# Platinum as a Metallic Material

## Examples of Applications in Temperature Measurement Technology

Due to its noble behavior and the associated stability of its physical properties, platinum is used in two important areas of temperature measurement technology: thermocouples and resistance thermometers.

### Thermocouples

Fig.1
Immersion thermocouple for steel making

Three technically significant thermocouples are based on physically pure platinum and platinum-rhodium alloys. Compared with non-precious metal thermocouples, these have the advantage that they can be used up to high temperatures under oxidizing conditions.

Type S (designation according to ANSI1)) consists of platinum with PtRh90/10 and can be used for long-term measurements up to 1,600 $°C$. Short-term measurements can even be conducted right up to the melting point of platinum (1,769 $°C$), which becomes significant when the temperature of molten metal is to be determined by means of immersion thermocouples.

Thermocouples of the Type R consist of platinum and PtRh87/13. These have a similar range of application to the Type S thermocouples, but demonstrate a somewhat greater sensitivity (EMF per $°C$).

For long-term applications at temperatures up to 1,800 $°C$, thermocouples of Type B are recommended which consist of PtRh94/6 and PtRh70/30. This type has, however, the disadvantage of a lower sensitivity than the other types, thus essentially preventing its use below about 600 $°C$

### Resistance thermometers

Fig.2
Platinum sensor

Platinum has proved itself to be the most suitable metal for the resistance thermometer. For high precision measurements, thermometers are used that are made from coils of physically pure platinum wire or ribbon. They are then encapsulated in ceramic or glass. These normally have a nominal resistance of 100$Ω$ at 0 $°C$ (Pt100). Particularly for applications requiring large numbers of units in which the cost in relation to the function is an essential aspect, thin-film temperature sensors come into their own. These often have a higher nominal resistance of 500$Ω$ or 1,000$Ω$ (Pt500, Pt1000). The thin-film sensors have a substrate of e.g. Al2O3 with a photolithographically structured platinum thin-film. The nominal resistance is achieved by means of the meander-form of the structured platinum film. For low temperature applications up to about 170 $°C$ the sensor can be mounted directly onto a printed circuit board by SMT technology. For higher application temperatures up to about 850 $°C$ the sensors are usually encapsulated in ceramic.

 1) ANSI: American National Standards Institute
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