A resistance temperature detectors, abbreviated as RTD, are sensors used to measure temperature by correlating the resistance of the element with temperature. Each RTD has a pre-specified resistance value at a known temperature which changes in a predictable fashion. By measuring the resistance of the element, the temperature of that element can be determined from tables, calculations or instrumentation.
Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. They are usually made of metallic elements or alloys such as copper, nickel, or nickel-iron. The most linear, repeatable devices are made of platinum, a precious metal that is suitable for temperature measurements over a wide operating range. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it.