RTDs (resistance temperature device) are temperature sensors that are commonly used in a variety of industrial applications including industrial boilers, petrochemical, exhaust gas monitoring and food processing. RTD sensors have a higher accuracy than thermocouples and thermistors over a wide temperature range, and are more stable over time.
Simply put, an RTD is a sensor whose resistance changes with temperature in a consistent repeatable manner. An RTD can provide highly accurate and consistent temperature measurements because the change in resistance of certain materials is so predictable. Most RTD sensors have a response time between 0.5 to 5 seconds and commonly feature a platinum based element, but can also be constructed with nickel or copper. RTDs made with platinum (also known as PRTs - platinum resistance thermometer) are used most often today due to their higher temperature capabilities, better stability and repeatability.
Probe type RTDs, such as the NOSHOK 900 Series, generally consist of a rigid probe with direct mounted connector or extension cable. Assembly type models, such as NOSHOK 910/915 Series, usually consist of a rigid probe assembled with a connection head (junction box). Direct immersion probes have the RTD protective sheath (the probe) welded to process fitting similar to temperature gauges - this offers better response but mechanical protection is limited. Access to a process under operation is also limited. Assemblies for Thermowells have the RTD probe typically spring loaded in the connection fitting - this ensures good .thermal contact and removes dead space in well tip.
RTD connection heads are available in cast aluminum, white polypropylene, and cast 316 Stainless Steel. White polypropylene is popular for sanitary and chemical applications, while Stainless Steel is often used in food, pharmaceutical, biotech and chemical applications. Aluminum and Stainless Steel are preferred for industrial applications. Intrinsically safe explosion-proof enclosures are also available for hazardous environments.
RTD terminal blocks are often used in junction heads for connecting sensors to signal wiring. Each position has 2 connections, one connection from sensor, the other for signal wire (Example: for a single 3 wire RTD, a 3 position terminal block is needed - 6 connections total - 3x2). RTDs can be used in conjunction with a thermowell to protect the instrument, and the probe can be replaced in some models without possibility for leakage. RTD Transmitters are used to convert the sensor signal to the industry standard 4 to 20 mA or to a voltage output signal, and allow signal calibration for precise outputs proportional to temperature.
RTDs are highly accurate, stable and repeatable temperature sensors, and are resistant to contamination under 660°C. Because of their high repeatability, they can accurately measure identical temperatures even when exposed to multiple heating and cooling cycles with nominal discrepancies, making them an ideal solution for many industrial applications. Click for more info on NOSHOK Industrial RTDs.