General principles of temperature measurement: study of J and K type thermocouples calibration

A.L. F. Coelho, C. C. O. Vieira, J.C. C. Campos

Resumo


This study aims to determine and analyze the calibration and hysteresis curves of K and J type thermocouples. From these studies were obtained the dynamic characteristics of the analyzed thermocouples. To determine the calibration and hysteresis curves the thermocouples were submitted to different temperatures in order to obtain the respective tension values. From the obtained voltage versus temperature curve it was possible to see that the generated voltage is linearly proportional to the difference of temperature between the hot and reference junctions. Then, the curve’s slope was obtained in order to determine the thermocouple’s Seebeck coefficient. The obtained values were50.4±1.1 µV/°C and 39.2±1.0 µV/°C for K and J type, respectively. Maximum hysteresis error of 3.71 % for J type and 3.04% for K type were obtained. Analyzing the hysteresis curves it was possible to conclude that this was a random error, since it is always present despite the experimenter’s level of caution. Submitting the K type thermocouple to a step temperature excitation it was possible to obtain a temperature versus time curve. From this curve the thermocouple’s time constant was determined to be 27.6 s. Mathematically, from an energy balance, it was determined that the J type thermocouple has a time constant of 21.8 s and the K type 27.1 s, that is, the J type thermocouple responds faster to temperature change.

Palavras-chave


medição de temperatura, termopar, curva de calibração, constante de tempo, erro de histerese.

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Referências


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