What is a thermocouple?
A thermocouple is a sensor used to measure temperature. Thermocouples consist of two wire legs made from different metals. The wire legs are welded together at one end, creating a junction. This junction is where the temperature is measured. When the junction experiences a change in temperature, a temperature dependant voltage is produced. The voltage can then be interpreted using thermocouple reference tables to calculate the temperature.
There are many types of thermocouples, each with its own unique characteristics in terms of temperature range, chemical resistance, and application compatibility. Type J, K, T, & E are “Base Metal” thermocouples, the most common type of thermocouple. Type R, S, and B thermocouples are “Noble Metal” thermocouples (due to high platinum content), which are used in high-temperature applications (see thermocouple temperature ranges for more details).
Thermocouples are used in many industrial, scientific, and OEM applications. They can be found in nearly all industrial markets: Power Generation, Oil/Gas, Pharmaceutical, Biotech, Cement, Paper & Pulp, etc. Thermocouples are also used in everyday appliances like stoves, furnaces, and toasters.
Thermocouples are typically selected because of their low cost, high-temperature limits, wide temperature ranges, and durable nature.
What are the different types of thermocouples?
There are multiple types and grades of thermocouples. Each type of thermocouple wire has a specific combination of metal alloys. This combination is what defines the type of thermocouple. For example, a type K thermocouple is made when a wire of Nickel-Chromium is welded to a wire of Nickel-Aluminium. The grade of the thermocouple is dependent not only on the combination of alloys used but also on the purity of those alloys. Learn more on our thermocouple wire page.
What are the thermocouple junction types?
There are four types of thermocouple junctions: grounded, ungrounded exposed, and ungrounded uncommon.
Grounded: A thermocouple is grounded when both thermocouple wires and the sheath are all welded together to form one junction at the probe tip. Grounded thermocouples have a very good response time because the thermocouple is making direct contact with the sheath, allowing heat to transfer easily. A drawback of the grounded thermocouple is that the thermocouple is more susceptible to electrical interference. This is because the sheath often comes into contact with the surrounding area, providing a path for interference.
Ungrounded: A thermocouple is ungrounded when the thermocouple wires are welded together but they are insulated from the sheath.
Exposed: A thermocouple is exposed when the thermocouple wires are welded together and are sticking out of an open probe tip. The response time is very quick, but exposed thermocouple wires are more prone to corrosion and degradation. Unless your application requires exposed junctions, this style is not recommended.
Ungrounded Uncommon: An ungrounded uncommon thermocouple consists of a dual thermocouple that is insulated from the sheath and each of the elements is insulated from one another.
What is mineral insulated cable?
A mineral insulated (M.I.) cable is used to create thermocouple probes. M.I. cable insulates thermocouple wires from one another and provides the metal sheath that surrounds them. M.I. cable has two (or four when duplex) thermocouple wires running down the middle of the sheath. The sheath contains highly compacted magnesium oxide to ensure the wires are properly insulated and separated. M.I. cable helps to protect the thermocouple wire from corrosion and electrical interference and provides probes with the greatest durability and service life. Learn more about our thermocouple sheath materials here.
What are the Special Limits of Error (SLE) vs. Standard Limits of Error?
Standard limits of Error: These thermocouples use standard “thermocouple grade” wire and make up the great majority of sensors.
Special Limits of Error: These thermocouples are made with a higher grade of thermocouple wire that is purer than standard wire, which increases their accuracy. They are more expensive than standard thermocouples. SLE thermocouples have twice the accuracy of standard limits.
What is System Error?
System error is calculated by adding the accuracy of the temperature sensor (thermocouple) and the accuracy of the meter used to read the voltage signal together. For example, a Type K thermocouple has an accuracy of +/- 2.2°C at 50°C. The meter has an accuracy of +/- 1.0°C. That means the total system error is +/- 3.2°C at 50°C.
How do thermocouples compare to RTDs?
Temperature Range: First, consider the difference in temperature ranges. Base metal thermocouples can reach 2,300°F and noble metal thermocouples can reach 3,100°F. Compare this to RTDs which have a standard maximum range of 400°F and an extended maximum range of 1,100°F.
Cost: A thermocouple has a lower initial cost compared to an equivalent RTD unit.
Accuracy, Linearity, & Stability: As a general rule, RTDs are more accurate than thermocouples. This is especially true at lower temperature ranges. RTDs are also more stable and have better linearity than thermocouples. If accuracy, linearity, and stability are your primary concerns and your application is within an RTD’s temperature limits, go with the RTD.
Durability: In the sensors industry, RTDs are widely regarded as less durable sensors when compared to thermocouples. However, Reotemp has developed manufacturing techniques that have greatly improved the durability of our RTD sensors. These techniques make Reotemp’s RTDs nearly equivalent to thermocouples in terms of durability.
Response Time: RTDs cannot be grounded. For this reason, they have a slower response time than grounded thermocouples. Also, thermocouples can be placed inside a smaller diameter sheath than RTDs. A smaller sheath diameter will reduce response time. For example, a grounded thermocouple inside a 1/16” diameter sheath will have a faster response time than an RTD inside a ¼” diameter sheath. The effect of sheath diameter on thermocouple response time, for both grounded and ungrounded, is shown in the chart on page 6.
What considerations should I make for hazardous locations or applications?
Safety is a primary concern when instruments are used in hazardous locations, especially in applications where the potential for explosion is involved due to a concentration of flammable gases, vapors, or dust. When specifying a sensor or any instrumentation circuit for use in a hazardous location, the user needs to exercise caution in complying with local and/or national electrical codes and safety regulations.
It is important to consider the failure modes of the instrumentation and any catastrophic effects these modes could have based on the environment they are used in. The integrity and suitability of any instrumentation for use in hazardous locations is ultimately the responsibility of the end-user and/or of those making the specification decisions.
Generally, when instrumentation is to be used in a hazardous location, two commonly used methods to minimize the risk of ignitions and explosions are intrinsically safe and explosion-proof systems. Intrinsically safe systems are centered around prevention whereas explosion-proof systems are focused on containment.
Intrinsically safe systems operate on low power and are designed to limit the thermal and electrical energy of the instrument and associated connections to a level where ignition is not possible. Also, the devices cannot store enough energy to cause a spark when energy is released.
Explosion-proof systems are based on the principle of containment. In other words, an explosion-proof enclosure prevents any generated flames, sparks, or hot gases from escaping. These devices are designed to contain, control, cool, and/or vent any possible ignition due to a failure mode, without igniting the surrounding atmosphere.
When it comes to RTDs and thermocouples, it is important to note that these temperature measurement sensors are defined as simple apparatus by the National Fire Protection Association (NFPA). This means that they simply operate on, store and/or generate too low-level energy, i.e. low amperage and low voltage, to cause an ignition. This status can change, however, depending on what the sensor is connected to or how hot the surface of the sensor assembly may get outside of the process. The required method of protection can vary depending on many different factors and the entire instrumentation system must be examined before use in hazardous locations.
In summary, RTD and thermocouple sensors are considered simple apparatus as defined in the National Electric Code NFPA 70 Article 100 (Rev. 2020). By definition, these sensors generate too low an energy to be an ignition source. For use in hazardous environments, the sensor is typically connected to an intrinsically safe or explosion-proof apparatus. Final application suitability and compliance with regional standards are to be determined and approved by the end-user.