Types of RTDs (Resistance Temperature Detectors) are widely used for temperature measurement due to their high accuracy and stability. RTDs work based on the principle that the resistance of metals changes with temperature. Depending on the material used, RTDs have different performance characteristics suitable for various applications in industries, research, and environmental monitoring.
What is An RTD?
An RTD (Resistance Temperature Detector) is a type of temperature sensor that measures temperature by correlating the resistance of the sensing element with temperature. As temperature increases, the electrical resistance of the RTD material (usually a metal) increases in a well-defined, nearly linear manner.
RTD Components
An RTD consists of several major components: the resistance element, the wiring method, the sheath and the housing. The most critical component is the resistance element, which directly determines the performance and accuracy of the RTD.
Resistive Element: This is the core component of the RTD and is usually made of materials such as platinum, nickel, copper or tungsten. The resistance of the resistive element varies with temperature. Temperature changes result in small changes in resistance, and by accurately measuring this change, the RTD is able to provide highly accurate temperature values.
- Size and construction
Wire-wound element: realized by winding fine wires around a ceramic or glass core, high precision but slightly slower response.
Thin-film elements: Made by depositing a thin metal film on a substrate, they are small and responsive, but slightly less stable.
Encapsulation: Elements can be exposed or placed in a protective sleeve, such as a metal or ceramic jacket, to increase resistance to mechanical damage and chemical corrosion.
- Self-heating effect
When the current through the resistive element is too high, the element itself will heat up and affect the measurement accuracy.
Solution: Measure with a low current (usually 1mA or less) and compensate for the resistance of the wire through a 3- or 4-wire connection.
- Errors and tolerances
RTD components are generally categorized into different accuracy classes:
Class A (Class A): High accuracy with an error of ±(0.15°C + 0.002|t|).
Class B: Normal accuracy with ±(0.3°C + 0.005|t|).
1/3 DIN and 1/10 DIN: Higher accuracy standards than Class A.
Wiring: RTDs can be wired in 2-wire, 3-wire, and 4-wire configurations. 2-wire configurations are simple and low-cost, but are susceptible to the effects of wiring resistance, which results in lower accuracy. 3-wire and 4-wire configurations can eliminate the effects of wiring resistance and provide higher measurement accuracy.
Sheath: The sheath is used to protect the RTD resistance element from environmental factors (e.g. corrosion, mechanical damage, etc.). The sheath material can be stainless steel, ceramic or other high temperature and corrosion resistant materials.
Shell: The shell is usually used to encapsulate all components of the RTD to ensure its safety and stability when used in harsh environments. The choice of housing material is closely related to the environment in which the RTD is used.
Types of RTD
Platinum RTDs
Platinum RTDs are the most commonly used type. Platinum RTDs have a very good linear temperature-resistance relationship, i.e., their resistance changes relatively uniformly when the temperature changes, making them suitable for high-precision temperature measurements. Platinum’s high stability and high temperature resistance make it one of the most commonly used RTD materials. Its resistance temperature coefficient α is stable, ensuring high accuracy at different temperatures. Platinum RTDs are mainly available in models such as Pt100 (100 ohms at 0°C) and Pt1000 (1000 ohms at 0°C), which offer high accuracy and durability.
Applications
- Industrial process control: monitoring and controlling temperatures in manufacturing processes.
- Laboratory research: to provide highly accurate temperature measurements for experiments.
- Calibration equipment: for calibration of high-precision temperature sensors.
Nickel RTDs
The nickel RTD is lower in cost and is suitable for applications in the medium temperature range. It has a high resistance temperature coefficient, i.e. the resistance changes more when the temperature changes, making it suitable for some applications that are sensitive to temperature changes. However, compared to platinum RTDs, nickel RTDs are less stable and tend to oxidize at high temperatures.
Applications
- Heating, ventilation and air conditioning (HVAC): monitoring and regulating temperature.
- Consumer electronics: temperature monitoring in home appliances.
- Automotive: for monitoring temperatures in engine and exhaust systems.
Copper RTDs
Copper RTDs have very good performance in the low temperature range due to their very linear resistance versus temperature, but their narrow temperature range and tendency to oxidize limits their use in high temperature environments.
Applications
- Low-temperature research: suitable for low-temperature experiments and research.
- Electrical equipment: for temperature monitoring of power equipment such as transformers.
- Refrigeration systems: for monitoring the temperature of cooling equipment.
Tungsten RTDs
Designed for very high temperature applications, Tungsten RTDs have a relatively low resistance temperature coefficient, an extremely high melting point and excellent high temperature performance.
Applications
- Metallurgical industry: for temperature control of high temperature furnaces and kilns.
- Aerospace: for temperature monitoring of high temperature equipment such as jet engines.
- Nuclear industry: for temperature monitoring in high radiation environments such as reactors.
RTD Material Type Quick Reference Table
| Material | Temperature Range | Accuracy | Resistivity (20°C) | Resistance Temperature Coefficient (α) |
| Platinum | -200°C to +850°C | High accuracy (±0.1°C to ±0.5°C) | 10.6 µΩ·m | 0.00385/°C |
| Nickel | -80°C to +300°C | Medium accuracy (±0.5°C to ±1°C) | 69.3 µΩ·m | 0.00651/°C |
| Copper | -200°C to +260°C | Medium accuracy (±0.5°C to ±1°C) | 1.68 µΩ·m | 0.00427/°C |
| Tungsten | -200°C to +1400°C | High accuracy | 5.6 µΩ·m | 0.0045/°C |
RTD Material Type Quick Reference Table
Temperature Coefficient of Resistance (α)
Definition: The temperature coefficient of resistance is the rate of change of resistance value with each 1°C change in temperature.
Expression:
- R100: resistance value at 100°C.
- R0: Resistance value at 0°C.
Summary
RTD types (RTDs) are versatile temperature sensors that can meet the needs of different application scenarios depending on material properties, resistance temperature coefficient and resistivity. Platinum RTDs are suitable for high accuracy and wide temperature range applications, nickel RTDs are a cost-sensitive temperature measurement solution, copper RTDs are suitable for low-temperature applications, and tungsten RTDs are designed for high-temperature environments. Understanding the characteristics, resistance temperature coefficient and resistivity of each RTD material can help in selecting the most suitable type of RTD to improve the accuracy and reliability of temperature measurements.
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