Thermocouple vs RTD

Difference Between RTD Sensor and Thermocouple

What are Thermocouples / RTDs

These two types of sensors might seen very similar on the surface, however they are in fact quite different. They have different operational mechanics, mainly one relies on current measurement, the other on voltage. Additionally, they find application in very different use-case scenarios and more often than not one is a lot better suited than the other for solving a particular problem.

RTD Temperature Sensors

An RTD (Resistance Temperature Detector) temperature sensor is a type of sensor used to measure temperature by correlating the change in resistance of the RTD element with the change in temperature. The basic principle behind its operation is the predictable increase in the resistance of certain materials, particularly metals, as their temperature rises. Current is run through the element and its change (based on the variation of the resistance) is measured and converted into a temperature measurement.

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Thermocouple Temperature Sensors

A thermocouple is a temperature sensor consisting of two different metals joined at one end. When there is a temperature difference between the joined end (known as the “hot junction” or “measuring junction”) and the other end (known as the “cold junction” or “reference junction”), a small voltage is generated. This voltage, known as the Setback voltage, is proportional to the temperature difference between the two ends. By measuring this voltage, the temperature can be determined.

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RTDs and their Application in IIoT

An RTD temperature sensor is known for its accuracy, stability, and repeatability across a wide range of temperatures. Given these characteristics, RTDs are suited for a variety of applications where precise temperature measurement is critical. Here are some scenarios and applications where an RTD temperature sensor would be beneficial:

  1. Industrial Process Control:

    • In chemical plants, RTDs can monitor reactor temperatures, ensuring reactions occur at desired rates and yields.
    • In oil refineries, RTDs monitor temperatures in distillation columns and other processes.

  2. Food & Beverage Processing:

    • RTDs can ensure that pasteurization occurs at the correct temperature, ensuring food safety.
    • They monitor temperatures in fermenting tanks in breweries and wineries.

  3. Pharmaceutical Manufacturing:

    • RTDs monitor and control temperatures in bioreactors, ensuring optimal conditions for drug production.
    • They ensure that storage and transport temperatures meet requirements for drug efficacy and safety.

  4. Laboratory Environments:

    • In scientific research, RTDs provide precise temperature measurements for experiments.
    • In calibration labs, RTDs are often used as standard references due to their accuracy.

  5. Heating, Ventilation, and Air Conditioning (HVAC):

    • RTDs ensure that buildings maintain desired climate conditions by providing feedback to control systems.

  6. Aerospace and Defense:

    • RTDs monitor engine and cabin temperatures in aircraft.
    • They’re used in military equipment to ensure systems operate within specified temperature ranges.

  7. Cryogenics:

    • Given their precision, RTDs can measure extremely low temperatures in cryogenic processes and storage.

  8. Semiconductor Manufacturing:

    • The manufacturing process for semiconductors requires precise temperature control, and RTDs play a role in ensuring these conditions.

  9. Renewable Energy Systems:

    • In wind turbines, RTDs can monitor gearbox and generator temperatures.
    • In solar thermal power plants, RTDs monitor the temperature of heat transfer fluids.
 

While these are just a few examples, it’s evident that RTDs are versatile and can be employed in any application requiring accurate and consistent temperature measurement. The choice between an RTD and another type of sensor, like a thermocouple, will often depend on the specific requirements of the application, such as the desired temperature range, accuracy, and environmental conditions.

As RTDs provide a higher degree of accuracy, stability and linearity they are more suitable for Industrial applications where precision of measurements is more important than being cost efficient.

3-Wire RTD Transmitter
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Thermocouples and Commercial IoT Applications

Thermocouples are temperature sensors that are robust, versatile, and capable of measuring a wide range of temperatures. Their simplicity, broad temperature capabilities, and relative cost-effectiveness make them suitable for many applications. Here are some scenarios and applications where thermocouple sensor are commonly used:

  1. Industrial Process Control:

    • In steel and metal production, thermocouples measure temperatures in furnaces, kilns, and casting processes.
    • In glass manufacturing, they monitor the temperature of molten glass.
    • In plastic industries, thermocouples control the temperature of extruders and molding machines.

  2. Food & Beverage Processing:

    • Thermocouples monitor the temperature of ovens, grills, and fryers in large-scale cooking processes.
    • They’re used in deep frying to ensure oil reaches and maintains the desired temperature.

  3. Combustion Systems:

    • Thermocouples are commonly found in residential and industrial gas appliances such as water heaters, ovens, and furnaces to detect flame presence and ensure safety.

  4. Laboratory Environments:

    • Thermocouples are often used in high-temperature ovens and furnaces in material testing.
    • They’re employed in experimental setups due to their wide temperature range and quick response.

  5. Power Generation:

    • In gas turbine systems, thermocouples measure exhaust gas temperatures.
    • They monitor temperatures in boilers and steam systems in power plants.

  6. Healthcare:

    • Thermocouples monitor the temperature of autoclaves used for sterilization.

  7. Automotive:

    • Thermocouples measure exhaust gas temperatures, especially in high-performance and diesel vehicles.
    • They’re used to monitor battery temperatures in electric vehicles.

  8. Semiconductor Manufacturing:

    • Thermocouples can be used in certain processes where rapid temperature change measurement is necessary.

  9. Environmental Monitoring:

    • Thermocouples can be used in geothermal research to measure temperatures deep within the Earth.
Thermocouples have the advantage of being relatively cheap and readily available, you can easily source a good quality, cost efficient sensor of this type in large quantities. This, coupled with their wider temperature range and fast response time makes them more suitable for commercial applications, where the measurements are not that specific and do not require the highest level of precision. Additionally, use-cases where a large number of temperature probes need to be deployed for example in field for monitoring soil/plants or over a large factory for environmental monitoring the costs can add up quickly and a cheap, easy to source sensor is the better solution.

Advantages and Disadvantages of RTD and Thermocouple

Both RTDs and thermocouples are widely used for temperature measurement, each with its own set of advantages and disadvantages. Here’s a comparison:

RTDs

Advantages:

  1. High Accuracy: RTDs are generally more accurate than thermocouples, especially over narrow temperature ranges.
  2. Stability: RTDs tend to be stable over time, offering consistent readings.
  3. Repeatability: RTDs can provide very consistent measurements for given temperatures.
  4. Linear Output: RTDs have a relatively linear output, making them easier to interpret and use in control systems.
  5. Long-term Reliability: With proper installation and protection, RTDs can maintain their performance for many years.

Disadvantages:

  1. Limited Temperature Range: RTDs usually have a narrower temperature measurement range compared to most thermocouples.
  2. Cost: RTDs, especially those made from platinum, tend to be more expensive than thermocouples.
  3. Fragility: RTDs can be more fragile than thermocouples, especially the wire-wound types.
  4. Slower Response Time: RTDs typically have a slower response time to temperature changes compared to thermocouples.
  5. Requires External Current Source: RTDs need an external current source for operation, which can introduce potential errors.
Thermocouple
NCD Industrial Wireless Thermocouple High Temperature Sensor

Thermocouple

Advantages:

  1. Wide Temperature Range: Thermocouples can measure a broader range of temperatures than RTDs, from very low to very high.
  2. Ruggedness: Thermocouples can be very durable and can operate in challenging environments where other sensors might fail.
  3. Fast Response Time: Due to their small thermal mass, thermocouples can respond rapidly to temperature changes.
  4. Cost-Effective: Thermocouples are often less expensive than RTDs.
  5. Simple Design: Thermocouples are passive devices that generate a voltage, and they do not require an external power source.

Disadvantages:

  1. Lower Accuracy: Thermocouples generally have lower accuracy compared to RTDs.
  2. Less Stable: Over time, especially at high temperatures, the thermocouple materials can degrade or become contaminated, leading to a drift in readings.
  3. Needs Reference: Thermocouples require a reference (cold junction) temperature to determine the absolute temperature at the sensing tip.
  4. Non-linear Output: The relationship between temperature and generated voltage is non-linear, requiring conversion methods or lookup tables.
  5. Susceptibility to Electromagnetic Interference: Long thermocouple leads can act as antennas, picking up electrical interference which can affect readings.

The choice between an RTD and a thermocouple depends on the specific requirements of the application, such as the desired temperature range, accuracy, environment, response time, and budget.

RTD vs Thermocouple

Summary

Both RTDs and Thermocouples find use in a wide-range of applications, some of which overlap. This does not mean they are interchangeable as their main characteristics are quite different. As a rule of thumb:

RTDs are more expensive and better suited for IIoT applications that cost is not a limiting factor and very precise, accurate and stable readings in a narrow temperature range are required. They are best suited for controlled environments where environmental protection level and robustness of the sensor itself is not essential.

Thermocouples are cheaper, easier to provision in large quantities and overall more versatile. They are suitable for measurements in a wide temperature range, they have higher response time and are more robust. However, they have lower accuracy, stability and precision, making them more suitable for applications with a wider-range of temperature range for example outdoor scenarios where environmental conditions are a playing factor (Thermocouples are more robust than RTDs).

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