Resistance Temperature Detectors (RTDs) available today can generally be categorized into one of two basic types of RTDs, depending on how their temperature sensing element is constructed. One type of RTD contains thin-film elements and the other type of RTD contains wire-wound elements. Each type is best suited for use in certain environments and applications.
The invention of a resistance thermometer was made possible by the discovery that the conductivity of metals decreases predictably with increases in their temperatures. The first-ever resistance thermometer was assembled from insulated copper wire, a battery and a galvanometer in 1860. However, its inventor, C.W. Siemens, soon discovered that a platinum element yielded more accurate readings at a much wider range of temperatures. Platinum remains the most commonly used material in temperature measurement using RTD sensing elements today.
Thin-film RTD elements are made by depositing a very thin layer of metal (usually platinum) onto a ceramic substrate material. The metal film is laser cut or etched into an electrical circuit pattern that provides the specified amount of resistance. Lead wires are then attached, and a thin protective glass coating is applied to the entire element.
This type of RTD is popular because of its ruggedness, reliability and low cost. Thin-film elements are more resistant to damage from shock or vibration than other types of RTDs. Their flat profile makes for design flexibility, enabling their use in many different industrial control and instrumentation applications. Multiple resistance, tolerance, size and shape options are also available.
ln the wire-wound type of RTD, the sensing element is comprised of a small coil of ultra-thin wire (typically platinum). This wire coil can be packaged inside a ceramic or glass tube (the most common construction) or the wire can be wound around the outside of a ceramic or glass housing material. Wire-wound RTD sensing elements can easily be made into pt100 probes by mounting them inside metal tubes or sheaths. This protects them from their surroundings and increases their durability. They can also be fitted into custom housings.
Wire-wound RTDs (especially the inner-coil type) are the most accurate type of RTD. Those with glass cores can readily be immersed in many liquids, while those with ceramic cores can be used to accurately measure extremely high temperatures. However, they are more expensive to manufacture than thin-film types, requiring skilled labor and advanced assembly facilities. They also tend to be more vibration-sensitive.