Development Of Temperature Control System For Guarded-hot-plate Apparatus

Thermal conductivity is one of the thermal properties of materials.Accurate measurement of thermal conductivity has very important theoretical significance and practical value.Guarded-hot-plate method is by far the most accurate absolute method to measure the thermal conductivity of adiabatic and thermal conductive materials.But in recent years,the National Institute of Metrology carried out the domestic thermal conductivity coefficient measurement results show that different laboratories,different sources of guarded-hot-plate method meter measurement deviation is up to ±15 %.Analyzing the comparison results,it is believed that a key reason for such obvious measurement differences is that the temperature measurement and control level of the guarded-hot-plate apparatus is not high.Because the thermal inertia of Guarded-hot-plate apparatus is very large,the temperature difference between the components interferes with the temperature control of the components and the overall device.A precise temperature control method and thermal control system are needed to improve the accuracy of thermal conductivity measurement by thermal conductivity meter.This thesis aims to study the optimization temperature control parameters of high temperature guarded-hot-plate method.The main contents of this paper are as follows:Based on the analysis of the basic principle of thermal conductivity measurement by the guarded-hot-plate method,the structure of the hot and cold plate and the distribution of the optimal position of the heating wire,the guarded-hot-plate apparatus is designed.The thermal control system is built by using precision platinum resistance thermometer,high precision thermometer,virtual PID controller and so on.The temperature control system software is designed based on Lab VIEW,and the functions of data acquisition and PID accurate temperature control are realized.Through the research of temperature control experiment and optimization parameter method,the accurate temperature control of thermal conductivity meter is realized in the range of normal temperature to 500 ℃.The stability and accuracy of temperature control are in line with the international most advanced NIST protective hot plate device.The main research results of this paper are as follows:1.Based on one-dimensional steady-state heat conduction theory and the principle of guarded-hot-plate method,a set of thermal conductivity apparatus with single sample is designed.The temperature control system is composed of two temperature control modes.The temperature of the metering plate is controlled by constant power and the temperature of the protective layer is controlled by PID.The temperature of the protective plate follows the temperature of the central plate,and the heat dissipation of the metering plate to the non-metering area is eliminated.2.A rapid temperature control strategy is designed.The control program is designed with segmented PID,and the heating sequence is designed in the overall test,so that the mutual influence between the various parts of the test device is reduced,and the temperature can be raised quickly and accurately controlled without obvious overshoot.3.The optimal parameter combinations corresponding to different temperature measuring sections are studied.The control parameters are optimized to make the target temperature rise quickly and stably.At the same time,the temperature of the edge protective layer,protective plate and cold plate can be controlled within ±0.01 ℃ for a long time.The extended measurement uncertainty is less than 1 %,which is consistent with the level of the most advanced NIST guarded-hot-plate apparatus in the world.In the measurement range of300 ℃～400 ℃,the system can control the temperature better after running for 6 hours and the temperature fluctuation of the circumferential protective plate,upper protective plate and cold plate can be maintained within 0.005 ℃.