Practical Resistance Thermometer Detectors (RTDs)

Contents

1. Contamination Considerations at High Temperatures
2. Platinum Purity and Construction
3. Sensor Design Flexibility
4. Summary

Resistance Thermometer Detectors (RTDs) rely on platinum sensor elements that must maintain a fully annealed condition and remain contamination-free to ensure high stability. Proper selection of support materials, sheath construction, and insulation is crucial to avoid sensor poisoning and mechanical strain.

Contamination Considerations at High Temperatures

• Below 250°C: Contamination is rarely an issue.
• Above 250°C: Materials such as base metals, mica, and borosilicate glass can react with or dissolve in the platinum sensor.
• Mitigation: Special mounting methods and hermetic sealing with oxygen-filled gas help keep reactive elements oxidized, reducing sensor damage.

Platinum Purity and Construction

• Industrial RTDs use lower-purity platinum wire than laboratory-grade sensors to improve mechanical robustness and contamination tolerance.
• The wire is typically doped with other metals to meet IEC and British standards for temperature-resistance specifications and tolerance limits.
• Sensors are designed to cancel thermoelectric voltages generated by dissimilar metals and to maintain adequate insulation resistance (per IEC 60751).
• Coil windings are made non-inductive, and current flow is controlled to prevent self-heating. Both DC and AC (up to 500 Hz) excitation is supported.
• Heat conduction along the sheath, wires, and insulators is minimized to ensure accurate readings.

Sensor Design Flexibility

• RTD assemblies can also use metals other than platinum.
• Shapes and sizes vary widely depending on application needs, including:
  • Large surface areas for fast response times
  • Very small sensors for precise point temperature measurement
  • Long or large sensors for temperature averaging over an area or length

This ensures users can distinguish connectors based on both type and temperature rating.

Summary

Practical RTDs are carefully constructed platinum-based sensors designed to maintain stability and accuracy under various conditions, especially at elevated temperatures. Their flexibility in shape and size allows them to be tailored for diverse industrial applications, balancing robustness, contamination resistance, and measurement precision.

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.