with an interval of repeated sampling intervals t1And measuring the complete relaxation signal intensity of a plurality of pure water standard samples with different masses, and establishing a standard measurement curve according to the complete relaxation signal intensity of each pure water standard sample and the mass of the pure water standard sample. The accumulated sampling times are not particularly limited, and may be selected according to actual conditions, and it should be noted that, in general, the accumulated sampling times cannot be too small, otherwise, the testing accuracy may be affected. Preferably, the number of accumulated samples may be 32 or more when performing measurement, for example: 64 times, 96 times, 128 times, 160 times, 192 times, 224 times, etc., preferably: 32 to 156 times, etc.
For the standard measurement curve, the mass of the pure water standards is linear with the full relaxation signal intensity of each pure water standard. Specifically, a standard curve equation between the complete relaxation signal quantity of the pure water standard sample and the quality of the pure water standard sample is established:
Yi-max＝kX+b (1)
wherein: y isi-maxRepresenting the full relaxation signal intensity of standard water samples of different masses;
x represents the mass of a pure water standard sample;
b is a constant term due to operational and instrumental errors.
In the present invention, the repeated sampling interval time refers to the interval time between two samplings, i.e. the time interval between adjacent repeated scans of the sample. For a repeated sampling interval time t1Wherein the interval time between each two adjacent samples is the same, i.e. the repeated sampling interval time t1Is the same. For example: the interval between the first and second samples is 1000ms, the interval between the second and third samples is also 1000ms, and the interval between the third and fourth samples is also 1000 ms.
Preferably, the oversampling interval time t1Is 1500 to 10000ms, preferably 2000 to 5000ms, for example: 3000ms, 4000ms, 6000ms, 7000ms, 8000ms, 9000ms, etc.
Further, the invention can obtain the complete relaxation signal intensity of a plurality of pure water standard samples with different masses through a plurality of times of measurement for a long time.
In some embodiments, considering the use of the instrument and the long time consumption, the present invention can preferably obtain the full relaxation signal intensity of a plurality of pure water standards with different qualities by the following methods: using free induction decay pulse sequence, at intervals of repeated sampling interval time tcThe nuclear magnetic resonance signal intensity of the pure water standard sample is measured, a relation curve of the nuclear magnetic resonance signal intensity of the pure water standard sample and the repeated sampling interval time is established, and a relaxation time constant of the pure water standard sample is obtained; the accumulated sampling times are not particularly limited, and can be selected according to actual conditions, and similarly, the accumulated sampling times cannot be too small, otherwise, the testing precision is affected. Preferably, the number of accumulated samples may be 32 or more when performing measurement, for example: 64 times, 96 times, 128 times, 160 times, 192 times, 224 times, etc., preferably: 32 to 156 times, etc.
At interval of repeated sampling interval time t1Measuring the nuclear magnetic resonance signal intensity of a plurality of pure water standard samples with different masses, and combining the relaxation time constants of the pure water standard samples to obtain the complete relaxation signal intensity of each pure water standard sample; the accumulated sampling times are not particularly limited, and can be selected according to actual conditions, and similarly, the accumulated sampling times cannot be too small, otherwise, the testing precision is affected. Preferably, the number of accumulated samples may be 32 or more when performing measurement, for example: 64 times, 96 times, 128 times, 160 times, 192 times, 224 times, etc., preferably: 32 to 156 times, etc.
Wherein the sampling interval time t is repeatedcEach interval time in (a) is different.
Specifically, the nuclear magnetic resonance signal intensity and repeated sampling of pure water standard sampleSample interval time tcThe relation curve of (A) is generally an exponential curve, which can be expressed as
M＝a×(1-exp(-r×tc)) (2)
Wherein M is the nuclear magnetic resonance signal intensity of pure water standard samples with different masses;
r is the relaxation time constant of the pure water standard sample;
a is the net magnetization constant, i.e. the maximum possible value of the magnetization;
tcfor repeatedly sampling the interval time tc。
From the exponential curve, the relaxation time constant r can thus be obtained. Then using the oversampling interval time t1Measuring a series of X's of different massesiNuclear magnetic signal intensity Y of pure water standard sampleiThen using the formula:
Yi-max＝Yi/(1-exp(-r×t1)) (3)
wherein: y isiThe nuclear magnetic resonance signal intensity of pure water standard samples with different masses;
r is the relaxation time constant of the pure water standard sample;
Yi-maxthe full relaxation signal intensities of pure water standards of different masses;
t1for repeatedly sampling the interval time t1。
And calculating to obtain the complete relaxation signal intensity of pure water standard samples with different masses according to the exponential curve.
Note that for the oversampling interval time tcWherein the interval time between each two adjacent samples is different, i.e. the repeated sampling interval time tcEach interval time in (a) is different. For example: the interval between the first sampling and the second sampling may be 1000ms, the interval between the second sampling and the third sampling may be 1500ms, and the interval between the third sampling and the fourth sampling may be 2000 ms.
The number of pure water standards of different mass is not particularly limited in the present invention, and is generally at least 2, but in order to obtain a more accurate standard measurement curve, the number of pure water standards of different mass may be determined to be 3 or more, for example: 4, 5, 6, 7, 8, 10, 20, etc. In addition, when obtaining the relaxation time constant of the pure water standard sample, the pure water standard sample used may be one of the pure water standard samples, or may be another pure water standard sample.
For a repeated sampling interval time tcEach interval time in (2) may be sequentially increased or decreased, and may also be irregularly confused. For example: the interval between the first sampling and the second sampling is 2000ms, the interval between the second sampling and the third sampling is 1000ms, and the interval between the third sampling and the fourth sampling is 4000 ms. Preferably, the sampling interval time t is repeated in order to obtain the relation curve convenientlycMay be sequentially increased or may be sequentially decreased.
In some specific embodiments, the time t is spaced for repeated samplingcThe step size of (2) can be generally 1000 to 3000ms, for example: 1200ms, 1500ms, 1800ms, 2000ms, 2200ms, 2500ms, 2800ms, and so forth. The step length is the repeated sampling interval time tcThe difference between two adjacent intervals.
In some specific embodiments, the oversampling interval time tcIs 1000 to 20000ms, preferably 1000 to 17000ms, for example: 3000ms, 5000ms, 8000ms, 10000ms, 12000ms, 15000ms, etc.
< sample to be tested and Water content thereof >
The detection method of the invention can be applied to various feasible woods, so the sample to be detected can be the existing feasible wood. Generally, in the test, wood needs to be subjected to pretreatment such as cutting. Specifically, wood is cut to a feasible size for testing as a sample to be tested. The size of the sample to be measured is not particularly limited in the present invention, and may be determined according to the size of the test tube of the instrument. Specifically, in the invention, a sample to be measured can be cut into small samples of 5-20 mm (tangential) x 5-20 mm (radial) x 5-20 mm (longitudinal), that is, the length of the sample to be measured is 5-20 mm, the width of the sample to be measured is 5-20 mm, and the height of the sample to be measured is 5-20 mm. As far as possible no visible defects are present for the sawn timber that has been cut.
Then repeatedly sampling the interval time t at intervals2And measuring the complete relaxation signal intensity of the sample to be measured, and then obtaining the content of the water in the sample to be measured according to the standard measurement curve of the pure water standard sample. The accumulated sampling times are not particularly limited, and can be selected according to actual conditions, and similarly, the accumulated sampling times cannot be too small, otherwise, the testing precision is affected. Preferably, the number of accumulated samples may be 32 or more when performing measurement, for example: 64 times, 96 times, 128 times, 160 times, 192 times, 224 times, etc., preferably: 32 to 156 times, etc.
For a repeated sampling interval time t2Wherein the interval time between each two adjacent samples is the same, i.e. the repeated sampling interval time t2Is the same. For example: the interval between the first and second samples is 1000ms, the interval between the second and third samples is also 1000ms, and the interval between the third and fourth samples is also 1000 ms. In addition, the inter-sampling time t is repeated2With a time t between repeated sampling1May be the same or different; for example: oversampling interval time t2Is 1000ms, the oversampling interval time t1May be 1000ms, 2000ms, 3000ms, etc.
Preferably, the oversampling interval time t2Is 1500 to 10000ms, preferably 2000 to 5000ms, for example: 3000ms, 4000ms, 6000ms, 7000ms, 8000ms, 9000ms, etc.
And finally, obtaining the water content of the sample to be detected based on the total mass of the sample to be detected and the content of water in the sample to be detected.
Specifically, the water content of the sample to be measured can be calculated according to the following formula:
A＝m1/(m0-m1)×100％ (4)
wherein A is the water content percent of a sample to be detected;
m1g is the content of water in a sample to be detected;
m0is the total mass of the sample to be tested, g.
Compared with the traditional test method, the method disclosed by the invention has the advantages that the test speed is high, the test result is accurate, the sample does not need to be subjected to complex chemical treatment and the like in the test process, the sample loss and damage do not exist, and the green and environment-friendly test concept is fully embodied.
Examples
Embodiments of the present invention will be described in detail below with reference to examples, but those skilled in the art will appreciate that the following examples are only illustrative of the present invention and should not be construed as limiting the scope of the present invention. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products commercially available.
Examples
1) The method comprises the steps of correcting the central frequency and the pulse width of the nuclear magnetic resonance instrument by using a nuclear magnetic resonance instrument (NMRC 12-010V, the magnetic field intensity is 0.28T and the probe aperture is 24mm, which are produced by NYME (New Meyer of Suzhou) and a free induction decay pulse sequence and using a standard oil sample, wherein the number of sampling points is 1024, the repeated sampling interval time is 3000ms, the accumulated sampling times are 64 times, the echo time is 0.1ms, the analog gain is 20, the digital gain is 3 and the pre-amplification gain is 2.
2) Placing a standard sample of pure water (distilled water) in a nuclear magnetic test tube for measurement, and obtaining repeated sampling interval time t by adopting a free decay pulse sequencecThe range of (1) is 1000-17000 ms, the step length is 2000ms, and the initial value of the attenuation curve is recorded as the nuclear magnetic signal intensity M (figure 1). In addition, free decaying pulse sequences test othersThe parameters are set as follows: the cumulative sampling number is 64 times, and the echo time is 0.1 ms. FIG. 1 is a schematic diagram illustrating a process for obtaining a nuclear magnetic signal of a substance using a free induction decay pulse sequence. The free decay curve of water is consistent with the free decay curve shape of the sample to be tested. Thus, in the present invention, FIG. 1 is a free decay plot of a pure water standard, which corresponds to the penultimate measurement point of FIG. 2 corresponding to 15000 ms.
According to the nuclear magnetic signal intensity and the repeated sampling interval time t of the pure water standard samplecI.e., the formula shown in the following formula (2):
M＝a×(1-exp(-r×tc)) (2)
wherein M is the nuclear magnetic resonance signal intensity of pure water standard samples with different masses;
r is the relaxation time constant of the pure water standard sample;
a is the net magnetization constant, i.e. the maximum possible value of the magnetization;
tcfor repeatedly sampling the interval time tc。
Thereby obtaining the relaxation time constant r of the pure water standard sample, and the nuclear magnetic resonance signal intensity and the repeated sampling interval time t of the pure water standard samples with different qualities of the embodimentcThe index curve of (A) is represented by the following formula (2-1):
M＝18951.31×(1-exp(-4.1883×10-4×tc)) (2-1)
wherein the fitting coefficient R of the exponential curve20.9975 (fig. 2), it can be shown that the nuclear magnetic signal intensity of the pure water standard has a high exponential fit to the oversampling interval.
And then 6 pure water standard samples with different masses are placed in a nuclear magnetic test tube for measurement, and the nuclear magnetic signal intensity of the pure water standard samples is obtained by adopting a free attenuation pulse sequence. Wherein the free decay pulse sequence parameters are set as: oversampling interval time t13000ms, 64 accumulated sampling times and 0.1ms echo time.
Then calculated according to the following formula (3):
Yi-max＝Yi/(1-exp(r×t1)) (3)
wherein: y isiThe nuclear magnetic resonance signal intensity of pure water standard samples with different masses;
r is the relaxation time constant of the pure water standard sample;
Yi-maxthe full relaxation signal intensities of pure water standards of different masses;
t1for repeatedly sampling the interval time t1。
Substituting the nuclear magnetic resonance signal intensity Y of pure water standard samples with different massesiRelaxation time constant r and repeated sampling interval time t of pure water standard sample1So as to obtain the complete relaxation signal intensity Y of pure water standard samples with different massesi-max。
And then carrying out linear fitting once according to the complete relaxation signal intensity of the 6 standard water samples and the quality of the corresponding pure water standard samples in a standard curve mode (1). Wherein, the standard curve equation (1) between the complete relaxation signal quantity of the pure water standard sample and the quality of the pure water standard sample:
Yi-max＝kX+b (1)
wherein: y isi-maxRepresenting the full relaxation signal intensity of standard water samples of different masses;
x represents the mass of a pure water standard sample;
b is a constant term due to operational and instrumental errors.
To obtain a fitted straight line (1-1):
Yi-max＝-40.6549+38021.3977×X (1-1)
the fitting straight line (1-1) is a standard curve, and the fitting coefficient R of the standard curve20.9994 (fig. 3), indicating a high linear fit between the amount of nuclear magnetic signal and the total amount of moisture for the pure water standards.
3) Respectively prepared from poplar (A)1、A2) Acacia senegantis (B)1、B2) Firepower nan (C)1、C2) A small sample (12 mm. times. 12mm) was sawn from the sawn timber after the timber processing.And (3) immediately placing the small sample in a nuclear magnetic test tube after measuring the mass of the small sample, and measuring the nuclear magnetic signal intensity of the sample by adopting a free attenuation pulse sequence. The free decay pulse sequence parameters are set to: oversampling interval time t2Is 3000 ms; the accumulated sampling times are 64 times; the echo time is 0.1 ms.
4) The obtained nuclear magnetic signal intensity of the sample to be measured is substituted into the standard curve (1-1), and the moisture mass of the sample to be measured can be obtained as (nuclear magnetic signal intensity + 40.6549)/38021.3977. The moisture content of the wood can be calculated as sample moisture mass/(sample mass-sample moisture mass) × 100%.
After the nuclear magnetic resonance scanning is finished, the sample is dried according to the method specified in GB/T (determination) to measure the moisture content, and the moisture content is compared with the result measured by the method, and the specific result is shown in Table 1.
TABLE 1
The measurement results of the embodiment prove that the time domain nuclear magnetic resonance technology can be applied to the measurement of the water content, the measured water content of the wood is closer to the measurement result of the drying method, the method for measuring the water content by using the nuclear magnetic resonance is a quick and convenient measurement method, and a quick and nondestructive detection method is provided for the measurement of the water content.
It should be noted that, although the technical solutions of the present invention are described by specific examples, those skilled in the art can understand that the present invention should not be limited thereto.
Having described embodiments of the present invention, the foregoing description is intended to be exemplary, not exhaustive, and not limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen in order to best explain the principles of the embodiments, the practical application, or improvements made to the technology in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.
Claims (10)
1. A method for measuring the moisture content of wood is characterized by comprising the following steps:
using free induction decay pulse sequence, at intervals of repeated sampling interval time t1Measuring the complete relaxation signal intensity of a plurality of pure water standard samples with different masses, and establishing a standard measurement curve according to the complete relaxation signal intensity of each pure water standard sample and the mass of the pure water standard sample;
at interval of repeated sampling interval time t2Measuring the complete relaxation signal intensity of a sample to be measured, and obtaining the content of water in the sample to be measured according to a standard measurement curve;
obtaining the water content of the sample to be detected based on the total mass of the sample to be detected and the content of water in the sample to be detected;
wherein the sampling interval time t is repeated1Is the same, the oversampling interval time t2Is the same, and the oversampling interval time t1And a oversampling interval time t2The same or different.
2. The method of measuring moisture content of wood according to claim 1, wherein the full relaxation signal intensity of the pure water standard is obtained by:
using free induction decay pulse sequence, at intervals of repeated sampling interval time tcMeasuring the nuclear magnetic resonance signal intensity of a pure water standard sample, establishing a relation curve of the nuclear magnetic resonance signal intensity of the pure water standard sample and the repeated sampling interval time, and obtaining a relaxation time constant of the pure water standard sample;
at interval of repeated sampling interval time t1Measuring the nuclear magnetic resonance signal intensity of a plurality of pure water standard samples with different masses, and combining the relaxation time constants of the pure water standard samples to obtain the complete relaxation signal intensity of each pure water standard sample;
wherein the sampling is repeatedInterval time tcEach interval time in (a) is different.
3. The method of measuring moisture content of wood according to claim 2, wherein the resampling interval time tcIs sequentially increasing or sequentially decreasing.
4. The method of water content in wood according to claim 3, wherein the repeated sampling interval time tcThe step length in the process is 1000-3000 ms.
5. Method for measuring moisture content in wood according to any of claims 2-4, characterized in that the repeated sampling interval time tcThe interval time in (1) is 500-25000 ms, preferably 1000-20000 ms, and more preferably 1000-17000 ms.
6. Method for measuring moisture content in wood according to any of claims 1-5, characterized in that the repeated sampling interval time t1Or the oversampling interval time t2Each interval time in (1) is 1500-10000 ms.
7. The method of measuring moisture content of wood according to claim 6, wherein the repeated sampling interval time t1Or the oversampling interval time t2Each interval time in the time interval is 2000-5000 ms.
8. The method of measuring moisture content of wood according to any one of claims 1 to 7, wherein the cumulative number of samplings per measurement is 32 or more.
9. The method for measuring the moisture content of wood according to any one of claims 1 to 8, wherein the measurement is performed by a time-domain nuclear magnetic resonance analyzer.
10. The method of measuring moisture content of wood according to claim 9, further comprising the step of instrumental calibration of the time domain nmr analyzer prior to the measurement.
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