Quick guide to selecting the right probe for process measurements in pipelines with flows

Response time differences between RTDs and thermocouples

Before examining the different constructions of temperature probes, it’s essential to highlight that the type of sensing element significantly influences response times.

RTDs (PT100 or Pt1000)

• Provide high accuracy and long-term stability, but their larger thermal mass as sensing elements results in slower response times.
• Ideal for stable and precise measurements in applications that don’t require rapid reaction times.

Thermocouples (Type K)

• Smaller thermal mass and direct thermal junctions enable faster responses to temperature changes.
• Best suited for processes with rapid thermal variations or high-temperature applications.

These fundamental differences must be considered when selecting the sensor type and probe configuration, as they directly affect measurement performance.

Response times and probe configuration comparisons for pipeline applications

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Clamp-on probes for cylindrical surfaces (RTD ASPT-S or Thermocouple ASTC-S)

Advantages

• Rapid response times due to direct contact with the pipe surface (not suitable for painted or surface-treated pipes), typically reacting in 5–15 seconds.
• Easy installation without requiring structural modifications to the pipeline.
• Suitable for temporary applications or quick monitoring tasks.

Disadvantages

• Sensitivity to the mounting method; poor thermal contact or inadequate insulation can lead to inaccurate readings (e.g., pipes with insulating paint).
• Lack of protection from external factors compared to probes with thermowells.

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Probes with Thermowells (RTD head connection ASPT-H or Thermocouple head connection ASTC-H with Thermowells made from bars ASTW)

Advantages

• Protect the sensor sheath, making them ideal for harsh environments where the process fluid might damage the sensor.
• More stable readings due to thermal inertia, less affected by rapid variations or fluctuations.
• Simplified maintenance: the temperature sensor can be replaced without draining or interrupting the process.

Disadvantages

• Slower response times due to the thermal inertia introduced by the thermowell and filling fluid, typically 20–40 seconds.
• Require structural changes; their installation requires a sleeve or an opening (threaded or welded) provided in the pipe.

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MgO-insulated probes, mineral oxide(RTD ASPT-W or Thermocouple ASTC-W)

Advantages

• Excellent balance of speed and durability; MgO insulation ensures fast thermal response (like clamp-on probes) while protecting the sensor from mechanical shocks and vibrations.
• High thermal resistance; suitable for temperatures up to 1600 °C (2912 °F ) depending on the thermocouple type and the sheath material, ensuring decent stability and precision.
• Versatile; can be used for immersion (with or without thermowell) or surface measurements depending on their construction geometry.

Disadvantages

• Sensitive to installation; when used without a thermowell, they require proper immersion depth to ensure good thermal contact.
• Limited direct protection; without a thermowell, they are more exposed to chemical or mechanical damage over time.

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Summary

The choice of sensor type and model depends on specific application needs:

• For quick response times and ease of installation, clamp-on probes are ideal but require proper setup.
• For harsh environments or situations demanding robust protection, thermowell-equipped probes provide greater reliability at the expense of slower response times.
• MgO-insulated probes offer a good compromise between speed, durability, and versatility, making them particularly suitable for high-performance applications without significant structural modifications.