In any process where accurate temperature monitoring is critical, from power generation and HVAC systems to food processing and pharmaceutical manufacturing, Resistance Temperature Detectors (RTDs) are one of the most trusted sensor technologies. Selecting the right RTD element is essential for achieving high performance, long-term stability and compliance with industry standards.
Ashcroft, a global leader in temperature and pressure instrumentation, has decades of experience helping customers specify, install and calibrate RTDs in demanding environments.
Read this article to learn about RTD sensing elements, how they work and how to select the right element based on industry requirements, construction, accuracy, wiring and environmental factors.
What is an RTD Sensing Element
An RTD element is the core sensing component of an RTD sensor. It's typically made from platinum and is designed to change its electrical resistance in a predictable way as temperature changes. The resistance is measured by a monitoring device, such as a transmitter or controller, and converted into a temperature reading.
The most common RTD elements include:
- Pt100: 100 ohms at 0 °C (32 °F)
- Pt1000: 1000 ohms at 0 °C (32 °F)
Ohm is the unit of measurement that is often represented with the symbol Ω.
Figure 1. OHM Unit of Measure
Built to Meet Strict International Performance Standards
Before discussing sensing element construction types or wiring methods, it’s important to understand the standard that governs RTD performance: IEC 60751. This international standard defines:
- The temperature coefficient of resistance (typically 0.00385 Ω/Ω/°C for platinum)
- Tolerance classes (e.g., Class B, A, AA)
- Resistance-temperature relationship curves (commonly referred to as "385 curves")
- Construction criteria for thin-film and wire-wound elements
Compliance with IEC 60751 ensures your RTD sensor will work predictably and interchangeably with other IEC-compliant components, such as transmitters, controllers and calibrators.
The Sensing Principle of RTDs
The RTD sensing principle is simple: as temperature increases, the electrical resistance of the platinum element increases in a well-defined, mostly linear way. For instance:
- A Pt100 element has 100.0 ohms at 0 °C.
- At 100 °C (212 °F) , it typically has 138.5 ohms, yielding a slope of 0.385 Ω/°C.
Because of this predictable behavior, RTDs are highly repeatable and offer exceptional measurement accuracy, especially when compared to thermocouples, which are more prone to drift and nonlinearities.
Types of RTD Elements: Wire-Wound vs. Thin-Film
RTD elements are built in different ways to meet varying performance needs. The two most common construction types are:
1. Wire-Wound RTD Elements
- Made with a fine coil of platinum wire wound around a ceramic or glass core.
- Offers the widest temperature range: –200 °C to +850 °C (–328 °F to +1562 °F).
- Highest accuracy and long-term stability, but generally more expensive and slower in response.
- Ideal for lab applications, aerospace, and high-precision process monitoring.
Figure 2: Wire Round Sensing Element
2. Thin-Film RTD Elements
- Platinum is deposited in a zig-zag pattern on a ceramic substrate, then protected with a glass or epoxy coating.
- Compact and cost-effective, with fast response times.
- Typical range: –50 °C to +400 °C (–58 °F to +752 °F).
- Used extensively in OEM sensors and industrial automation.
Figure 3. Thin Film Sensing Element
Choosing the right construction depends on your temperature range, durability needs and cost constraints.
Understanding RTD Accuracy and Tolerance Classes
Tolerance classes define how closely an RTD element’s output aligns with the ideal resistance-temperature curve. The tighter the class, the better the accuracy, but also the higher the cost and manufacturing complexity.
Figure 4. RTD Accuracy and Tolerance Classes (per IEC 60751)
For regulated industries like pharmaceuticals or food, Class AA is often required. For general process control, Class A or B may suffice.
PT100 vs. PT1000: Which Resistance Value is Right?
RTDs are most commonly available in Pt100 and Pt1000 variants.
Figure 5. Two Pt100 and Pt1000 Variants
Because Pt1000 elements generate a larger voltage drop, they are less impacted by lead wire resistance, making them ideal for 2-wire configurations and long-distance installations.
RTD Wiring Configurations: 2-, 3-, or 4-Wire?
The wiring method you choose for your RTD installation has a direct effect on measurement accuracy, particularly with lower resistance types like Pt100. There are three primary wiring configurations for RTDs:
- The 2-wire RTD. This is the most straightforward configuration, using a single lead wire from each end of the sensing element to the monitoring device. It is also the least accurate, as the total resistance measured includes both the element and the lead wires.
- The 3-wire RTD. By far, the most common circuit design for RTDs used in industrial processes. This configuration includes two wires, one side of the sensing element and one wire connected to the other side. This arrangement nulls the lead resistance of the two wires connected to the side of the element, increasing the measurement accuracy, so it is well suited for process applications.
- The 4-wire RTD. This configuration offers the highest level of accuracy. The 4-wire circuit connects two wires to each end of the sensor element, fully eliminating the influence of lead wire resistance from the temperature reading. It is more complex and costly than the 2-wire and 3-wire options, but is ideal for applications demanding the highest precision.
Figure 6. RTD Wiring Types
Key Selection Criteria: Match Element to Environment
When selecting an RTD element, keep these application-driven factors in mind:
Housing Compatibility and RTD Assemblies
RTD elements don’t operate alone. They’re embedded into protective housings and probes that suit the environment. Ashcroft offers a range of assemblies such as:
- S81 RTD Temperature Probe: Made with MI cables and is designed for high-vibration and industrial use.
- The S50 Industrial RTD: Made with MI cables, this instrument is designed to work with thermowells and is and is well suited for OEM applications, remote monitoring, hazardous locations, and extreme service environments.
Assemblies must match not only the process conditions but also the wiring method and installation type (e.g., direct mount, remote, sanitary, threaded, or flanged).
Ready to Learn More?
Now you understand why selecting the right RTD sensing element involves more than simply picking a resistance value. It is also about matching your system’s needs for accuracy, speed, durability and compliance. If you want to deepen your knowledge further, we have several related articles about temperature products:
- Which thermocouple lead wire is best for my application?
- How an RTD works
- When to use an RTD or Thermocouple
In the meantime, download our guide to learn more about Ashcroft's temperature measurement solutions.
