Study And Improvement Of Automatic Calibration System For Thermocouple

It is generally recognized that the measuring accuracy of the thermocouple falls down after long-term application of it so that the periodic calibration of the thermocouple is strongly recommended.With the development of digital instruments and computer technology,the calibration of the thermocouple is becoming increasingly automatic.The thermocouple automatic calibration system mainly consists of the thermocouple calibration furnace,the digital multi-meter,the scanning switch,the temperature controller and the computer equipped with control software.This paper aims to study the measuring principle of the thermocouple as well as the composition and operation mode of thermocouple automatic calibration system.We also discuss some unfavourable situations that we meet in our practical work regarding the operation of the thermocouple.For instance,the uniform temperature field of calibration furnace often proves to be out-of-tolerance under the conditions of low temperature or overloaded setting mode.In order to improve the uniformity of the furnace to some extent,we adopt three section temperature control mode.We also summarizes the strengths and weaknesses of several operation methods of the reference temperature of thermocouple,including freezing point constant temperature method,compensating wire method,room temperature compensation method and compensating bridge method in this paper.By writing the automatic calibration software,we achieve the goal of debugging the whole system.The methods stated above were tested on the calibration devices of working base metal thermocouple during my work at Shanghai Institute of Measurement and Testing Technology which proved to be stable,effective in improving the uniform temperature field.The improvement of system performance is verified by evaluating the uncertainty of the improved system.