HWX-II (Constant Power Plane Heat Source Method) Intelligent Thermophysical Parameter Tester

This instrument provides an intelligent method for testing thermophysical parameters using a constant power planar heat source method. This system has the characteristics of high measurement accuracy, high degree of automation, and easy operation. The test results show that the testing errors of thermal conductivity and thermal conductivity are both less than ± 4%.
1: Summary
Thermophysical parameters of matter are one of the macroscopic physical quantities of matter, and are important fundamental parameters for various scientific research and engineering design. It includes thermal conductivity, thermal conductivity, specific heat, thermal expansion coefficient, and thermal emissivity, among which thermal conductivity and thermal conductivity are the main indicators of material thermophysical parameters.
At present, most of the instruments produced in China for measuring the thermophysical parameters of solid materials use potentiometers and ammeters to measure the thermal capacity of heaters and the potential and related parameters of thermocouples, and manually calculate the thermal conductivity and thermal conductivity. Its disadvantages are low degree of automation, poor universality, complex adjustment process, and significant influence of human factors on test results. The thermal conductivity tester produced abroad has a complex structure, is expensive, and is not easy to promote and use. Therefore, there is an urgent need to develop an automated instrument with high degree of automation, easy operation, fast experimental speed, high accuracy, and strong universality for measuring the thermal and physical parameters of substances.
There are many testing methods and corresponding testing instruments for measuring the thermal conductivity and thermal conductivity of materials. The testing method used in this instrument's "Intelligent Thermophysical Parameter Testing System" is the testing principle, implementation, and results of the constant power plane heat source method.
2: Testing principle
The fixing and heating parts of the specimen in the thermal physical parameter testing system using constant power planar heat source method
Sample 1, Sample 2, and Sample 3 are made of the same material that is fastened together with different thicknesses. The thickness of specimen 1 is δ, the thickness of specimen 2 is x1, and the thickness of specimen 3 is δ+x1. Place a pair of thermocouples between specimen 1 and specimen 2, as well as between specimen 2 and specimen 3, to measure the temperature rise of the upper and lower surfaces of specimen 2. Place a constant power flat heater between specimen 2 and specimen 3. If the length and width of specimen 2 are 8-10 times its thickness, the power of the heater is constant, and the heat capacity of the heater is zero. Under these conditions, specimen 2 can be regarded as an infinitely large flat wall and has no internal heat source. Connect the heater power supply, and the heater symmetrically provides heat to both the upper and lower surfaces, with q0 kcal/m2 on each side. At the moment when the flat heater is powered on, the initial temperature of the three test materials is uniformly equal to T. As time τ increases, the test materials will heat up, and the heat flow gradually transfers to the two sides away from the heater. During this process, the temperature change only occurs in the direction perpendicular to the flat heater.

Figure 1 Schematic diagram of heating and fixing parts of the test material
Under the above conditions, the thermal conductivity λ and thermal conductivity α of the test material can be calculated according to the following formula [1]:
Thermal conductivity coefficient
Thermal conductivity coefficient
In the formula ξ 2x1- directly retrieve from the table based on the measured quantity
θ (0, τ 0) - The temperature rise in the central area of the contact surface between specimen 2 and the planar heater at time τ 0.

Implementation of Three Testing Methods
According to the testing principle, the testing device consists of three parts: the specimen and specimen fixture, the heating system, and the data acquisition and processing of the microcontroller (Figure 2).

(The newly produced microcontroller control system has been changed to computer or laptop control)
Figure 2 Schematic diagram of thermophysical reference measurement device
The specimen is divided into three pieces, with the middle piece being thinner and the two sides being thicker. Add thermocouples between specimens and fix them with fixtures. The heating system includes a heater and a voltage regulator to generate stable heat. The microcontroller system processes data according to the algorithm provided by the testing principle and displays and prints the results.
The constant power planar heat source method for measuring material thermophysical parameters requires further exploration in terms of method principles and experimental techniques. To accurately determine the thermophysical parameters of materials, in addition to accurately measuring temperature and time, the following experimental conditions must also be met: (1) the tested sample is uniformly isotropic and its physical properties are constant; (2) The length and width of the sample are 8-10 times the thickness, indicating that the sample is semi infinite and has a uniform initial temperature; (3) Constant power planar heat source; (4) The heat capacity of the heater is zero. If the above conditions are not met, measurement errors will inevitably occur. Therefore, it is necessary to analyze and correct these error factors, control experimental conditions appropriately, and improve experimental equipment in order to obtain high accuracy [2-4].
3.1 System Hardware Design
The constant power planar heat source method intelligent thermophysical parameter testing system is a new generation testing system based on the 8031 microcontroller. It uses the microcontroller to perform various calculations on the measurement data, eliminating or reducing errors caused by interference signals, analog circuits, and human factors.
The system hardware consists of sensors, preamplifiers, channel control circuits, analog-to-digital conversion circuits, keyboard display and control circuits, print drivers and control circuits, microcontroller systems, system monitoring and backup protection circuits, system and heater power supplies, heaters, sample clamps, and other components. The hardware composition of the system is shown in Figure 3.

Figure 3 Hardware composition diagram of the testing system
The temperature voltage characteristic curve of the thermocouple is in exponential form, and this system uses a microcontroller calculation method to perform linear correction on it. In addition, the thermocouple calibration table is based on the thermocouple cold junction temperature being equal to 0 ℃. If the cold junction temperature is not equal to 0 ℃, the thermoelectric potential will change with the cold junction temperature, so it is necessary to calibrate the thermocouple temperature measurement circuit. This system uses AD590 integrated temperature sensor to compensate for the cold end of thermocouples. The hardware circuit and preamplifier circuit schematic of the thermocouple cold junction compensator based on the thermocouple connection law and the intermediate temperature law are shown in Figure 4.

Figure 4 Schematic diagram of thermocouple cold junction compensation and preamplifier circuit
The analog-to-digital conversion circuit adopts ICL7135 dual integration A/D converter, which can set various working states through the keyboard and display them in different forms on the monitor. The main function of the printing control circuit is to control the mechanical action of the micro print head. The system monitoring backup protection circuit is designed to prevent the microcontroller system from instantaneous power failure, power grid undervoltage, and software "runaway".
3.2 Software Design
The system software is an important component of the testing system, which includes modules such as system management, data computation, printer management, parameter settings, data acquisition and filtering, thermocouple thermoelectric potential temperature conversion, interrupt handling, clock management, etc. By organically combining them in a certain hierarchical structure through the system management module, various functions can be completed. The flowchart of the system management software is shown in Figure 5.

Figure 5 System management software flowchart
4 Test results
The thermal conductivity of polyurethane foam was measured using an intelligent thermophysical parameter testing system, and compared with the traditional testing instrument. The specifications of the tested materials are: test material 1:200 × 200 × 65mm; test material 2:200 × 200 × 22mm; test material 3:200 × 200 × 90mm. The test results are shown in Table 1. The experimental results show that the intelligent testing system has better reproducibility, mean square error, and relative error in measuring thermal conductivity values than traditional testing instruments. After considering the factors that affect the accuracy of the test, the thermal conductivity test error is less than ± 4%.
Table 1 Thermal conductivity test results of polyurethane foam (W/m. ℃)
Type of flat heat source method instrument
Intelligent Thermophysical Parameter Testing System for Experiment Times Traditional Thermal Conductivity Testing System
1 0.027985 0.02824
2 0.027947 0.02737
3 0.027836 0.02840
4 0.027875 0.02861
5 0.027652 0.02785
Average value 0.027859 0.02809
Mean square deviation 1.159776 × 10-4 4.39299 × 10-4
Relative error 3.18% 4.04%