The criterion for selecting a material to make an RTD is:
- The material must be malleable so that it can be formed into small wires
- It must have a repeatable and stable slope or curve.
- The material should also be resistant to corrosion.
- The material should be low cost.
- It is preferred that the material have a linear resistance verses temperature slope.
Some of the common RTD materials are platinum, copper, nickel, Balco (an alloy of 70% nickel and 30 % iron).
These metals have the advantage that they can be manufactured to a very high degree of purity and are, consequently, available with highly reproducible temperature/resistance characteristics.
These metals can also be drawn to a fine diameter wires required in resistance thermometry.