1. How do thermocouples work?
Thermocouples are widely used, low cost temperature sensors. They are formed from two dissimilar metal wires welded together at one end, forming a temperature sensitive region known as a thermocouple junction. A small temperature dependent voltage difference occurs across the junction due to the thermoelectric effect. By measuring the voltage difference, the temperature can be determined. Many types of the thermocouples are available, each made of different metals. The types are denoted by letters, e.g. K type or J type. Different thermocouple types have varying measurement ranges, accuracy and output sensitivities. Although it is possible to purchase bare thermocouple wires, most are manufactured with an electrically insulating layer and a protective outer braiding. Furthermore, thermocouples normally include a protective metal sheath covering the thermocouple junction.
Since the voltage difference generated by the thermoelectric effect is very small, the thermocouple output must be amplified before it can be read by the process controller. However, connecting the thermocouple wire to an amplifier (or any other electronic device) results in the formation of two additional and unwanted thermocouple junctions between the ends of the wires and the amplifier. The unwanted junctions, referred to as cold junctions, generate their own voltage differences and are therefore a large source of error. To eliminate the cold junction voltages, the amplifier performs a process known as cold junction compensation. This requires a special amplifier called a thermocouple amplifier. Each amplifier is compatible with a specific thermocouple type, though many amplifiers are programmable. Whenever extending the length of a thermocouple (or attaching a thermocouple connector), ensure that the extension wire is made of the same two metals as the thermocouple, to avoid creating additional cold junctions.
2. General characteristics of thermocouples
Thermocouples are low cost and high reliability but low accuracy temperature sensors, used across almost every industry. Their main advantages are their large measurement ranges (-270 to 1,500 °C, depending on thermocouple type) and fast response times. They are passive sensors, meaning that they do not require an external voltage supply. Their output sensitivity ranges from 0.01 – 0.07 mV/°C depending on the thermocouple type. They are available in a wide range of wire diameters, the choice of which strongly effects response time. Furthermore, they are available with different electrical insulation and protective braiding materials, the choice of which effects cost and allowable operating temperature.
3. Input and output signals
Thermocouples are passive sensors and therefore do not require a supply voltage. They output a small voltage, approximately proportional to the process temperature, which must be amplified to be read by the process controller (though many controllers include built-in thermocouple amplifiers). The amplifier is known as a thermocouple amplifier.
4. Applications of thermocouples
Thermocouples are the most popular type of temperature sensor. They are used across almost every industry, with many of their applications overlapping with those of RTD sensors, e.g. within the process manufacturing industry. Thermocouples are the first choice for measuring extreme temperatures (both high and low) such as in furnaces, jet engines and cryogenics. Furthermore, bare thermocouple wires prove popular for applications requiring fast response times.
5. Typical specification
|Measurement range||See chart below|
|Accuracy||See chart below|
|Output voltage||mV signal|
|Passive / active||Passive|
|Contact / non-contact||Contact|
|Sheath diameter||3-8 mm typical|
|Sheath length||25-1,000 m typical|
Temperature range, accuracy and color code for common thermocouple types
Service temperatures of common insulation and braiding materials
|PVC (insulation)||-10 to 105 °C|
|PFA (insulation)||-70 to 250 °C|
|Fiber glass (insulation)||-70 to 400 °C|
|Ceramic (insulation)||0 to 1,200 °C|
|Stainless steel (braiding)||600 °C|
|Inconel (braiding)||1,200 °C|
6. Purchasing tips
- Thermocouple type: K type thermocouples are the most common type and therefore a good default choice for most applications. Other thermocouple types are useful in more demanding applications e.g. if there is a requirement for higher accuracy or sensitivity. The table below shows which thermocouple performs best across several selection criteria. Note that R and S type thermocouples are particularly expensive due to them containing platinum.
- Response time: Due to their thermal mass, thermocouples require a finite time to arrive at the fluid temperature. Exposed thermocouples made of thin wires have exceptionally fast response times. Conversely, sheath protected, ungrounded thermocouples with large diameter wires have slow response times. Fluid thermal conductivity and velocity also effect response time e.g. response times in water are faster than in air. Response times are defined in terms of time constants. The time constant is the time taken for the temperature difference between the thermocouple and the fluid to decay by 63.2% (e.g. from 100 °C to 46.8 °C). The 2nd time constant is the time taken for the temperature difference to decay by a further 63.2%, i.e. from 63.2% to 86.5%. As such, the time taken for the temperature difference to decay from 0% to 63.2% is equal to the time taken from 63.2% to 86.5%. The first 5 time constants are illustrated in the graph below, where 0 represents the initial sensor temperature and 100% represents the fluid/process temperature.
Time constants of thermocouple wires for initial probe temperatures of 38 °C
- Grounded, ungrounded or exposed: Thermocouples are available either with or without protective metal sheaths. Those without sheaths are referred to as exposed junction thermocouples. They have a much faster response time but are more delicate and vulnerable to oxidation, particularly Galvanic corrosion. Thermocouples protected with sheaths maybe either be grounded (to the sheath) or ungrounded. Grounding provides better thermal contact and therefore faster response times. However, grounding creates a ground loop, which can increase electrical noise.
- Insulation and braiding materials: Most thermocouple types are capable of measuring temperatures in excess of 1,000 °C. However, the electrical insulation and protective braiding often limit the practical maximum operating temperature. For example, PVC insulated K type thermocouples may only operate up to a maximum temperature of 105 °C, despite the K type thermocouple wire being capable of operating at up to 1,260 °C (see material temperature limit table).
- Sheath material: Sheaths are commonly made of stainless steel. However, Inconel is recommended for applications above 600 °C because stainless steel becomes vulnerable to corrosion at high temperatures.
- Sheath length and diameter: Thermocouple sheaths are typically 3-8 mm in diameter and 25-1,000 mm in length, though other sizes are also available.
- Special limits of error: Each thermocouple type can be purchased in ‘special limits of error’ meaning that higher purity metals are used, resulting in improved accuracy.
7. Advantages of thermocouples
Thermocouple temperature sensors:
- Are very low cost and highly robust temperature sensors.
- Have extremely large measurement ranges, from cryogenic to temperatures in excess of 1,500 °C. The practical temperature range depends on both the thermocouple type and choice of insulation/braiding material.
- Are available with very thin wires (down to 0.025 mm), enabling very fast response times, with time constants of under 0.01 seconds possible in some fluids.
- Do not experience self heating because they are passive sensors
8. Disadvantages of thermocouples
Thermocouple temperature sensors:
- Are of low accuracy compared to RTD resistance thermometers.
- Emit a very small output voltage (0.01-0.07 mV/°C depending on type), making them sensitive to electrical noise.
- Exposed thermocouples are susceptible to galvanic corrosion due to two dissimilar metals touching.
- Require more expensive signal processing because of the need for signal amplification and cold junction compensation.
- Have lower long term stability than RTD resistance thermometers.
9. Application tips
- Thermowells: A thermowell is type of protective sheath which performs the same function as a regular thermocouple sheath but is not permanently attached to the thermocouple. It enables the thermocouple to be removed from a pipe or vessel without any down time or leakage of fluid because the thermowell remains in place providing a reliable fluid tight seal.
- Calibration for improved accuracy: A significant cause of error in thermocouples is material impurity within the thermocouple wire, leading to accuracy being far lower than repeatability. Accuracy can therefore be improved by calibrating the thermocouple and using the calibration data for determining temperature.
- Differentiating between grounded and ungrounded thermocouples: It is possible to determine whether a thermocouple is grounded by using a multi-meter to test for electrical continuity between the sheath and either thermocouple wire. If there is electrical continuity is means that the thermocouple is grounded.
- Wire end temperature: Thermocouple amplifiers measure ambient temperature to perform cold junction compensation. To maintain accuracy, the ends of the thermocouple wires (or thermocouple connector if one is used) must remain at ambient temperature. Choosing thermocouples with long and thin wires reduces the rate of heat transfer, helping to maintain the wire ends at ambient temperature.
- Avoiding thermal shunting: Thermal shunting occurs when the presence of the thermocouple effects the measured temperature by absorbing (or dissipating) heat. The susceptibility to thermal shunting is application dependent. Thermal shunting can be minimized by using smaller diameter thermocouple wires and sheaths.
- Avoiding linearity error: The thermocouple output is not completely linear, which causes measurement error. High end digital thermocouple readers avoid linearity error by using built-in thermocouple tables for determining temperature from the output voltage.