RTDs are designed based on the thermoresistive effect of metals. Although any metal could be used to measure temperature, the metal selected should have a high melting point and be corrosion resistant. Materials most commonly used for RTDs are platinum, copper, nickel, and molybdenum because of their chemical stability, highly reproducible electrical properties, availability in a near pure form (which ensures consistency in the manufacturing process), and a very predictable, near-linear R–T relationship. Table 1 shows the features of these metals.
Platinum is by far the most common RTD material because of its long-term stability in air, broad temperature range, ease of manufacture, and reasonable cost. In fact, platinum RTDs are used to define the International Practical Temperature Scale (IPTS) from the melting point of hydrogen (−259.34°C) to the melting point of silver (+961.78°C). Platinum RTDs are available with alternative nominal resistance R0 values at 0°C of 10, 25, and 100 Ω. Pt100 is dominant with the practical range from −240°C to 850°C, although special versions are available for up to 1000°C.
Nickel RTDs are preferred in cost-sensitive applications such as air conditioning and consumer goods. They are generally manufactured in higher resistance values of 1 or 2 kΩ with a simple two-wire connection (rather than the three- or four-wire connections common with platinum types). Nickel is less chemically inert than platinum and thus is less stable at higher temperatures. Nickel RTDs are normally used in the environmental temperature range and in clear air.
RTDs are recognized for their excellent linearity throughout their temperature range, while maintaining a high degree of accuracy, robustness, long-term stability, and repeatability. Some RTDs have accuracies as high as 0.01 Ω at 0°C. Common industrial RTDs drift less than 0.1°C per year, and others are stable and drift within 0.0025°C per year.