RTD Sensor Materials

Choosing the right material for a Resistance Temperature Detector (RTD) is critical for ensuring accuracy, stability, and repeatability. While several metals meet the fundamental requirement of a predictable resistance-to-temperature relationship, only a few are practical for industrial temperature sensing.

Common RTD Materials

1. Copper (Cu)
   Copper has a near-linear resistance-temperature relationship and is inexpensive, making it a viable choice for specific applications. However, its low electrical resistivity and susceptibility to oxidation at elevated temperatures limit its use.
• Typical range: -100°C to +180°C
• Best suited for: Average temperature measurements over a long stem length
• Limitations: Poor stability and repeatability over time
2. Nickel (Ni) and Nickel Alloys
   Nickel-based RTDs offer high resistivity and a steep resistance-temperature slope, resulting in high sensitivity. However, their non-linear behavior and strain sensitivity can reduce measurement accuracy. A distinct inflection point at the Curie temperature (~358°C) complicates calibration.
• Typical range: -100°C to +180°C
• Advantages: High sensitivity, low cost
• Drawbacks: Non-linearity, mechanical sensitivity, and complexity near the Curie point
3. Platinum (Pt)
   Platinum is the industry standard for RTD sensors due to its exceptional stability, purity, and chemical resistance. As a noble metal, it maintains accuracy across a wide temperature range and exhibits a consistent, well-characterized resistance-temperature curve.
• Typical range: -200°C to +650°C (commonly used within -50°C to +500°C)
• Benefits:
• Excellent long-term stability
• High purity and reproducibility
• Predictable R vs. T curve (though not perfectly linear)
• Considerations:
• More expensive, though only small amounts are required
• Susceptible to contamination at high temperatures
• Requires proper heat treatment to remove vacancy defects
4. Other RTD Materials
• Molybdenum film: Used for stable sensing between -50°C and +200°C
• Semiconductor-based thermistors: Offer a wide temperature range and high sensitivity, but lack standardisation and are not typically used in precision RTD applications.
• Germanium: Effective below 100K, especially around 10K where platinum’s resistance is too low
• Carbon-glass and Rhodium-Iron: Specialised RTDs for cryogenic applications, including temperatures approaching 0.5K. These materials offer high sensitivity at ultra-low temperatures but are complex to calibrate and typically used in research environments.

Summary

For most industrial and scientific applications, platinum RTDs offer the best balance of accuracy, stability, and reliability across a broad temperature range. While copper and nickel have niche uses, platinum remains the material of choice, particularly for standardised RTDs like the widely used Pt100 sensor.

Further Reading

RTD vs Thermocouple – Choosing the Right Sensor
Explore the features and characteristics of thermocouples and RTDs

RTD Output Tables
View Resistance versus Temperature tables for all Pt100 sensors.

What are the RTD colour codes?
Explore RTD colour codes and wiring configurations.