Temperature Measurement in Industrial Processes: RTD and Thermocouple Selection Guide

1. Introduction: The challenge of temperature measurement for plant reliability

Precise temperature measurement is a fundamental aspect in almost every industrial process. It directly influences product quality, process reliability, energy efficiency and the service life of system components. Incorrect temperature detection can lead to significant operational disruptions, production downtimes and safety risks. The choice of the appropriate temperature sensor – resistance thermometer (RTD) or thermocouple – is crucial and requires a deep understanding of the respective measurement technologies, their specifications and the specific process requirements.

In the DACH manufacturing industry, where the highest standards for efficiency and reliability apply, the careful selection and implementation of temperature measuring devices is critical. This manual highlights the technical principles, standards, selection criteria and best practices in order to provide system operators and maintenance technicians with well-founded decision-making support and thus ensure long-term system reliability.

2. Fundamental principles of temperature measurement technology

2.1. Resistance thermometer (RTD)

Resistance thermometers are based on the physical principle that the electrical resistance of metals depends on temperature. The most common RTDs are platinum resistance thermometers (Pt-100, Pt-1000), whose resistance increases as the temperature increases (positive temperature coefficient). Platinum is characterized by high linearity, stability and reproducibility over a wide temperature range.

• Pt100: Has a resistance of 100 ohms at 0 °C. The temperature coefficient is typically α = 0.00385 Ω/(Ω °C) according to IEC 60751.
• Pt1000: Has a resistance of 1000 ohms at 0 °C. This enables lower measuring currents and reduces the self-heating error, which is particularly advantageous in applications with high accuracy requirements.

For measurement, a constant current is sent through the sensor and the resulting voltage drop is measured. To eliminate the influence of line resistance, 3- or 4-wire circuits are usually used. A 4-wire circuit offers the highest precision because it completely compensates for the line resistance.

2.2. Thermocouples

Thermocouples use the Seebeck effect, a thermoelectric effect in which an electrical voltage (thermo-EMF) is created in a circuit consisting of two different metals when the two contact points have different temperatures. This voltage is proportional to the temperature difference between the measuring point (hot end) and the reference point (cold end).

The level of the thermal EMF depends on the material and is determined by the specific combination of metals used. Common thermocouple types for industrial applications are:

• Type K (Nickel-Chrome/Nickel-Aluminium): Widely used, temperature range approx. -200 °C to +1250 °C. Sensitivity approx. 41 µV/°C at 0 °C.
• Type J (iron/copper-nickel): Range approx. 0 °C to +750 °C. Sensitivity approx. 55 µV/°C at 0 °C. Suitable for vacuum or reducing atmospheres.
• Type N (Nicrosil/Nisil): Range approx. -270 °C to +1300 °C. Improved stability and oxidation resistance compared to Type K.

A crucial requirement for thermocouples is cold junction compensation (CJC), as the measured voltage represents a differential temperature. The CJC corrects the reference junction temperature to a reference value (usually 0 °C) to determine the actual temperature at the measuring point.

3. Technical specifications and standards

3.1. Standards for RTDs

The most important standard for platinum resistance thermometers is the IEC 60751 (Industrial Platinum Resistance Thermometers and Platinum Temperature Sensors). It defines the resistance-temperature relationship and the tolerance classes:

• Class AA: Most accurate class, typ. ±(0.1 + 0.0017 |t|) °C.
• Class A: High accuracy, typ. ±(0.15 + 0.002 |t|) °C.
• Class B: Standard accuracy, typ. ±(0.3 + 0.005 |t|) °C.

These tolerances indicate the maximum permissible deviation at a certain temperature 't' in °C. The typical measuring range for Pt100/Pt1000 is between -200 °C and +850 °C.

3.2. Standards for thermocouples

For thermocouples, the IEC 60584 (Thermocouples – Part 1: Reference functions) is decisive. It specifies the standardized thermal EMF values for various types of thermocouples and defines tolerance classes:

• Class 1: Higher accuracy. For type K, e.g. ±1.5 °C or ±0.004 |t| (which is bigger).
• Class 2: Standard precision. For type K e.g. ±2.5 °C or ±0.0075 |t| (which is bigger).

The maximum temperature ranges vary greatly depending on the type. While type K can be used up to +1250 °C, special high-temperature types such as type R or S (platinum-rhodium) can reach up to +1700 °C.

3.3. General standards and certifications

Additional standards are relevant for both sensor types, including:

• DIN EN 50212: Installation and use of industrial thermocouples.
• VDI/VDE 3512: Temperature measurement in technical practice – basics and applications.
• EN 60079-0 ff.: ATEX guidelines for use in potentially explosive areas.
• CE marking: Conformity with European directives.

The material selection for thermowells (e.g. 1.4571 for standard applications, Inconel 600 for high temperatures and corrosive media) must correspond to the process conditions (pressure, chemical resistance, abrasion).

4. Selection and Sizing Guide

The decision between RTD and thermocouple depends on a variety of process parameters. The following guide will help you make an informed choice.

4.1. Critical selection criteria

• Temperature range: RTDs are more precise in the range of -200°C to +600°C (up to +850°C with reduced stability). Thermocouples are the preferred choice for temperatures above +600°C to +1700°C.
• Accuracy and long-term stability: RTDs offer significantly higher accuracy and stability than thermocouples. A Class A Pt100 has a maximum deviation of ±0.15°C at 0°C, while a Class 1 Type K thermocouple tolerates ±1.5°C or 0.4% of reading at 0°C.
• Response time: Thermocouples generally have a faster response time due to their lower mass and more direct contact point. They often have an advantage in processes with rapid temperature changes. An exposed thermocouple solder joint can achieve a response time of under 0.1 seconds, while a shrouded RTD requires 1-5 seconds.
• Robustness and Vibration: Thermocouples are mechanically more robust and resistant to vibration and shock because they are made of simpler wire construction. RTDs, especially wire-wound RTDs, can be damaged if exposed to strong vibration.
• Costs: Thermocouples are generally cheaper to purchase than RTDs, especially for higher temperature ranges and longer measuring lines.
• Cable routing: With RTDs, the cable resistances must be compensated for using 3- or 4-wire connection technology, which can increase the installation effort and wiring costs. For thermocouples, the measuring leads must be made of thermocouple materials (compensation or extension leads) and the cold junction compensation must be carried out correctly.
• Ambient influences: Chemical aggressiveness, electromagnetic interference (EMC), pressure and abrasion must be taken into account when choosing the protective tube material and design. DIN EN 61000-4-2 ff. regulates the EMC requirements.

4.2. RTD vs thermocouple decision table

5. Installation and commissioning best practices

Correct installation is essential for the accuracy and longevity of temperature sensors. The following points should be noted:

• Insertion depth: The sensor insert must be immersed sufficiently deeply in the measuring medium to ensure a representative temperature measurement. As a rule of thumb, immersion is at least 10 times the diameter of the protective tube.
• Protective tubes (thermal sleeves): Always use suitable thermal sleeves that can withstand the process pressure, chemical aggressiveness and flow speed. They protect the sensor from mechanical damage and enable replacement without interrupting the process. Dimensioning according to ASME PTC 19.3 TW or DIN 43772 is the guiding principle here.
• Cabling:
• RTDs: Use shielded cables to minimize electromagnetic interference. Make sure you have the correct 3- or 4-wire connection to compensate for the line resistance. Avoid kinks or mechanical stress on the connecting wires.
• Thermocouples: Only use thermocouple or compensating cables of the appropriate type. These cables have the same thermoelectric properties as the thermocouple itself to avoid additional measurement errors caused by thermoelectric voltages at unwanted junction points. Make sure polarity is correct.
• EMC protection: Measuring lines should be laid separately from power lines to avoid inductive and capacitive coupling. Additional shielding and potential equalization must be implemented in accordance with VDE 0100 or DIN EN 50178.
• Vibration Damping: In environments with strong vibrations, sensors with flexible connectors or special vibration dampers should be installed to prevent wire breaks.
• Moisture protection: The connection heads and terminal points must be protected from moisture to prevent leakage currents and corrosion. Protection types according to EN 60529 (IP codes) must be observed.

6. Failure modes and root cause analysis

Detecting and analyzing errors in temperature sensors is crucial for quick troubleshooting and restoring process stability.

6.1. RTD failure modes

• Interruption (Open Circuit): The resistance of the sensor approaches infinity. The cause is often a wire break in the measuring insert or at the connection terminals, often due to vibrations or mechanical overload. Visually recognizable by an implausible, extremely low or no temperature display.
• Short Circuit: The resistance is very low (near 0 ohms). The cause is usually damage to the insulation of the supply lines or the sensor element, often due to overheating or chemical corrosion. The display will then be an unrealistically high temperature.
• Drift/Aging: The resistance-temperature characteristic curve shifts over time. This leads to systematic measurement errors. Causes include thermal cycles, contamination of the platinum or mechanical stress. Regular calibration will reveal this. The typical drift of a Pt100 is <0.05 °C per year under normal operating conditions.
• Incorrect line resistance: In 2-wire applications, a variable line resistance leads directly to measurement errors. In the case of 3- or 4-wire circuits, errors in the connection plan or defective compensation lines are the cause.

6.2. Thermocouple failure modes

• Interruption (Open Circuit): A wire break in the thermocouple or at the soldering point means that thermal EMF is no longer generated. The measuring device typically displays the cold junction temperature or a lowest limit value. Often due to thermal fatigue, corrosion or mechanical impact.
• Decalibration/aging: The thermoelectric properties of metals change under the influence of temperature. This leads to a shift in the characteristic curve and thus to incorrect measured values. Causes include high temperatures over a long period of time, chemical contamination or oxidation. With type K, a green discoloration (green red) can occur due to oxidation of the nickel at temperatures between 800 °C and 1050 °C under reducing conditions. This leads to negative drift in the display.
• Incorrect cold junction compensation: Inaccurate detection of the cold junction temperature leads to systematic errors.
• Wrong polarity: If the connection is reversed, the temperature difference is measured in the opposite direction.
• Ground connection (Ground Loop): With grounded thermocouples, if there is insufficient insulation, loop currents can arise that distort the signal.
• Short circuit in the protective tube: If the protective tube is damaged, the thermocouple can come into contact with the wall and lead to an incorrect measuring point.

7. Predictive maintenance and condition monitoring

Predictive maintenance strategies for temperature sensors aim to detect potential failures early and avoid unplanned downtime. Condition monitoring plays a central role here.

• Regular calibration: The most important measure. Sensors should be checked at defined intervals (e.g. annually or semi-annually, depending on criticality) against certified reference sensors or in temperature baths/dry block calibrators. Calibration according to DIN EN ISO/IEC 17025 ensures traceability to national and international standards. Typical test points could be 0°C, 100°C, 300°C and 600°C. The drift is documented.
• Insulation resistance measurement: The measurement of the insulation resistance between the conductors and the protective tube (sheath) is particularly informative for RTDs and sheathed thermocouples in humid or aggressive environments. A falling insulation resistance indicates the beginning of insulation damage, which can lead to short circuits or leakage currents. According to IEC 60751, the minimum insulation resistance is 100 MΩ at 20 °C.
• Trend analysis of the measured values: Monitoring of the sensor outputs over a longer period of time. Significant deviations from expected values ​​or in comparison to redundantly installed sensors can indicate the beginning of drift or a defect.
• Response time check: Occasional check of the response time. An extension may indicate deposits on the protective tube or degradation of the sensor itself.
• Visual Inspection: Regular inspection of thermowells for corrosion, erosion, mechanical damage or discoloration, especially on thermocouples, which may indicate decalibration.
• Residual current monitoring: Integrated diagnostic functions in modern measuring transducers can detect and signal leakage currents or wire breaks.

8. Comparison table: Detailed sensor variants

The following table provides a detailed comparison of common industrial temperature sensor variants.

9. Conclusion

The informed selection between resistance thermometers and thermocouples is an engineering task that has a decisive influence on the reliability and efficiency of industrial systems. While RTDs impress with their high accuracy and stability at moderate temperatures, thermocouples are the preferred solution for high-temperature applications and processes with rapid temperature changes where slightly lower precision can be tolerated.

Consideration of specific process conditions, compliance with relevant standards such as IEC 60751 and IEC 60584, and implementation of best installation and maintenance practices are essential. As a reliable partner, UNITEC-D GmbH offers a wide range of certified temperature measurement solutions that meet the highest demands of the DACH manufacturing industry.

For detailed product information and a comprehensive selection of certified temperature measurement solutions, visit our e-catalogue at: https://www.unitecd.com/e-catalog/

10. References

• IEC 60751: Industrial platinum resistance thermometers and platinum temperature sensors.
• IEC 60584-1: Thermocouples – Part 1: Reference functions.
• DIN EN 50212: Industrial thermocouples – installation and use.
• VDI/VDE 3512: Temperature measurement in technical practice.
• Endress+Hauser: Technical documentation and application manuals for temperature measurement technology.