CN101320007A - Measuring device for thermal conductivity of materials by probe method - Google Patents

Abstract

The invention discloses a needle probing method material thermal conductivity measuring device based on the principle of a line heat source. The couple and the heating wire are placed inside the probe tube, and the probe inserted into the measured material sends the temperature potential signal to the microcomputer processing system for processing, transformation and display; the regulated power supply provides a constant voltage for the electric heating wire. The heating power is constant, and its voltage range is controlled by the microcomputer processing system. The invention improves the measurement application range of the hot wire method, and the probe method can be used for any substance that can be inserted into the probe. , Just insert the probe into the material to be tested, and then the computer can automatically calculate the program, and the result can be seen on the computer, so it has the characteristics of convenient measurement and high precision.

Description

Measuring device for thermal conductivity of materials by probe method

technical field

The invention relates to a material thermal conductivity measurement device, in particular to a material thermal conductivity measurement device using a probe method.

Background technique

At present, a hot-wire method measuring instrument is used to measure the thermal conductivity of materials, but the existing hot-wire method mainly has the following problems: (1) the measurement range is narrow, and only a single material such as gas or liquid can be measured; (2) the operation Complicated, poor repeatability, accuracy cannot be guaranteed.

Contents of the invention

The present invention aims at the problem of narrow measuring range and complex operation of the existing hot wire method measuring instrument, and proposes a probe method material thermal conductivity measuring device, which transmits the data to the computer by using the probe method for heating and temperature measurement , and through the program set in the computer, the thermal conductivity of the material can be displayed in real time, the measurement range is wide, the data is accurate, and the operation complexity is reduced by relying on the computer.

Basic principle of the present invention: when heating an infinitely large object with a heating wire, the line heat source can be regarded as composed of countless point heat sources, and each point heat source continuously releases heat with a constant power. Then after a period of time, due to heat conduction, the temperature rise at a distance r of the off-line heat source in a medium with constant properties is:

$\mathrm{<span\; class="notranslate">\; \&Delta;T</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="4"\; label="Q"\; filenames="A20081004025300071.png"\; state="\{\{state\}\}">Q</figure-callout></span>}}{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; \&pi;\&lambda;</span>}}{<span\; class="notranslate">\; \&Integral;</span>}_{{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}^{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}\frac{{\mathrm{<span\; class="notranslate">\; e</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; u</span>}}}{\mathrm{<span\; class="notranslate">\; u</span>}}\mathrm{<span\; class="notranslate">\; du</span>}$

in:

$\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="r"\; filenames="A20081004025300071.png"\; state="\{\{state\}\}">r</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>}$

$\mathrm{<span\; class="notranslate">\; \&beta;</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; r</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\sqrt{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; at</span>}<span\; class="notranslate">\; )</span>}}$

In the formula:

Q: Power per unit length of the line heat source (w/m);

λ: thermal conductivity (w/m.K);

α: Thermal diffusivity (m2/s);

t, t0: time, initial time;

p: density (kg/m3);

Cp: specific heat (J/kg°C)

After sorting and deforming the above formula, when the diameter of the probe is small enough or the test time is long enough, then the above formula can be transformed into:

$\mathrm{<span\; class="notranslate">\; \&Delta;T</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="4"\; label="Q"\; filenames="A20081004025300071.png"\; state="\{\{state\}\}">Q</figure-callout></span>}}{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; \&pi;\&lambda;</span>}}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; ln</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; t</span>}}{{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>$

This formula is the principle equation based on the probe method to measure the thermal conductivity of materials.

According to the characteristics of the heating wire of the device we designed, as long as the size of the material to be tested is greater than 1.905mm (the diameter of the electric heating wire is 0.0762mm, then 0.0762mm×25=1.905mm), the material to be tested can be regarded as infinite large objects.

Based on the theoretical principle of the above-mentioned hot wire source, the technical solution of the present invention is: a probe method material thermal conductivity measuring device, which is characterized in that it includes a probe, a microcomputer processing system and a stabilized power supply, and the probe includes an electric heating wire , probe tube and thermocouple, thermocouple and heating wire are placed inside the probe tube, the probe inserted into the material to measure the temperature of the material will input the temperature signal into the microcomputer processing system, which will be processed, transformed and displayed by the microcomputer processing system The regulated power supply provides a constant voltage to the heating wire in the probe to make the heating power constant, and the voltage range of the regulated power supply is controlled by a microcomputer processing system.

The ratio of the length of the probe to the outer diameter is greater than 25, the heating wire adopts constantan whose resistance changes with temperature, and the surface of the constantan wire is coated with a polytetrafluoroethylene insulating layer, and the electric heating wire is folded in half Then insert it into the probe tube with the same length as the probe tube. The thermocouple is a T-type copper-constantan insulated thermocouple, the temperature measuring point of the thermocouple is arranged in the small hole in the middle of the probe tube, and the middle of the probe tube is filled with insulating thermal silica gel.

The beneficial effect of the present invention is that: the present invention improves the measurement application range of the hot wire method, and the probe method can be used for substances that can be inserted into the probe, regardless of solid, liquid, or gas; Repeatability and accuracy; because the computer is used to directly display the measurement results, only need to insert the probe into the material to be tested to go through the automatic calculation program of the computer, and the results can be seen on the computer, so it has the advantages of convenient measurement and high precision. high feature.

Description of drawings

Fig. 1 is the principle schematic diagram of probe method material thermal conductivity measuring device of the present invention;

The probe method material thermal conductivity measurement device is shown in Figure 1, including three parts: probe, microcomputer processing system and regulated power supply. The main function of the probe is to insert the measured material to heat it and measure the temperature of the material; the probe includes an electric heating wire 3, a probe tube 4 and a thermocouple 5, and the thermocouple 5 and the electric heating wire 3 are placed in the probe tube 4 When measuring, the temperature signal measured by the probe is input into the microcomputer processing system 1 through the data acquisition board connected with the microcomputer processing system 1 . The main function of the microcomputer processing system 1 is to control the whole test system, that is, to collect the temperature measured by the probe, process the temperature data, and control the voltage of the stabilized power supply 2 . The function of the voltage stabilizing power supply 2 is to provide a constant voltage for the heating wire, so that when the heating wire is heated, the heating power is constant.

The length of the selected probe is 53mm, the outer diameter is 0.7mm, and its length-to-diameter ratio is 75.7, which makes the ratio far greater than 25, which satisfies Blackwell's conditions well (the length-to-diameter ratio must be greater than 25). The heating wire 3 adopts constantan (0.0762 mm in diameter) whose resistance changes with temperature. The heating wire 3 is folded in half and inserted into the probe tube 4. The length of the heating wire 3 is the same as that of the probe tube 4. The surface of the heating wire 3 is coated with polytetrafluoroethylene insulation. Layer; thermocouple 5 adopts T-type copper-constantan insulated thermocouple (diameter 0.0762mm), and the temperature measuring point of thermocouple 5 is arranged in the small hole in the middle of probe tube 4, and does not lead out from the small hole. The middle of the tube 4 is filled with insulating heat-conducting silica gel, and the root of the probe tube is fixed on the insulating plate.

Using the principle of the hot wire method to measure the temperature rise of the probe at different times, the slope of the logarithm of the temperature rise with time is obtained, which is brought into the simplified formula calculated by the hot wire method, and then calculated according to the heating power of the probe The thermal conductivity of the material can be calculated. All calculation modes are set in the microcomputer processing system, and the material to thermal conductivity can be calculated quickly by obtaining the temperature-time curve combined with the heating power. The characteristics of the measuring device are that it is easy to use, has a wide range of applicable temperature, can measure frozen materials, is applicable to a wide range of materials, and has high precision.

Claims (4)

1, a kind of probe method material thermal conductivity measuring device, it is characterized in that, comprise probe, microcomputer processing system and stabilized voltage power supply, described probe comprises electric heating wire, probe tube and thermocouple, thermocouple and heating wire Placed inside the probe tube, the probe inserted into the measured material to measure the temperature of the material sends the temperature potential signal into the microcomputer processing system, which is processed, transformed and displayed by the microcomputer processing system; the regulated power supply supplies the electric heater in the probe The wire provides a constant voltage to keep the heating power constant, and the voltage range of the regulated power supply is controlled by the microcomputer processing system. 2. The probe method material thermal conductivity measuring device according to claim 1, characterized in that the ratio of the probe length to the outer diameter is greater than 25. 3. The device for measuring thermal conductivity of materials by probe method according to claim 1, wherein the electric heating wire used for the probe is constantan wire, and the surface of the constantan wire is coated with polytetrafluoroethylene for insulation layer, the electric heating wire is folded in half and then inserted into the probe tube with the same length as the probe tube. 4. The device for measuring thermal conductivity of materials by probe method according to claim 1, characterized in that the thermocouple is a T-type copper-constantan insulated thermocouple, and the temperature measurement point of the thermocouple is arranged in a small hole in the middle of the probe tube. In the hole, the middle of the probe tube is filled with insulating thermal silica gel.

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