A device and method for measuring the thermal conductivity of insulating materials

Abstract

This invention discloses a device and method for measuring the thermal conductivity of insulating materials. By placing the reference end of a thermocouple in the reference box of the material under test, aligning it with the position of the thermocouple's measuring end in the test box, this invention ensures that the measuring and reference ends are affected identically by external environmental temperature disturbances. When measuring the thermal conductivity of a material in high or low temperature environments, since the ambient temperature cannot be completely stable, when the ambient temperature fluctuates, the thermocouple reference and measuring ends are under the same heat transfer boundary conditions, and the effects will be synchronous. Furthermore, this invention ensures that when the hot wire is heated, the temperature of the thermocouple reference end is unaffected by the heating of the hot wire, thus guaranteeing that the thermoelectric potential measured by the thermocouple is caused by the temperature rise of the material in the test box.

Description

Technical Field

[0001] This invention belongs to the field of measuring instrument and equipment technology, specifically relating to a device and method for measuring the thermal conductivity of insulating materials. Background Technology

[0002] Thermal conductivity is an important parameter for measuring the thermophysical properties of materials and is one of the key performance indicators required for thermal calculations and the design of various industrial furnaces. There are many methods for measuring thermal conductivity, but the methods vary depending on the material, the application, the range of thermal conductivity, the geometry of the sample, the testing conditions (temperature, atmosphere, etc.), the required accuracy of the data, the measurement cycle, and the cost. The hot-wire method is currently one of the most widely used testing methods.

[0003] In existing technologies, the hot-wire method primarily involves inserting a linear thermal conductor and a thermocouple parallel between two samples, locally heating the hot wire, and determining the thermal conductivity by measuring the temperature rise at a certain distance from the hot wire. In this method, the two ends of the thermocouple are located at a point between the two samples and at a point outside the samples, respectively. The temperature difference between the two ends is obtained by measuring the thermoelectric potential between them, and then the thermal conductivity of the material is calculated. However, in this method, both ends of the thermocouple are affected by the hot wire, and because they are significantly affected by ambient temperature disturbances, the accuracy and precision of the test results decrease. Summary of the Invention

[0004] The purpose of this invention is to provide a device and method for measuring the thermal conductivity of insulating materials that provides accurate and precise test data, and is easy and accurate to operate.

[0005] The objective of this invention can be achieved using the following technical solution: A device for testing the thermal conductivity of insulating materials, characterized in that: it includes a test box for the material to be tested, a reference box for the material to be tested, a thermocouple, and a hot wire; both the test box and the reference box consist of two pieces of the material to be tested placed parallel to each other in an insulating shell; the reference end of the thermocouple is placed in the reference box, and the measuring end of the thermocouple is placed in the test box, with the position of the reference end in the reference box being consistent with the position of the measuring end in the test box; the hot wire is located on the center line of the two pieces of the material to be tested in the test box, and the thermocouple is parallel to the hot wire.

[0006] Furthermore, the distance between the thermocouple and the hot wire is 10-15 mm, which ensures that the temperature rise of the thermocouple measuring end is relatively stable and that the temperature rise is caused by the heat transfer of the material being measured.

[0007] Furthermore, the outer shell of the thermal insulation material is covered with a layer of heat insulation material. The thermal insulation shell and the heat insulation material can reduce the impact of ambient temperature disturbances during the measurement process.

[0008] Furthermore, the heating wire is a resistance wire with a small temperature coefficient, and the heating wire is provided with constant heating power by a constant current source.

[0009] Furthermore, the reference end of the thermocouple is located in the exact middle of the two test materials in the reference box; the temperature measuring end of the thermocouple is located in the exact middle of the two test materials in the test box; thus ensuring that the two ends of the thermocouple are not affected by the external environment, and are closer to an ideal state.

[0010] Another objective of this invention is to provide a method for testing the thermal conductivity of insulating materials. The method comprises a testing device for the thermal conductivity of insulating materials, a microcontroller, a digital switch, a dummy load, and a constant current source to form a measurement system. The thermoelectric potential signal obtained from the thermocouple and the control terminal of the digital switch are respectively connected to the microcontroller. The constant current source is connected to the hot wire and the dummy load via the digital switch. The specific testing steps are as follows:

[0011] 1) Determine the temperature coefficient α of the thermocouple and the resistance R0 per unit length of the hot wire;

[0012] 2) Connect the measurement system. First, connect the constant current source to the dummy load via a digital switch, set the heating current of the constant current source, and preheat the measurement system.

[0013] 3) After the thermoelectric potential signal is detected to be stable and without fluctuation, the constant current source is switched to the hot wire via a digital switch, the current I of the constant current source is recorded, and the thermoelectric potential signal ε(t) at time t and the thermoelectric potential signal ε(2t) at time 2t are recorded at the same time.

[0014] 4) Data Processing: The temperature rise of the material under test over time is θ(t) = ε(t) / α; once θ(t) and θ(2t) are determined, the following can be calculated: According to the calculated value E1(x) can be determined by consulting the international standard ISO 8894-2:2007(E), and then the thermal conductivity can be calculated.

[0015] Furthermore, the power of the dummy load is matched to the power of the hot wire, for example, it can be a resistance wire with the same resistance value as the hot wire. The dummy load eliminates the impact of device heating (such as chip heating) on ​​the measurement system. High heating power of the hot wire can cause the constant current source and other devices to heat up; therefore, the measurement system needs to be preheated to a stable temperature before measurement can be performed. However, to ensure the stability and uniformity of the internal temperature of the test box before measurement, the system cannot be preheated by heating the hot wire. The dummy load solves this problem. Moreover, when the power of the hot wire and the dummy load are matched, the current will not jump when the constant current source switches from the dummy load to the hot wire, and the measurement signal will not be interfered with.

[0016] Furthermore, in step 3), multiple sets of ε(t) at time t and the corresponding ε(2t) at time 2t are recorded. Then, in step 4), multiple sets of λ values ​​are obtained. The average of the multiple sets of λ values ​​is taken as the final thermal conductivity of the insulating material, which further reduces experimental and calculation errors.

[0017] Compared with existing technical solutions, the technical solution provided by this invention can achieve the following beneficial effects:

[0018] 1) By setting up the reference box for the material under test, the reference end of the thermocouple is positioned within the reference box, consistent with the position of the measuring end of the thermocouple within the test box. This ensures that the measuring and reference ends are affected identically by fluctuations in ambient temperature. When measuring the thermal conductivity of a material in high or low temperature environments, since the ambient temperature cannot be completely stable, the reference and measuring ends of the thermocouple are under the same heat transfer boundary conditions, and the effects will be synchronous when the ambient temperature fluctuates.

[0019] 2) The setting of the reference box for the material under test ensures that the temperature of the reference end of the thermocouple is not affected by the heating of the hot wire when the hot wire is heated, thus ensuring that the thermoelectric potential measured by the thermocouple is caused by the temperature rise of the material in the test box of the material under test.

[0020] 3) Using a dummy load can eliminate the impact of device heating (such as chip heating) on ​​the measurement system. The high power of the hot wire heating causes the constant current source and other components to heat up. Therefore, the measurement system needs to be preheated to a stable temperature before measurement, and the constant current source output and thermoelectric potential signal should be stable. However, to ensure the stability and uniformity of the internal temperature of the test box before measurement, preheating the system by heating the hot wire is not feasible. Setting a dummy load solves this problem. Furthermore, when the power of the hot wire and the dummy load are matched, the current will not jump when the constant current source switches from the dummy load to the hot wire, and the measurement signal will not be interfered with. Attached Figure Description

[0021] Figure 1 A schematic diagram of the structure of Embodiment 1 of the present invention;

[0022] Figure 2 System connection diagram of the test method in Embodiment 2 of the present invention;

[0023] 1. Thermocouple; 2. Test box for material under test; 3. Upper sheet of material under test; 4. Temperature measuring end of the thermocouple; 5. Lower sheet of material under test; 6. Hot wire; 7. Reference end of the thermocouple; 8. Reference box for material under test. Detailed Implementation

[0024] To more clearly illustrate the purpose and advantages of the present invention, the following embodiments, in conjunction with the accompanying drawings, will provide a more detailed description of the invention. The specific embodiments described below are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0025] Example 1

[0026] See appendix Figure 1 This invention discloses a device for testing the thermal conductivity of insulating materials, characterized by comprising a test box 2 for the material to be tested, a reference box 8 for the material to be tested, a thermocouple 1, and a hot wire 6. Both the test box and the reference box consist of two pieces of the material to be tested (3 and 5) placed parallel to each other within an insulating shell. The reference end 7 of the thermocouple is placed in the reference box, and the measuring end 4 of the thermocouple is placed in the test box, with the position of the reference end in the reference box coinciding with the position of the measuring end in the test box. The hot wire is located at the center line of the two pieces of the material to be tested within the test box, and the thermocouple is parallel to the hot wire. The thermocouple is a low-resistance resistance wire, and the heating source is a constant current source. An insulating shell of insulating material is added to the outside of the reference box to ensure that the measurement is not affected by the external environment. The specific working principle is as follows:

[0027] In an infinitely large sample, there is an infinitely thin and long linear heat source. The temperature change over time at various points within the sample can be determined using differential equations and the corresponding single-valuedness conditions. The heat conduction equation is:

[0028]

[0029] In the formula, r is the distance from the temperature measuring point inside the sample to the hot wire (m), and a is the thermal diffusivity of the material (m). 2 / s), λ is the thermal conductivity of the material (W / (m·K)), t is the heating time of the hot wire (s), q is the power per unit length of the hot wire (W / m), and θ(r,t) is the temperature rise of the sample at point r at time t (K).

[0030] Since r is fixed in this invention, θ(r,t) is abbreviated as θ(t).

[0031] If the heating power q of the hot wire is constant, then the temperature rise of the sample at point r at time t can be determined as:

[0032]

[0033] In the formula, E1(x) is the exponential integral:

[0034]

[0035] ratio It is a single-valued function of x, based on the ratio x can be calculated, and thus E1(x) can be calculated.

[0036] From equation (2), we can know that:

[0037]

[0038] The values ​​of and E1(x), and the corresponding function table between them, can be found in Table 2 of the international standard ISO 8894-2:2007(E), as follows:

[0039]

[0040]

[0041] In the experiment, we can measure θ(t), so we can obtain θ(2t) / θ(t), which, according to equation (4), is the value of θ(t). The value of E1(x) can be obtained by looking up this value in the table; then, the thermal conductivity λ can be calculated according to equation (2):

[0042]

[0043] Example 2

[0044] See appendix Figure 2 This invention discloses a method for testing the thermal conductivity of insulating materials. The method employs an insulating material thermal conductivity testing device, a microcontroller, a digital switch, a dummy load, and a constant current source to form a measurement system. The thermoelectric potential signal obtained from the thermocouple and the control terminal of the digital switch are respectively connected to the microcontroller. The constant current source is connected to the hot wire and the dummy load via the digital switch. The specific testing steps are as follows:

[0045] 1) Determine the temperature coefficient α of the thermocouple and the resistance R0 per unit length of the hot wire;

[0046] 2) Connect the measurement system. First, connect the constant current source to the dummy load via a digital switch, set the heating current of the constant current source, and preheat the measurement system.

[0047] 3) After the thermoelectric potential signal to be detected is stable and without fluctuation, the constant current source is switched to the hot wire via a digital switch, the current I of the constant current source is recorded, and the thermoelectric potential signal ε(t) at time t and the thermoelectric potential signal ε(2t) at time 2t are recorded simultaneously.

[0048] 4) Data Processing: The temperature rise of the material under test over time is θ(t) = ε(t) / α; once θ(t) and θ(2t) are determined, the following can be calculated: According to the calculated value E1(x) can be determined by consulting the international standard ISO 8894-2:2007(E), and then the thermal conductivity can be calculated.

[0049] We use resistance wires of the same material and length as the hot wire as a dummy load, so that when the current source switches from the dummy load to the hot wire, the current will not change abruptly and the measurement signal will not be disturbed.

[0050] We can measure multiple sets of t and 2t values, calculate multiple λ values, and then calculate the average to obtain the final, more accurate λ value.

[0051] Furthermore, the microcontroller can be connected to a computer, and the data collected by the microcontroller can be directly transmitted to the computer to calculate the thermal conductivity in real time.

[0052] The above specific examples illustrate the present invention and are merely for the purpose of aiding understanding, not limiting the scope of the invention. Those skilled in the art can make various simple deductions, modifications, or substitutions based on the concept of the present invention. These deductions, modifications, or substitutions also fall within the scope of the claims of this invention.

Claims

1. A method for testing thermal conductivity of an insulating material, characterized in that, The insulation material thermal conductivity testing device, a single-chip microcomputer, a digital switch, a dummy load, and a constant current source form a measurement system; the thermoelectric potential signal obtained by the thermocouple and the control end of the digital switch are connected to the single-chip microcomputer, and the constant current source is connected to the hot wire and the dummy load through the digital switch; the insulation material thermal conductivity testing device comprises a material testing box, a reference box, a thermocouple, and a hot wire. The test box and the reference box are placed in parallel in the heat preservation shell. The reference end of the thermocouple is placed in the reference box, the temperature measuring end of the thermocouple is placed in the test box, the position of the reference end in the reference box is consistent with the position of the temperature measuring end in the test box, the hot wire is located on the center line of the upper and lower test materials in the test box, and the thermocouple and the hot wire are parallel. The specific test steps are as follows: 1) determining the temperature coefficient of the couple and the resistance of the hot wire per unit length ; 2) Connect the measurement system, connect the constant current source to the dummy load through the digital switch, set the heating current of the constant current source, and preheat the measurement system. 3) After the thermoelectric potential signal is detected to be stable and without fluctuation, the constant current source is switched to the hot wire via a digital switch, the current I of the constant current source is recorded, and the thermoelectric potential signal at time t is recorded at the same time , and the thermoelectric potential signal at time 2t ; 4) Processing data: the temperature rise of the material to be tested over time is ; when determining and , the value of can be calculated; according to the calculated value , the international standard ISO 8894-2:2007(E) is consulted, and can be determined, so as to calculate the thermal conductivity .

2. The insulation material thermal conductivity testing method of claim 1, wherein the distance between the thermocouple and the hot wire is 10-15 mm.

3. The insulation material thermal conductivity testing method of claim 1, wherein the heat preservation shell is covered with a layer of thermal insulation material.

4. The insulation material thermal conductivity testing method of claim 1, wherein the hot wire is a resistance wire with a small temperature coefficient, and the hot wire is provided with a constant heating power by the constant current source.

5. The insulation material thermal conductivity testing method of claim 1, wherein the reference end of the thermocouple is located at the center of the two test materials in the reference box, and the temperature measuring end of the thermocouple is located at the center of the two test materials in the test box.

6. The insulation material thermal conductivity testing method of claim 1, wherein the power of the dummy load matches the power of the hot wire.

7. The method of claim 1, wherein the step 3) records a plurality of t time point and corresponding 2t time point , and the step 4) obtains a plurality of values, and the plurality of values are averaged as the final thermal conductivity of the insulation material.

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