DESIGN OF METAL HEAT CONDUCTIVITY MEASUREMENT USING PROBE METHOD

This study aims to design a tool that can measure the heat conductivity of metals by the Probe method. The design method used a heatflowing metal beam and an LM35 temperature sensor to measure temperature changes on the probe. This measurement system design is based on conduction heat transfer; meanwhile, the metal used is aluminum and carbon steel. For each tested metal, an LM35 temperature sensor is placed on the metal to find out that a temperature change has occurred. The conductivity value can be obtained from the temperature difference between the probe and the metal being tested, the temperature change of the probe, the temperature change of the metal being tested, and the heating time. This value is processed and calculated by the microcontroller into the value of heat conductivity. The measurement results show that the average heat conductivity value is 214.93 W/mC for aluminum and 53.81 W/mC for carbon steel.


INTRODUCTION
The thermal conductivity [1,2] of material is essential for planning heat exchanger equipment in factories, power plants, and the machining world. The nature of heat conductivity plays a role in the process of heat transfer conduction of different magnitude for each material, so it will be easier to choose the material to be used in the design depending on its function for heating, cooling, or also for heat-insulating. From an engineering point of view, the main problem is determining the rate of heat transfer at the specified temperature difference. It's determined by the test using the equipment that can be easily used according to demands.
By testing the heat conductivity of materials, which is one of the important physical properties to show how quickly the heat that flows in a particular material, we can find out whether a material can be classified as a conductor or as an insulator. Materials that have a high value of heat conductivity can be used as a conductor [3], and vice versa is used as an insulator. Therefore, it is necessary to design and measure the heat conductivity of materials and tests them.
The conductivity meter [4] is very needed in the testing laboratory. The more industries that require the heat conductivity of the made product, the more often the industry asks the value of heat conductivity in the testing laboratory. In addition to the lab and industry, researchers, in particular, who want to research new materials to be used as insulators [5], or new alloys for metals, also require heat conductivity measuring devices that are practical and easy to make.
Measurement of heat conductivity is critical in several engineering applications. The properties of this material are used as a measure of the used material quality. Specialized laboratories for measuring heat conductivity are needed [6]. The direction of measurement is also important for heat conductivity measurement, which is higher in the axial direction [7]. The rate of heat flow in the axial direction in two and a half times higher than the other direction [8].
This probe method has been developed previously [9,10]. The measurement of heat conductivity was done using a probe rod from hot-flowing metal and a temperature sensor in the middle of the probe heater to determine the temperature changes of the probe. The measurement results were not presented, only presents the calibration results of measuring instruments that have been made. For this reason, this research will design a heat conductivity measurement tool and prove the accuracy of the heat conductivity measurement results on metals by the probe method.

METHOD
The device made in this study is using heat propagation principle, conduction. Heat flows because there is a temperature difference between two objects. The heat flows from high temperature to low temperature. The system monitoring is designed uses CodeVisionAVR [11,12], which controls the traffic of this device system. CodeVisionAVR acts as a program | 71 SPEKTRA: Jurnal Fisika dan Aplikasinya Volume 5 Issue 1, April 2020 that instructs the system to work. CodeVisionAVR is then stored in Atmega8L. This device is equipped with various electronic components in carrying out the process on the system.
The used heat conductor in this tool is a conductor media. In principle, this tool serves to measure the value of metal conductivity. The using tool starts with heating the probe by a heater installed inside the probe until the probe is hot enough, according to a predetermined temperature, then, the probe can be affixed to the tested metal. The heat will transfer from the hotter probe to the cooler tested metals. So the tested metal at a specific time has a higher temperature than originally. The time required for the tested metal to reach a predetermined temperature will be calculated using a formula into the value of heat conductivity. The temperature sensor used is IC LM35 [13]. This sensor reads the initial temperature of the tested metal before it is heated. The sensor also reads the temperature of the heated probe and ensures that heat has been spread evenly across the probe so that the measurement can be started by measuring the temperature between the probe and the tested metal. The results of temperature and time measurements will be processed and calculated by the Atmega8L microcontroller [14], then the result, the heat conductivity, is displayed by the LCD. FIGURE 1 shows the design of heat conductivity measuring devices in the block diagram.

RESULT AND DISCUSSION
Measurements were made to determine the value of heat conductivity in metals. The metals used as the test sample are aluminum and carbon steel, meanwhile, the probe uses copper with a content > 99.9% of Cu. The probe is beam-shaped and has a mass of 0.236 kg, a 80 mm length, a 19 mm width, and a thickness of 20 mm. The heater that used as the probe is given a voltage of 12 V and a current of 2 A. The experiment results are presented in  T1 = the different temperature of the probe, before and after heating ( o C) T2 = the temperature difference of the tested metal, before and after it is heated ( o C) t = the heating time (s) m2 = the mass of the metal being tested (kg) c2 = the specific metal heats tested (J/kg. o C)

Metal c (J/kg. o C)
Copper 390 Aluminum 910 Carbon Steel 490    For the heat conductivity value to be more accurate, the temperature sensor used must have a higher-resolution value. is due to the rust on carbon steel 19 mm, which causes the value of heat conductivity on carbon steel 19 mm to fall.

CONCLUSION
In this research, the design of heat conductivity measurement using a probe method has been carried out. The probe used is in the form of blocks and is made of copper (content > 99.9% Cu). Metals used as testing materials are aluminum with two kinds of thickness and carbon steel with two kinds of thickness. The developed tool was able to show differences in the heat conductivity measurement results of the two different metals. Errors or inaccuracies of the measurements occur because this probing method requires a very flat cross-section of the probe and tested material. If it is not entirely flat, there will be a cavity that causes entering air, so the conduction process is not perfect, and it takes quite a long time for heat to move from the probe to the tested metal.