Resistive Temperature Devices (RTD) Information
Resistive temperature device (RTD) elements are wire windings or other thin-film serpentines that exhibit changes in resistance with changes in temperature. 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. RTD elements use two basic sensing technologies. Wire-wound devices consist of a coil of insulated wire wrapped around a ceramic or glass core. Thin-film devices also use ceramic substrates, but are smaller than traditional wire-wound devices. Multi-element products combine two or more RTD elements in a single sensor/transducer package to provide redundancy in case the primary element fails. Resistive temperature device (RTD) elements are often used with RTD probes and interfaced to RTD temperature transmitters, devices that convert resistance measurements to current signals.
Types of Sensors
Resistive temperature device (RTD) elements use two types of sensors: Standard (Class B) and Precision (Class A). Both types are defined by the International Electrotechnical Commission (IEC), a global organization that prepares and publishes standards for electrical and electronic technologies. Class B sensors are the industry standard for platinum RTD elements. They use a single ice-point calibration and provide an accuracy of .3° C for temperature measurements at 0° C. Class A accuracy is multi-point and more precise, providing tighter control of the temperature coefficient and an accuracy of .3° C at 0° C. Although the American Society for Testing Materials (ASTM) also uses Class A and Class B designations, these categories specify different permissible deviations than IEC standards.
Specifications for resistive temperature device (RTD) elements include operating temperature, nominal resistance, response time, minimum accuracy, length, and width or diameter. Operating temperature is the range of temperatures in which devices are designed to operate. Typically, the upper and lower 5% of the operating range is non-linear. For wire-wound RTD elements, nominal resistance is the electrical resistance of the wire coil. Response time is dependent upon the media in which devices are immersed. Because there is no single standard, response time is an approximation and varies widely among devices. Minimum accuracy is the allowable temperature difference from a given temperature. For rectangular resistive temperature device (RTD) elements, length is the longer side of the RTD housing while width is the shorter side. For circular devices, diameter is an important dimension to consider. Certifications and approvals for RTD elements include IASTM E1137, IEC 751, and DIN EN 60751.