Resistance Temperature Detectors (RTDs) are among the most reliable and accurate devices used for temperature measurement. They operate on the principle of the predictable change in electrical resistance of certain materials as a function of temperature. Because of their accuracy, repeatability, and long service life, RTDs are widely used in industrial applications ranging from energy production to chemical processing, food safety, and HVAC systems.
WORKING PRINCIPLE
The fundamental principle of RTDs is that the electrical resistance of a conductor varies with its temperature. This relationship is approximately expressed by the following formula:
R(T) = R0 [1 + α (T – T0)]
Where:
- R(T): Resistance at temperature T (Ω)
- R0: Resistance at reference temperature (commonly 0 °C) (Ω)
- α: Temperature coefficient of resistance (1/°C)
- T: Measured temperature (°C)
- T0: Reference temperature (°C)
Platinum-based RTDs, such as Pt100, provide high accuracy and stability because of their well-defined α value.
COMMON MATERIALS
- Platinum (Pt): The most widely used material due to its stability and accuracy across a broad temperature range. Common variants include Pt100 (100 Ω at 0 °C) and Pt1000 (1000 Ω at 0 °C).
- Nickel (Ni): Less expensive but limited in temperature range.
- Copper (Cu): Good linearity but prone to oxidation.
STRUCTURAL DESIGNS
- Wire-wound RTDs: Extremely accurate, suitable for laboratory applications.
- Thin-film RTDs: Compact, fast response time, often used in industrial processes.
- Industrial probe type RTDs: Housed in stainless steel, suitable for process piping, boilers, and industrial equipment.
ACCURACY AND STANDARDS
RTD accuracy classes are defined by IEC 60751.
Class A: ±(0.15 + 0.002 × |T|)
Class B: ±(0.30 + 0.005 × |T|)
COMPARISON WITH THERMOCOUPLES
- RTDs: High accuracy, stable, typically limited to -200 °C to 600 °C, more expensive.
- Thermocouples: Wider temperature range (-200 °C to 1800 °C), faster response, lower cost, but less stable.
APPLICATIONS
- Chemical and petrochemical industries
- Power plants (turbine and boiler monitoring)
- Food industry (pasteurization, fermentation)
- HVAC systems (climate control and energy management)
- Laboratories (calibration and reference measurements)
MAINTENANCE AND CALIBRATION
RTDs are long-lasting but require periodic inspection and calibration to maintain accuracy. This includes mechanical protection, regular verification, and monitoring of wiring and connections.
CONCLUSION
Resistance thermometers (RTDs) are essential tools for precise and reliable temperature measurement in modern industries. With proper material selection, installation, and regular calibration, they provide years of stable performance, ensuring process efficiency, safety, and compliance.