With noble metal thermocouples, we usually mean type R, S or B. These are thermocouples made of two dissimilar alloys of platinum and rhodium. The use of platinum has a number of technical advantages, but also a number of properties we need to take into account when designing instruments based on the use of sensors made out of these alloys.
Apart from its appealing looks, platinum is a fascinating substance, being on one hand the hardest metal and at the same time very ductile. This means it has a soft feel to it, until you try to cut it. For thermocouple applications, and incidentally as thermowell, it has an outstanding, sometimes unsurpassed, resistance to high temperature oxidation and acids. The measurement itself is very accurate when compared to the more common types such as type K or N.
At first glance the initial price of an instrument based on platinum wire seems high. In many cases however there is a possibility to do a trade-in, e.g. with our noble metal return program. The value of the returned platinum is so high that the real cost of ownership turns out to be lower than that of other alloys. We don’t re-weld old wire; the platinum and rhodium are returned to their state as molecules participating in the raw material for the production of new alloys or maybe jewelry.
When designing an instrument based on platinum, even though the material has appealing high-end properties, special care is essential. For example, repeated stress or strain needs to be avoided. Especially repeated compressive strain may lead to grain boundary slip, resulting into what is often described as bamboo-effect.
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