Base metal thermocouples drift rate dependence from thermoelement diameter

2014 Joint IMEKO TC1-TC7-TC13 Symposium: Measurement Science Behind Safety and Security

Pavlasek, P.^1 ; Duris, S.^1 ; Palencar, R.^1

Slovak University of Technology, Faculty of Mechanical Engineering, Bratislava, Nam. Slobody

17, Slovakia^1

关键词: Irreversible changes;    Long term stability;    Manufacturing process;    Material constant;    Production quality;    Safety and healths;    Stability issues;    Systematic changes;   

Others  :  https://iopscience.iop.org/article/10.1088/1742-6596/588/1/012016/pdf
DOI  :  10.1088/1742-6596/588/1/012016

来源: IOP

【 摘 要 】

Temperature measurements are one of the key factors in many industrial applications that directly affect the quality, effectiveness and safety of manufacturing processes. In many industrial applications these temperature measurements are realized by thermocouples. Accuracy of thermocouples directly affects the quality of the final product of manufacturing and their durability determines the safety margins required. One of the significant effects that affect the precision of the thermocouples is short and long term stability of their voltage output. This stability issue occurs in every type of thermocouples and is caused by multiple factors. In general these factors affect the Seebeck coefficient which is a material constant, which determines the level of generated voltage when exposed to a temperature gradient. Changes of this constant result in the change of the thermocouples voltage output thus indicated temperature which can result in production quality issues, safety and health hazards. These alternations can be caused by physical and chemical changes within the thermocouple lead material. Modification of this material constant can be of temporary nature or permanent. This paper concentrates on the permanent, or irreversible changes of the Seebeck coefficient that occur in commonly used swaged MIMS Type N thermocouples. These permanent changes can be seen as systematic change of the EMF of the thermocouple when it is exposed to a high temperature over a period of time. This change of EMF by time is commonly known as the drift of the thermocouple. This work deals with the time instability of thermocouples EMF at temperatures above 1200 °C. Instability of the output voltage was taken into relation with the lead diameter of the tested thermocouples. This paper concentrates in detail on the change of voltage output of thermocouples of different diameters which were tested at high temperatures for the overall period of more than 210 hours. The gather data from this testing was used to establish the relation between the level of EMF drift and the lead diameter of the thermocouple thermoelements. Furthermore this data was also used to create a drift function which mathematically expresses the dependency between the drift rate and the diameter of the thermocouple leads.