What is an RTD?
An RTD — or resistance temperature detector — is a preferred tool for measuring temperatures. This device is used in a large range of industrial applications where accurate temperature data is needed regularly. The RTD comes with multiple advantages that aren’t included in other temperature probe sensors, such as thermocouples. Because of their high accuracy and wide operating range, RTDs are usually the preferred method, even though they require a slightly larger investment.
An RTD delivers a special type of input signal, much like a digital or analog signal. Just as you could input a digital or analog signal into a programmable logic controller (PLC), you can input an RTD into a PLC. This would allow for the PLC to monitor temperature through the RTD.
RTDs are a relatively simple and reliable device that you can count on for getting the most accurate temperature reads, as long as the devices are maintained with a recommended calibration schedule.
How do RTDs work?
RTDs are built with a pretty straightforward design that gives a reliable read, particularly between a certain, wide temperature range. An RTD works by using the correlation between a metal’s temperature and the metal’s resistance in order to provide a read on the temperature. There are several metals that can be used in an RTD, each with a specific temperature to resistance ratio. The resistive element can come in several different shapes, like as a wire, or as a thin film of metal.
Measurement through an RDT happens at the element. This portion of the device, made up of the metal in one of the aforementioned shapes, will collect the resistance data required for an accurate temperature reading. When the element is inserted to measure the process temperature, the metal inside of the element heats up, and as its temperature increases so does the metal’s resistance to the electrical flow. As the RTD resistive element grows warmer, its electrical resistance increases and gets measured in ohms. RTDs are passive devices, so in order to provide a reading, they need to receive a small electrical current from an outside source.
RTD elements are often made with platinum, nickel, or copper. The different metals vary in their correlation between resistance to electrical flow and their change in temperature. Knowing the measurement of each metal’s resistance lets you determine the temperature through the RTD.
Different types of RTD elements
There are multiple different element types that can be used to read temperature through resistance in the RTD. These vary from which metal is used for the element, to the design of the element. The type of elements needed in an RTD can be determined by what instrument you’ll use to read the sensor.
Thin-film element In an RTD with a thin-film element, the resistant metal — usually platinum — is adhered as an extremely thin layer onto a substrate plating. This substrate is usually made of ceramic. The entire film is then covered in a protective glass to help protect the element. This type of RTD element is not as stable as an alternative like a wire-wound element and has a limited temperature range.
Wire wound element An RTD with a wire-wound element is made with a wire of pure metal — whether it’s copper, platinum, or nickel — wrapped around a ceramic core. The element is generally fragile, so it’s protected with the sheath to help increase its longevity.
This wire-wound design provides the greatest accuracy and can measure a wider range of temperatures, so it is often preferred for these reasons. However, if the fragile element becomes stressed or damaged, it can cause tiny errors in measurement over time.
Coiled element Although wire-wound elements used to be the go-to, coiled elements have now become the most common type of RTD in the industry. Like the wire-wound element, the coiled element uses a wire coil, but this one is kept in its coiled shape with the help of some mechanical design. The coil is housed in a ceramic tube and packed within a fine ceramic powder, allowing the element to move without compromising its shape. This way, the RTD can continue to provide highly accurate measurements without the potential of stress or damage to the element that could affect the accuracy of the reading.
Why use an RTD?
There are several reasons why one might choose to use an RTD over an alternative like a thermocouple. The biggest draw to an RTD is that this tool provides a highly accurate temperature read. They’re widely considered as one of the most accurate temperature sensors around. In cases where temperature measurement is absolutely critical, and RTD is definitely the best option. To maintain this level of accuracy, an RTD should be calibrated as needed.
Part of what keeps the RTD so accurate in so many use cases is the tool’s stability and repeatability. As long as an RTD is calibrated correctly, you’ll always get the same, exact read for the temperature. This is because the known resistance of any of the given metals that may be used in the element will always remain the same.
RTD tools have a wide operating range, particularly when they’re designed with platinum elements. For this reason, as well as others, platinum is a common metal used in RTDs. RTDs can provide accurate measurements for temperatures as high as 650°C, which makes them better alternatives to options like thermistors.
However, any higher than that and it may be best to opt for a thermocouple instead, which can tolerate even higher temps. Anything above 660º C and the metal in an RTD can be contaminated by the protective sheath and give off an inaccurate reading.
In most industrial applications, an RTD is the best way to go when it comes to getting an accurate temperature read. They’re essential for critical accuracy, great for high temps, and are fairly immune to electrical noise. Because of their sheath covering, RTDs are also good options in harsh environments where their casing will keep them protected.
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Phil Wiseman is Chief Marketing Officer at Alliance Calibration. He earned a B.S. in Chemical Physics from Centre College. Phil is an ASQ Certified Quality Auditor and ASQ Certified Manager of Quality/Organizational Excellence.