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Thermal elements

There are several possibilities to measure termperatures in everyday life such as quecksilver- and alcoholthermometers, liquid crystal foils or electronic devices with very specific temperaturesensors.

All those devices only work belov 400°C. But there’s always the problem to measure the temperature at a specific point. This is where thermal elements are used. Thermal elements are a combination of two different metals or alloys. A tension emerges at their point of contact which is preferably soldered or welded. This tension is called electromotive force, also called EMF.

The tension is always proportional to the temperature and therefore it is possible to measure temperatures from -200°C to +100°C with a NiCr/NiAl combination and from 0°C to 2200°C with a W5/W26 combination.

Thermal element

The object that will be covered below is a conical component that simulates a rocket head. It is its task to determine the heating of the outer skin in a wind channel as gapless as possible. A high-grade steel frame assembled with conventional thermal elements has some disadvantages:

the expensive production process as well the the indirect placement on the surface.

A more elegant solution has been found through the production of a galvanical thermal element.

Thin NiCr wires were placed on a aluminium main frame for that. After a specific preprocessing the main frame was nickelplated (2mm) and the wireends were galvaniced in an adhering manner at the same time.

After the aluminium was etched out again only a conical nickelshell remained. This nickelshell with its 25 NiCr-wires and as many thermal elments is perfect for a precise measurement of temperatures on a rocket head that is tested in a wind canal.

This method makes possible to install several mesuringpoints on a very little space with just one leakage.

A combination of constantan and copper is, of course, also possible.