Development Of A Double-guarded Hot Plate Thermal Conductivity Coefficient Measuring Instrument In Low Temperature

With the development of industry and the appearance of energy shortage, many kinds of heat insulators appear, so there are more demands on their performance. The methods of measurement of thermal conductivity are also improved. Guarded hot plate thermal conductivity measuring method with characteristics of wide measurement range and non-debugging before experiments, is used internationally.But with the fast development of thermal insulation technology and appearance of many new kinds of heat insulators, it is necessary to obtain material insulating data under any temperature conditions to make the insulating structure more economical and reasonable. The purpose of this dissertation is to develop thermal conductivity coefficient instrument under low temperature (temperature range: -20~50°C). This paper introduces the design and some experimental results of a new guarded hot plate thermal conductivity measuring instrument. The instrument, which has high measuring precision, is made up of double guarded hot plate apparatus and intelligent measuring-controlling system. The measuring results can be printed out and stored. Data can also be read when it is working, which are showed in graph and text format. Compared with similar instruments, the measuring speed is inproved to by a factor of 1/3. The most important thing is that it can work with no operator when it is working, and can be operated by people with no thermal knowledge.The main contents of the dissertation are as follows:1. After collecting the information of present thermal conductivity instruments, we put forward the characteristics they have and the aspects our instrument improved.2. Deduced the formula of thermal conductivity through analyzing the one-dimensional steady thermal theory of plate, and analyzed the factors which the thermal conductivity may be changed with. Designed the mechanical structure and drew them, which were prepared for the next experiments.3. Designed the temperature measuring circuit, heating circuit and cooling circuit for the three kinds of plates which is the heating plate, the guiding plate and the cooling plate.4. The cooling plate was used of semiconductor, which can cool the plate steady between -20℃and normal temperature. And the temperature error is lower than 0.1℃.5. PID technology was used in this instrument, and it solved the problem of manual operation, besides its presion was high, for the temperature error was lower than 0.5℃.6. Compile the program which was designed for the instrument using VB, which included the PID algorithm. It made the circuit communicate with the PC. This part realized the functions such as displaying the temperature of the three plates and the experimental result, printing, and saving experimental results in real-time.