RTD and thermocouples are temperature devices used in industrial temperature measurements. The main difference between them is their working mechanism.  

In a thermocouple, a voltage is generated at the junction of two different metals when there is a temperature difference between that junction and the other end. In RTD, the operation is based on the principle that the electrical resistance of a metal changes with temperature. To measure the temp, it measures the resistance change.

RTD is a more accurate and stable device, while thermocouples are durable, respond fast, and have a high-temperature range.

What is RTD (resistance temperature sensor)? 

An RTD is a passive temperature sensor that operates on the principle that the electrical resistance of certain materials changes consistently with temperature. RTDs provide accurate and stable temperature readings suitable for moderate temperature ranges. They are typically made of pure metals like platinum, including its most popular type PT1000. Platinum is the most accurate material that is also resistant to corrosion. Other materials RTD is made of are nickel (cheaper) and copper (cheaper and with the fastest response time).

What is a Thermocouple?

A thermocouple is a temperature sensor consisting of two metal wires joined at one end. It operates based on the Seebeck effect, where the junction of the two metals produces a voltage that varies with temperature. Thermocouples are used for measuring a wide range of temperatures, including very high temperatures, and are known for their fast response and durability in harsh environments.

The materials for thermocouple are:

• Type K: Chromel-Alumel
• Type J: Iron-Constantan
• Type T: Copper-Constantan
• Type E: Chromel-Constantan
• Type N: Nicrosil-Nisil

Pros and Cons of RTD vs. Thermocouple

• Accuracy: RTD > Thermocouple
• Temperature Range: Thermocouple > RTD
• Cost: RTD > Thermocouple
• Durability in Harsh Conditions: Thermocouple > RTD
• Response Time: Thermocouple > RTD

RTDs (Resistance Temperature Detectors)

1. High accuracy: RTDs provide very accurate temperature readings, especially in moderate temperature ranges compared to thermocouples. The common accuracy is 0.1 C, but up to 1 degree Celsius is possible. RTDs are excessively better than thermocouples at lower ranges.
2. Stable and repeatable: They offer excellent long-term stability and repeatability of measurements.
3. Linear response: RTDs exhibit a highly linear response over their temperature range.
4. Low drift: Over time, RTDs experience less drift in their readings.

1. Limited high-temperature range: RTDs are not suitable for highly high-temperature applications. A common range is approximately -200°C to 850°C, but many RTDs are designed for up to 400 °C only.
2. More expensive: They are generally more pricey than thermocouples due to materials and construction. The prices can be 2x-3x of a thermocouple, depending on the model.
3. Fragile: RTDs are more fragile and less durable in harsh environments.
4. Slower response time: They have a slower response time compared to thermocouples.

Thermocouples

1. Wide temperature range: Thermocouples can measure a broader range of temperatures, including very high temperatures. A common type of a thermocouple Type K has a temperature range of -270°C to 1372°C. Some specialized types, like Type B ( 0°C to 1820°C), can handle even higher temperatures.
2. Fast response time: They respond quickly to temperature changes and are suitable for dynamic processes. Some thermocouples take milliseconds to respond, three times faster than RTD.
3. Durable: More rugged and can withstand harsh conditions, including vibration and shock.
4. Cost-effective: Generally less expensive than RTDs.
5. Simple and versatile: Easy to install and use in various industrial applications.

1. Less accurate: Thermocouples are less accurate compared to RTDs, especially at lower temperatures. The typical accuracy is between 1 C and 4 C. This depends on different factors, like sensor oxidation, which thermocouples are known for.
2. Non-linear output: The temperature-voltage relationship is non-linear, which can complicate measurements. The output is the so-called ‘S’-type plot.
3. More prone to drift: They can experience more drift over time, affecting long-term reliability. They are also less stable.
4. Requires reference junction: A reference junction or compensation is necessary for accurate readings.

Applications of RTD vs. Thermocouple

To sum up, RTDs are used in industrial applications where precision is more important, while thermocouples are used in industrial applications where high temperatures are more important. Based on that, here is the breakdown:

RTD applications

1. Laboratories and precision testing
2. Pharmaceuticals and biotechnology
3. Food and beverage industry
4. HVAC systems
5. Industrial processes
6. Clean rooms and environmental chambers
7. Semiconductor manufacturing

Thermocouple applications

1. High-temperature industrial processes
2. Furnaces and boilers
3. Refrigeration systems
4. Power generation
5. Automotive industry
6. Aerospace and aviation
7. Steel and metal manufacturing
8. Glass industry
9. Plastic and polymer processing
10. Ammonia uses and sulfur recovery units
11. High-pressure and high-vibration applications

Real-life use cases For RTD and Thermocouple

Use Case for RTDs (Resistance Temperature Detectors)

Scenario: Pharmaceutical Manufacturing

Background: A pharmaceutical company requires precise temperature control in its manufacturing process to ensure the quality and efficacy of its drugs. Given that in the world of drug making, it's super critical to keep a close eye on temperature because even a slight change can mess with the chemical reactions and potentially compromise the medicine’s effectiveness, RTDs are recommended.

Application of RTDs:

• Monitoring fermentation process: RTDs are used to monitor and control the temperature in fermentation tanks. RTDs are super precise, making sure temperatures stay just right for those microbes to thrive.
• Quality control: In the quality control labs, RTDs provide accurate temperature readings for tests that require specific thermal conditions.
• Environmental monitoring: RTDs are also employed to monitor the temperature in storage areas where drugs are kept to ensure they are stored under optimal conditions.

Benefits:

• RTDs are vital in pharma because they nail down the exacting temperature control.
• RTDs react in a predictable way so that you can dial in the exact temp.
• The long-term stability of RTDs minimizes the need for frequent recalibration.

Use Case for Thermocouples

Scenario: Steel Manufacturing Plant

Background: A steel manufacturing plant requires temperature monitoring in environments with extremely high temperatures, such as in furnaces and during the casting process.

Application of Thermocouples:

• Furnace temperature monitoring: Thermocouples measure the temperature inside furnaces where steel is melted. These temperatures can be extremely high, well within the measurement range of thermocouples.
• Molten steel temperature measurement: During the casting process, thermocouples are used to monitor the temperature of molten steel to ensure it is within the proper range for casting.
• Equipment monitoring: Thermocouples are crucial for monitoring equipment temps to keep operations safe and on track.

Benefits:

• Thermocouples can measure the very high temperatures in steel manufacturing, which are beyond the capability of most other sensors.
• Thermocouples are great at keeping up with the quick shifts in heat you'd find in industrial settings, ensuring nothing gets too hot or cold without noticing.
• These sensors are tough enough to handle the extreme heat and rough shakes you'd find in a steel mill.