Design And Development Of An Automatic Verification System Of Thermocouple Based On National Regulations

Being an efficient sensor for contact temperature measurement, thermocouple has been widely used in many fields such as petroleum, chemical, steel, metallurgy,machine making, aerospace, and so on. Accuracy of thermocouples plays a vital role in quality of manufacture and research, and the quantity transfer of temperature.In traditional verification of thermocouple, temperature controlling, values reading, calculating, and results obtaining are all operated by human beings, which leads to a heavy and complicated work. In general, the vreification of four thermocouples can be finished by a skilled metrological verifier in a working day of eight hours. It is also difficult to control the desired oven temperature during the measurement. Meanwhile, the influence of artificial reading and disturb in the process makes the gross error more evident. In order to raise the accuracy of the verification and the work efficiency, it’s significant to establish an automatic verification system for thermocouples that ensures accurate and reliable value of measurement and monitor the measurement in real-time. Based on JJG35-1996 and JJG141-2013,technical indices of system hardware and software function satisfy the requirements of the use of statutory metrological verification institutions.The automatic verification system is divided into two parts, hardware and software. The hardware, which is used to measure the indication error of different thermocouples above 300℃, consists of thermocouple furnace, temperature controller,PID temperature indicator-controller, digital multimeter, scanning switch, computer,and printer. Among these, thermocouple furnace, temperature controller and zero-thermostat provide the constant temperature field; digital multimeter and scanning switch compose the data acquisition unit, which transfer the data into the computer for data processing, displaying and saving, and thus printing the verification record and certification. The automatic verification can then be realized.The computer of the automatic verification system and the software are the core of controller and verification system. The main functions of the software can be enumerated as follows: system parameter settings, verification parameter settings,equipment communication inspecting, process monitoring, data reading, data calculating, and result judging, etc. The original data record can also be saved and consulted.In this work, two verifications of different types of thermocouple are finished to test the working progress and verification efficiency of the system. An example of an S working noble metal thermocouple measurement is conducted to analyze the main source of uncertainty according to the specifications. In accordance with the obtained expanded uncertainty, it can be concluded that the values at every point are all less than a third of the amplitude of maximum permissible error with the corresponding model. Results show that the automatic verification system is properly designed, and the accuracy of measurement can meet the demand for routine verification work of thermocouples.