What Is an RTD? Element, Pt100 and Accuracy

An RTD, short for Resistance Temperature Detector, is a precision temperature sensor that measures temperature by detecting changes in the electrical resistance of a metal element, most often platinum. RTDs are widely used in industrial testing, engine development, battery thermal management, aerospace applications, and laboratory environments where accurate, repeatable temperature data is essential.

How Does an RTD Work?

As temperature increases, the electrical resistance of the metal element inside the sensor increases in a known, repeatable way. The measuring instrument passes a small current through the element, reads the resulting voltage, calculates resistance, and converts it to temperature. Unlike a thermocouple, which generates its own voltage through the Seebeck effect, an RTD is a passive sensor and always needs an external excitation current.

The most common element materials are platinum, nickel, and copper. Platinum is preferred for precision work because it is chemically stable and its resistance curve is nearly linear over a wide range.

What Is a Pt100? Are RTD and Pt100 the Same?

A Pt100 is a type of RTD, not a synonym for the term. It uses a platinum element with a resistance of 100 ohms at 0°C. A Pt1000 is the same design at 1,000 ohms, which makes lead wire resistance a smaller share of the total. Under IEC 60751, the standard temperature coefficient is 0.00385, so a Pt100 reads about 138.5 ohms at 100°C.

RTD Elements: Wire-Wound vs. Thin-Film

Wire-wound elements use fine platinum wire supported in a ceramic or glass body. They offer the widest temperature range and the best long-term stability, which makes them the choice for high-accuracy and higher-temperature work.

Thin-film elements deposit a platinum layer on a ceramic substrate and trim it to the target resistance. They are smaller, less expensive, respond faster, and tolerate vibration well because there is no free wire to move. Their upper temperature limit is lower than wire-wound elements.

2-Wire, 3-Wire, and 4-Wire RTDs

Because an RTD is read as a resistance, the resistance of its lead wires adds to the measurement. For a Pt100, one ohm of lead resistance reads as roughly 2.6°C of error, so the wiring configuration matters.

2-wire: Lead resistance adds directly to the reading. Suitable only for short runs or loose accuracy requirements.
3-wire: A third lead lets the instrument measure and subtract lead resistance, assuming the leads match in length and gauge. This is the standard industrial configuration.
4-wire: Two leads carry the excitation current and two separate leads sense the voltage across the element, so lead resistance has no effect. This is the configuration for laboratory and reference measurements.

RTD Accuracy: Tolerance Classes

IEC 60751 defines tolerance classes that state how far a Pt100 may deviate from the true temperature. Class A is ±(0.15 + 0.002·t) °C and Class B is ±(0.30 + 0.005·t) °C, where t is the temperature in °C. At 0°C that is ±0.15°C for Class A and ±0.30°C for Class B. At 200°C it is ±0.55°C and ±1.30°C. Each class applies only over a defined temperature range, which differs for wire-wound and thin-film elements, so a specification should state both the class and the range.

For comparison, a standard-limit Type K thermocouple is specified to the greater of ±2.2°C or 0.75% of reading, which is about ±2.2°C at 200°C.

Self-Heating and Response Time

The excitation current that makes the measurement possible also warms the element slightly. Instruments keep the current small, typically 1 mA or less for a Pt100, and the error is usually well inside the tolerance class. In still air or gas, where heat cannot escape quickly, it can become significant.

Response time depends on the mass of the element and sheath. A sheathed RTD responds more slowly than a fine-gauge exposed-junction thermocouple, which is the main reason to choose a thermocouple for fast transients.

RTD vs. Thermocouple: Which Do You Need?

RTDs are the better choice when:

Measurement accuracy within a fraction of a degree is required
The application operates in a moderate temperature range (nickel RTDs to approximately 300°C; platinum RTDs to approximately 600°C)
Repeatability across multiple test runs is a priority

Thermocouples are the better choice when:

Temperatures exceed the RTD operating range
The application involves high vibration, shock, or harsh mechanical environments
Faster response time is required
Cost and simplicity are factors

Many test programs use both: RTDs for reference and ambient points where accuracy sets the quality of the dataset, and thermocouples for hot, fast, or hard-to-reach points. For thermocouple types and selection, see Type J vs. Type K Thermocouple: How to Choose and K-Tec’s custom thermocouple manufacturer page.

What Factors Should Engineers Consider When Specifying an RTD?

Temperature range: Define minimum and maximum process temperatures before selecting element material and construction.
Accuracy class: Class A or Class B per IEC 60751. Tighter tolerance increases cost but may be required for calibration-critical applications.
Wiring configuration: 2-, 3-, or 4-wire, matched to the lead length and your instrument.
Response time: Sheath diameter, element construction, and installation depth all affect it.
Vibration and shock: Engine testing and dynamometer work call for careful mounting and, often, a thin-film element.
Sheath material: Stainless steel, Inconel, and other materials resist different media.
Termination: Lead wire type, connector, and length must fit your data acquisition system and installation.

RTD FAQ

What are the different types of RTDs?

RTDs are classified by element construction (wire-wound or thin-film), element material (platinum, nickel, or copper), nominal resistance (Pt100 and Pt1000 are the most common), and wiring (2-, 3-, or 4-wire).

How do I check whether an RTD is working?

Measure the element’s resistance at a known temperature, such as an ice bath at 0°C, where a Pt100 should read close to 100 ohms, and compare it with the IEC 60751 table for that temperature. An open circuit, a short, or a reading far from the table points to a failed sensor or damaged leads. Insulation resistance to the sheath is also worth checking on sensors that have seen moisture or heavy vibration.

Why is a 3-wire RTD used?

The third lead lets the instrument compensate for lead wire resistance, removing most of the error that a 2-wire connection would add.

Custom RTDs from K-Tec Systems

K-Tec Systems manufactures custom RTDs and thermocouples to exact customer specifications at our Ferndale, Michigan facility. Every assembly is built to your defined temperature range, element type, tolerance class, wiring configuration, sheath material, and termination requirements. Our thermocouples are produced to ISO/IEC 17025 standards, and all orders include a Certificate of Conformance.

We serve engineers and procurement teams in the automotive, aerospace, defense, medical, and drone industries. Whether you need a single prototype or a production run, K-Tec’s engineering team will work with you from specification through delivery.

Learn more about K-Tec’s thermocouple and RTD manufacturing capabilities or contact our team directly at 248-414-4100 or email contact@k-tecsystems.com.