Sensor Response Times

Contents

1. What Determines Response Time?
2. Response Times: Thermocouples vs. RTDs
3. Summary

All temperature sensors have a finite response time — the delay between a temperature change and the sensor's output reflecting that change. This delay becomes critical in dynamic environments where fast, accurate response is needed for:

• Control systems
• Alarms and safety cutouts
• Switching operations

If the sensor lags behind the actual temperature shift, system performance may be compromised.

What Determines Response Time?

The inherent response time of a sensor is largely dictated by its physical construction and thermal properties. A common testing method involves plunging the sensor from ambient conditions into fast-moving water at a different temperature — enabling comparison between sensor designs.

Key Thermal Property: Thermal Diffusivity

A sensor’s thermal diffusivity determines how quickly temperature changes travel through its body. It is given by the formula:

Thermal Diffusivity = k/(c x ρ)

Where:

• k = thermal conductivity
• c = specific heat capacity
• ρ = density

Faster response is achieved with:

• High thermal conductivity
• Low specific heat
• Low density

How to Improve Sensor Response Time

While material constraints and environmental requirements often limit design flexibility, the following practical steps can help improve response times:

• Minimize thermal resistance at the sensor-medium boundary
• Shorten the thermal path (reduce mass and distance to sensing element)
• Use the smallest sensor that still meets durability and accuracy needs

Response Times: Thermocouples vs. RTDs

Thermocouples

For thermocouples, the junction design is key:

• Grounded junctions (welded directly to the sheath tip) provide faster response.
• Ungrounded/insulated junctions are slower but electrically isolated.

Typical Thermocouple Response Times

In a test from 20°C to 100°C in moving water:

• 0.25 mm OD mineral-insulated thermocouple (ungrounded): ~0.015 seconds to reach 63.2% of the step change
• 10.8 mm OD thermocouple (ungrounded): ~9 seconds
• Grounded junctions respond about twice as fast in each case

iLarger thermocouples respond more slowly due to increased thermal mass.

Resistance Temperature Detectors (RTDs)

RTD response time depends on:

• Sensor thermal mass (lower = faster)
• Surface area-to-volume ratio (higher = better)
• Contact quality with the medium, often limited by protective housings

Unlike thermocouples, RTD response is affected by the entire sensor stem, especially when insulation or environmental sealing is required.

Typical RTD Response Times

• Sealed RTDs: 0.2 to 0.5 seconds
• Thin-film RTDs: offer slightly faster response
• Large, industrial RTDs in sensor pockets: can take several minutes to respond

Summary

Sensor response time is critical in fast-changing environments. It depends on the sensor's thermal properties and design:

• Thermocouples respond fastest when grounded and small in diameter.
• RTDs benefit from low thermal mass, high surface area, and good contact—but are often slowed by their protective housings.

Design choices such as sensor size, mounting method, and junction type can significantly impact performance. Where milliseconds matter, always consider response time alongside accuracy and durability.

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.