RTD Standards & Tolerances: Understanding IEC 60751

Contents

1. What Does the Standard Cover?
2. Tolerance Classes: Class A vs Class B
3. Nominal Resistance & Alpha Coefficient
4. Additional IEC 60751 Requirements
5. Summary

The international standard IEC 60751 defines the performance and tolerance requirements for industrial Resistance Temperature Detectors (RTDs), particularly platinum RTDs (Pt100), over the temperature range -200°C to +850°C.

What Does the Standard Cover?

According to IEC 60751, an RTD includes:

• The sensing resistor (typically platinum)
• Any protective sheath (if present)
• Internal connecting wires
• External terminals for connection to a measurement system
  Mounting hardware and connection heads may also be included.

While construction methods are not specified, the standard does cover essential electrical and performance parameters that impact accuracy and usability.

Tolerance Classes: Class A vs Class B

IEC 60751 defines two main tolerance classes that specify allowable deviation from the standard resistance-temperature curve.

• Class A sensors offer higher precision but are only applicable up to 650°C for standard 100-ohm RTDs.
• Class A is not valid for 2-wire configurations, due to additional lead resistance uncertainty.
• Standard RTDs conforming to these classes can be interchanged easily, which is highly beneficial for maintenance and system upgrades.

Nominal Resistance and Alpha Coefficient

Most platinum RTDs are designed with:

• A nominal resistance of 100 ohms at 0°C (Pt100) or 10 ohms at 0°C for high-temperature applications
• An alpha (α) coefficient of 0.00385 Ω/°C, which defines the slope of the resistance vs. temperature curve

Devices with 10-ohm elements are typically used above 600°C, where heavier gauge wire improves stability.

Additional IEC 60751 Requirements

The standard specifies characteristics relevant for industrial use, including:

• Compatibility with both DC and AC measurement systems (up to 500 Hz)
• Insulation resistance
• Response time and thermal response
• Self-heating effects
• Immersion and thermoelectric errors
• Mechanical performance: vibration, pressure, and temperature cycling
• Temperature limits

Manufacturers may also disclose additional parameters such as:

• Thermometer capacitance and inductance
• Capacitance to earth
• Internal wire resistance
• Immersion depth (recommended and minimum)
• Self-heating coefficients and thermal response time

These disclosures help users make informed decisions based on their application's specific needs.

Summary

IEC 60751 ensures interchangeability, accuracy, and system compatibility for platinum RTDs used in industrial settings. Understanding the standard’s tolerance classes, wire configurations, and disclosure requirements is essential for specifying and installing reliable temperature measurement systems.

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.