**3.4 Noble metal types**

Thermocouples employing platinum and platinum-rhodium alloys for their thermoelement (Noble-metal thermocouple types B, R and S) have been used for many years and exhibit a number of advantages over the base metal types. They are most resistant to oxidation, their thermoelements have higher melting points, and they have generally been found to be more reproducible at elevated temperatures in air. They are therefore used when higher accuracy and longer life is sought, though more experience with lower thermopowers.

Of all the standardized thermocouples, Type **S,** Platinum-10% rhodium (+) vs. Platinum (-) is the oldest and perhaps the most important. Type B, Platinum-30% rhodium (+) vs. Platinum-6% rhodium (-), was adopted as a standard type in the US in the late 1960s primarily to serve requirements in the 1200 to 1750oC range. At elevated temperatures, it offers superior mechanical strength and improved stability over types R and S, and it exhibits comparable thermopower. Its thermopower diminishes at lower temperatures and is vary small in the room-temperature range.

Identification of noble metal thermocouple wires is made difficult by the fact that all alloys are nearly identical in colour and all are non-magnetic. Sometimes it is possible to distinguish the positive wire from the negative one for types R or S by observing the 'limpness' of the wires. Pure platinum wires tend to be slightly more soft, or limp, while the rhodium-alloyed conductors are a little stiffer, enough so to permit identification. The differences, however, are subtle, and it is not possible to tell one rhodium alloy from another by these means. Proper connections for these thermocouples can be reliably determined by gently heating the junction and observing the resulting polarity on a sensitive indicator.
