Casing Metals for RTDs
A resistance temperature detector or RTD, is a sensor that can measure accurate temperatures by calculating the resistance of the unit with the temperature in the container. Most RTDs are made from thin wires wrapped around a ceramic core. To protect the unit, the resistance temperature detector is placed inside a core. The materials used to make the core vary, and have different strengths and weaknesses. The following five metals are used to make the outside core for RTDs:
Platinum: Platinum is the most commonly used metal to wrap an RTD. Platinum is a flexible metal that provides the greatest temperature accuracy of any other RTD container. Platinum is a precious metal, so the cost of this kind of sheath can be expensive.
Tungsten: Tungsten is a tough metal that is sometimes used to protect RTDs. Tungsten is ideal for using with probes that are in harsh conditions, such as constantly agitating liquids or harsh outdoor environments. It is rare to see tungsten cases, however.
Nickel: Nickel is another commonly-used casing for RTDs. Nickel is inexpensive, which makes it a popular choice for applications where extreme temperature accuracy is not required. Nickel cases are used in everyday industrial applications where small temperature inaccuracies are acceptable, which is usually the case for any process except the mixing of highly volatile ingredients.
Balco: Balco is a rare material that is sometimes used to case RTDs. This casing is made from a mixture of 70 percent nickel and 30 percent iron. Balco is the upgraded version of pure nickel RTDs, and has a much higher linearity than pure nickel. However, balco is used less often than platinum, because platinum is more accurate and lasts longer.
Copper: Copper RTDs are used in cases where the electronic current of the material is necessary to receive an accurate reading. Copper RTDs have a predictable resistance change with the rise and fall in temperature, which make it an inexpensive alternative to platinum RTDs.