What is a Resistance Temperature Device?
Resistance temperature devices (RTDs) provide accurate process temperature readings within thermal fluid systems. Typically made from platinum, RTDs use known mathematical relationships between resistance and temperature to measure a fluid’s heat.
RTDs go by a variety of names, including resistance temperature detector, pt-100, platinum resistance temperature detector, and resistance thermometer. Regardless of what they’re called, these devices generate accurate, repeatable readings at temperatures up to 900° F, making them ideal tools for monitoring temperatures within fluid systems.
HOW DOES AN RTD SENSOR WORK?
As the temperature increases in a metal, resistance also increases. RTDs use this relationship to measure the heat of a process fluid.
For some metals such as platinum, this relationship is nearly linear and remains stable across a wide temperature range. As a result, the resistance measured in a platinum element corresponds closely to its temperature-and therefore the temperature of the surrounding fluid, when installed in a thermal fluid system. Copper wires transmit the actual RTD element’s resistance to a measuring instrument, which computes the associated temperature.
HOW MANY WIRES SHOULD BE USED IN AN RTD?
The number of wires is an important consideration in RTD design. With a theoretical ideal wire, the resistance registered by the measuring instrument would equal the resistance of the platinum element. In reality, the wires themselves have their own levels of resistance which must be compensated for to ensure accurate readings.
For example, a two-wire system does nothing to account for the resistance of the wires, leading to a less accurate temperature reading. Three- and four-wire RTDs, on the other hand, are carefully designed to cancel out the resistance of the wires, drastically increasing accuracy. As a basic rule of thumb: the more wires, the more accurate the reading. However, three-wire systems are sufficient for most common industrial applications.
WHAT ARE THE COMPONENTS OF AN RTD?
The two critical elements of an RTD are the platinum resistance element, which functions as the sensor, and the wire configuration, which communicates that sensor’s reading. Other important elements include:
- Outside diameter (OD). The OD measures the wire and its surrounding insulation-typically no thicker than 0.5″.
- Tubing material. The tube housing is often made of stainless steel, although more temperature-resistant metals might be chosen depending on the application.
- Process connection. Various types of pressure fittings can be used as the process connection with many of the options resembling those used in thermocouples.
- Cold end termination. At the cold end of the wire, an RTD can terminate into a plug, wire, or terminal head, among other options, depending on the measuring instrument.
RTDs offer many benefits when used in thermal fluid systems:
- Highly accurate measurements
- Stable metal construction
- Consistent repeatability
- Functional in high temperature ranges
RTD VS. THERMOCOUPLE
RTDs are often confused with thermocouples, and while similar, thermocouples derive temperature measurements from the voltage change across two metals rather than the resistance of one.
Since their functionalities are so similar, it can be difficult to choose between a thermocouple and an RTD. As a general rule, RTDs yield more accurate and repeatable measurements thanks to their use of a highly stable metal sensor. They can also be easier to calibrate for the same reason.
Thermocouples, by comparison, are more complicated to configure and still generate less accurate results. However, there are a couple of reasons to choose a thermocouple over an RTD in certain applications. Thermocouples function at higher temperatures than RTDs and they can also be substantially cheaper, making them an excellent option where pinpoint accuracy isn’t necessary.
Thermal Fluid Systems carries and uses both types of sensors, and our expert staff can help you determine which is best for your application.
THERMAL FLUID SYSTEMS AND HIGH-PERFORMANCE RTDS
At Thermal Fluid Systems, we are experts in the design and construction of thermal fluid systems. One of our core considerations for every project is effective regulation, and RTDs present a means to accurately assess system metrics.