Introduction to RTD's (Resistance Temperature Detectors)

Contents

1. How it Works
2. History
3. Key Requirements for Accurate RTDs
4. The RTD Resistance Equation
5. The α Coefficient and Fundamental Interval
6. Summary

Resistance Temperature Detectors (RTDs) work on the principle that the electrical resistance of a conductor changes with temperature. As temperature increases, atomic vibrations and electron scattering within the metal increase, which causes its resistance to rise in a predictable way. This relationship can be used to accurately measure temperature—provided the sensing material is stable and pure.

How it Works

In simple terms, metals like platinum exhibit a reliable resistance-temperature relationship. While impurities and lattice defects can also affect resistance, these influences are mostly temperature-independent and manageable in high-purity materials.

RTDs offer several practical advantages over thermocouples:

• Absolute Measurement: No need for a reference junction or cold junction compensation.
• Simpler Wiring: Standard copper cables can be used between the sensor and instrumentation, reducing complexity.

History

The concept dates back to the 1860s, when Sir William Siemens first proposed using temperature-dependent resistance for sensing. Early versions used platinum, but the design suffered from instability due to mechanical strain and contamination.

It wasn’t until 1899 that platinum resistance thermometry became reliable, thanks to improved materials and construction methods pioneered by Callendar.

Key Requirements for Accurate RTDs

To achieve stable and accurate measurements, RTD elements must:

• Use high-purity metals (usually platinum).
• Be kept in a fully annealed condition to avoid physical changes.
• Be protected from contamination to prevent chemical alterations.
• Be mechanically supported to minimise strain from thermal expansion.

Manufacturing RTDs requires balancing sensitivity with robustness to ensure performance even in harsh industrial environments.

The RTD Resistance Equation

For platinum RTDs, the resistance–temperature relationship is predictable and follows a polynomial form:

Above 0ºC a second-order (quadratic) polynomial is sufficient:
R_t /R₀ = 1 + At + Bt²

Below 0°C (for higher accuracy) a more accurate third-order (cubic) polynomial is used:
R_t /R₀ = 1 + At + Bt² + Ct³(t-100)

Therefore:
t = (1/α)(Rt - R0)/R0 + δ(t/100)(t/100 -1)

Where:

• R_t = Resistance at temperature t
• R₀ = Resistance at 0°C
• t = Temperature in °C
• A, B, C = Calibration constants

The constants A, B, and C in these equations are typically determined by calibration for high-accuracy sensors, but in industrial applications, standardised values as defined in IEC 60751 are commonly used:

• A = 3.90802 × 10^-3
• B = –5.802 × 10^-7
• C = –4.2735 × 10^-12

The α Coefficient and Fundamental Interval

A key parameter in defining platinum RTDs is the alpha coefficient (α), which represents the average temperature coefficient of resistance between 0°C and 100°C:

α = (R₁₀₀ – R₀) / (100 × R₀)

This equation compares the resistance at 0°C (R₀₀) and 100°C (R₁₀₀) — a difference known as the fundamental interval. For commercial RTDs, this fundamental interval is typically 38.5 Ω, giving an α value of 3.85 × 10^-3/°C.

The alpha coefficient is influenced by both the purity and annealed condition of the platinum sensing wire. Higher-purity, fully annealed platinum can achieve alpha values between 3.925 × 10^-3/°C and 3.928 × 10^-3/°C, providing better linearity and accuracy — although it is more sensitive to strain and contamination.

To ensure consistency, the IEC 60751 standard defines tables of resistance vs temperature values for RTDs, based on:

• R₀ = 100 Ω at 0°C
• R₁₀₀ – R₀ = 38.5 Ω (fundamental interval)
• α = 3.85 × 10^-3/°C
• Available in tolerance Class A and Class B

These tables form the backbone of industrial RTD calibration, enabling reliable measurement across a wide range of temperatures.

Summary

RTDs measure temperature by tracking how resistance changes with heat. Platinum is the most widely used material due to its stability, purity, and repeatable characteristics. RTDs are accurate, require no reference junction, and use standard wiring—making them ideal for many industrial applications.

The relationship between temperature and resistance is described by polynomial equations, and standardised in IEC 60751, which defines calibration constants and tolerance classes. For accurate performance, the RTD element must remain chemically stable, physically unchanged, and free from strain or contamination.

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.