Thermocouple is a standard to measure temperatures and is widely used in industry and academia. Thermocouples are made by welding two wires with different materials at one end. The temperature of the welded junction is obtained by measuring the voltage between the two open ends. The physical principle of thermocouple is based on thermoelectric effect (Seebeck effect) where the temperature difference can be converted to electricity. The temperature and voltage conversion are tabulated into standard tables for various types of thermocouples made with different materials. These tables (or sometimes polynomial fitted equations) are often used to refer temperature from a measured voltage.
Thermocouples only perform point measurement of the junction temperature. The junction of the most commercially available thermocouples are mostly sphere-like but the geometry is not in a well-defined shape. The junction is also normally bigger than its adjoining wires.
This could introduce two problems:
1. When thermocouple is used to measure fluid temperature such as hot gas stream, there is more uncertainty in calculating heat transfer when the junction does not have a well-defined geometry;
2. The junction may have different temperature of its adjoining wires due to different radiation loss.
The merits of making a thermocouple with cylindrical junction is to solve the two problems to improve the accuracy of temperature measurement.
The following video shows how I welded the thermocouple under a microscope using a micro-torch.
The video is also available on youtube:
http://www.youtube.com/watch?v=HUriLTnkCQw
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