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A thermocouple is an essential type of transducer that transforms thermal energy into electrical energy. It is constructed by connecting wires made of different metals to create a junction. When the temperature at this junction changes, a voltage is produced, which can be measured to determine the temperature accurately.
This device operates on the principle of the Seebeck Effect, which indicates that when different metals form a junction, they produce a small, detectable voltage in response to temperature changes at that junction. The voltage generated is influenced by the degree of temperature change and the characteristics of the metals involved.
Comprising two insulated wires of distinct metals connected to a measuring device, a thermocouple serves as both a safety and monitoring instrument for a wide array of processes and machinery, ensuring reliable temperature readings to maintain proper functioning.
Below is an illustration depicting how a thermocouple works. As the temperature rises at the junction of the wires on the left, the associated change in temperature is shown on the gauge on the right.
Thermocouple assemblies are specifically designed for application in tough, extreme, and challenging environments. Selecting the right thermocouple involves considering factors such as temperature range, environmental conditions, and the type of material being measured. Moreover, the thermocouple's dimensions and form are customized to suit particular applications, ensuring optimal accuracy and responsiveness.
In a thermocouple, the two wires are joined to form a junction where one wire, connected to the body of the thermocouple, acts as the hot or measuring junction that measures the temperature. The other wire connects to a reference junction, also known as the cold junction, which is kept at a known temperature. The thermocouple determines the unknown temperature by comparing the voltage generated at the hot junction with the reference temperature at the cold junction.
The idea of a thermocouple is based on three principles of effect discovered by Seebeck, Peltier, and Thomson.
The Seebeck effect occurs when two different metals are joined together at two junctions, creating an electromotive force (emf). This emf varies depending on the types of metals used and the temperature difference between the junctions. The Seebeck effect is the principle behind the operation of thermocouples, where the generated voltage is used to measure temperature differences.
An electromotive force (emf) is generated in a circuit when two dissimilar metals are joined to form two junctions, due to the temperature difference between these junctions. The varying temperatures cause a voltage to develop across the junctions, which can be measured and used to determine temperature changes.
The Thomson effect occurs when heat is absorbed or released along the length of a conductor that has different temperatures at its ends. This effect is related to the flow of electrical current through the conductor and the temperature gradient along it. Essentially, the Thomson effect describes how the temperature of the conductor changes in response to the electric current and its distribution along the rod.
The circuit of a thermocouple is illustrated in the image below, where wires A and B, made of different metals, are joined to form a junction. The two junctions are maintained at different temperatures, generating a Peltier emf in the circuit. This emf is a function of the temperature difference between the two junctions.
Electrons are responsible for carrying both heat and electricity. When one end of a copper wire is heated, electrons move towards the cooler end, creating a temperature gradient along the wire. This movement of electrons converts heat into electrical energy. The same principle, as discovered by Volta and Seebeck, is utilized in thermocouples to measure temperature differences.
When the junctions of a thermocouple are at different temperatures, a millivolt signal is generated, which is unique to the specific pair of conductor materials used. This signal is defined by the International Electrotechnical Commission’s standards IEC 1977. Thermocouples made according to these standards are standardized, ensuring they are interchangeable regardless of the manufacturer or country of origin.
For a thermocouple to provide accurate measurements, it requires cold junction compensation, typically achieved using an ice or water bath to set the reference temperature. This ensures that the two ends of the thermocouple are maintained at a consistent temperature, allowing accurate comparison between the hot junction and the cold junction, as shown in the diagram above. Additionally, a thicker thermocouple wire can measure higher temperatures but tends to have a slower response time.
If the temperatures of the junctions in a thermocouple are identical, an equal and opposite electromotive force (EMF) will be generated at each junction, resulting in zero current flow through the circuit. However, when the junctions have different temperatures, the EMF will not cancel out, and current will flow through the circuit, similar to how heat flows through a copper wire. The magnitude of the EMF and the resulting current depend on the types of metals used and the temperature difference between the two junctions. This voltage is measured by a meter to determine the temperature difference.
The EMF generated in a thermocouple circuit is very small, typically measured in millivolts, and requires a highly sensitive instrument for accurate measurement. A measuring or reading instrument is essential to amplify the millivolt signal, interpret it as a temperature reading, and display the result. Commonly used instruments include galvanometers and voltage-balancing potentiometers, with potentiometers being the most frequently used for this purpose.
A potentiometer, also known as a pot or potmeter, measures potential difference by comparing an unknown voltage to a reference voltage, offering high precision in its measurements. It is a three-terminal variable resistor that functions as an adjustable voltage divider, allowing for accurate control and adjustment of voltage levels.
A galvanometer measures very small electric currents and is often used to detect null deflection or zero current. It provides precise readings of minute electrical currents by indicating when the current is balanced to zero, making it a valuable tool for sensitive electrical measurements.
For a thermocouple to provide an absolute temperature measurement, it must be referenced to a known temperature, such as the freezing point, at the other end of the sensor cable. The hot junction acts as the measuring point, while the cold junction, shown in the diagram below, serves as the reference point where a cold junction compensation chip is located. Although the cold junction temperature may fluctuate, it provides a necessary reference. To ensure a constant temperature, the cold junction can be stabilized by immersing it in water or ice.
Ambient air can affect the reference temperature of a thermocouple. To counteract this, the system can be calibrated and adjusted using a reference junction compensation device. This device helps to maintain accurate measurements by compensating for variations in ambient conditions.
A thermowell is designed to protect a thermocouple from process media by encasing it in a closed tube or solid bar-stock that is mounted within the media. Thermowells are commonly used with fluids and pressure lines in refineries and chemical plants to extend the life of thermocouples. They facilitate the replacement of a thermocouple without requiring a process shutdown. Various types of thermowells are available, depending on the application, including:
Thermowells are also categorized by the method used to connect them to a thermocouple or thermistor. These connection types can include:
The differences between thermocouples are determined by the types of alloys used to produce their wires. The choice of metal wire depends on factors such as the temperature range to be measured, the environmental conditions, and the required mechanical strength. Thermocouples can be connected in three different ways: exposed, ungrounded or insulated, and grounded.
A thermocouple can be enclosed in a sheath to protect it from the atmosphere and minimize the risk of corrosion. Common sheath materials include stainless steel, Inconel, and Incoloy. Inconel and Incoloy are registered trademarks of Special Metals Corporation and are types of nickel alloys. The temperature ranges for the various types of sheaths are detailed in the chart below.
It is low-cost, offers good flexibility, provides fair electrical performance, and serves as a general-purpose material.
It has a high cost, high temperature rating, excellent chemical resistance, and superior electrical properties, but it exhibits poor cut-through resistance.
It boasts excellent physical, electrical, and mechanical properties across a wide range of temperatures, making it suitable for applications involving extreme heat and vibration. It maintains its mechanical properties even under the harshest conditions.
It is low-cost, offers excellent electrical properties, has high flammability, and is stiffer than vinyl.
It is excellent for high-temperature applications and suitable for use in environments where there is a possibility of hot spots.
It is used in commercial ovens and furnaces, and can monitor ambient temperatures in fireboxes, kilns, and grills. Its temperature range extends from -58°F to 2200°F.
A jacket can be applied over the primary insulation when additional mechanical protection is required. For vinyl insulation, the jacket is typically made of nylon, while polyethylene is used for vinyl or nylon insulation. This conductor jacket serves as a mechanical barrier, preventing shorting and providing extra durability.
The extension wires connect the sensor wire to the measuring instrument and are made from the same metals as the thermocouple wires. Typically, these extension wires are composed of a copper alloy and have a similar electromotive force (EMF) thermal coefficient as the thermocouple, ensuring accurate temperature measurement.
The four most common types of thermocouple circuitry are standard single, average, thermopile, and delta.
A standard single thermocouple consists of two dissimilar wires joined together at a measuring junction.
An average thermocouple configuration involves two or more thermocouples connected in parallel to a common cold junction. If the resistances of the thermocouples are equal, the electromotive force (EMF) will represent the average temperature of each junction.
A thermopile consists of a series of thermocouples connected in series. The electromotive force (EMF) generated by the thermopile is the sum of the EMFs from each individual thermocouple junction.
A delta thermocouple, also known as a differential thermocouple, features two similar wires joined to a dissimilar wire, with measuring junctions at different temperatures. The electromotive force (EMF) generated is the difference between the temperatures of the two junctions, known as the differential temperature. In this setup, one of the thermocouple junctions must be ungrounded, and a differential measuring instrument is required to measure the temperature difference accurately.
Thermocouples are available in various types, each suited for different applications, and are identified by a system of letters. Each type has distinct characteristics and temperature ranges. The differences between thermocouple types are based on their durability, temperature range, resistance, and specific applications.
The most commonly used thermocouple type features a grounded construction, selected primarily for its speed, as it responds approximately 50% faster than ungrounded types. In this design, the two wires are welded to the side of the metal probe sheath, and the tip of the probe completes the circuit.
The ungrounded type of thermocouple is typically the second choice, with its junction isolated from the sheath material. This isolation method results in slower response times compared to grounded types. However, ungrounded thermocouples generally have a longer lifespan, interface more easily with instrumentation, and are less susceptible to ground loop problems.
The least commonly used thermocouple is the exposed type, where the thermocouple protrudes from the sheath and is directly exposed to the environment. It has the fastest response time but is limited to applications that are dry, non-corrosive, and non-pressurized. Due to its exposed element, this type is more susceptible to damage and corrosion.
Common types of thermocouples include Types C, B, E, J, N, K, R, T, and S, which use base metals such as iron, copper, nickel, platinum, rhodium, and chromel. Each thermocouple consists of two different metals joined to form a junction, with each junction operating at a different temperature.
Type C thermocouples are constructed from tungsten and rhenium and are designed for applications involving extremely high temperatures, up to 4200°F (2315°C). They are typically used in hydrogen, inert, or vacuum atmospheres to prevent oxidation and failure. These thermocouples are equipped with protective sheaths made of materials such as molybdenum, tantalum, and Inconel, and feature insulators made of alumina, hafnia, and magnesium oxide.
Type E thermocouples feature chromel (a nickel-chromium alloy) as the positive leg and constantan (a copper-nickel alloy) as the negative leg. They operate within a temperature range of -330°F to 1600°F (-200°C to 870°C) and offer excellent electromotive force (EMF) versus temperature values. Type E thermocouples are suitable for sub-zero temperatures and are typically color-coded red or purple. They perform well in inert environments but need protection in sulfurous conditions.
Type J thermocouples use iron as the positive leg and constantan as the negative leg. They are commonly used in oxidizing, vacuum, inert, and reducing atmospheres, with injection molding being a typical application. Type J thermocouples need close monitoring because the iron leg can rust over time. They have a temperature range of 32°F to 1000°F (0°C to 760°C) and are color-coded red or white. Their lifespan can be limited when exposed to consistently high temperatures.
Type K thermocouples feature chromel for the positive leg and alumel for the negative leg. Alumel is an alloy primarily composed of nickel, with small amounts of aluminum, silicon, and manganese. These thermocouples are suitable for use in inert or oxidizing environments and have a temperature range of -300°F to 2300°F (-200°C to 1260°C). They exhibit EMF variations at temperatures below 1800°F (982°C), which can limit their effectiveness in some inert environments. Type K thermocouples are color-coded red or yellow.
Type N thermocouples use nicrosil, a nickel-chromium alloy, for the positive leg and nisil, a nickel-silicon-magnesium alloy, for the negative leg. They operate within a temperature range of 32°F to 2300°F (0°C to 1260°C) and are color-coded red or orange. Known for their exceptional resistance to green rot and hysteresis, Type N thermocouples are commonly used in refineries and the petrochemical industry.
Type T thermocouples consist of copper as the positive leg and constantan as the negative leg. They have a temperature range of -330°F to 700°F (-200°C to 370°C) and are color-coded red or blue. These thermocouples are well-suited for inert atmospheres and are resistant to decomposition. Type T thermocouples are commonly used in food production and cryogenic applications.
Noble metal thermocouples, also known as platinum thermocouples, include Types B, R, S, and P. These thermocouples use precious metal elements and are known for their high accuracy at very elevated temperatures. They also offer a long lifespan, making them suitable for demanding applications where precision and durability are essential.
The Type B thermocouple is designed for extremely high-temperature applications and boasts the highest temperature limit among all thermocouples. It offers exceptional accuracy and stability, utilizing an alloy combination of Platinum (6% Rhodium) and Platinum (30% Rhodium). The temperature range for Type B thermocouples extends from 2500°F to 3100°F (1370°C to 1700°C).
Type R thermocouples feature legs made of platinum with 13% rhodium and platinum, and have a temperature range of -58°F to 2700°F (-50°C to 1450°C). They are generally more expensive than Type S thermocouples due to the higher rhodium content. Type R thermocouples are known for their excellent accuracy and are commonly used in sulfur recovery processes. They offer similar performance to Type S thermocouples and are suitable for both high and low-temperature applications due to their stability.
Type S thermocouples are employed in high-temperature applications within the BioTech and pharmaceutical industries, as well as in low-temperature scenarios due to their precision and stability. They operate within a temperature range of -58°F to 2700°F (-50°C to 1450°C).
Type P thermocouples have a temperature response curve similar to that of Type K at high temperatures and can be used in oxidizing atmospheres with a temperature range up to 2300°F (1260°C). To connect a Type P thermocouple to the measuring instrument, a Type K extension wire is typically used.
Thermocouples are popular temperature sensors due to their broad temperature range, durability, and affordability. They are utilized in a variety of applications, including home appliances, industrial processes, electric power generation, furnace monitoring and control, food and beverage processing, automotive sensors, aircraft engines, rockets, and spacecraft.
Their compact size, rapid response time, and ability to withstand shocks and vibrations make thermocouples ideal for precise temperature control and measurement.
Below is a description of some of the various applications for thermocouples:
Thermocouples are ideal for the food industry due to their ability to provide accurate temperature readings quickly. They can be used at various stages of production to ensure proper cooking or storage conditions. Food production thermocouples typically consist of a two-piece unit: a handheld readout and a detachable probe. The probe contains two wires connected at the tip. Flat-headed probes are used to measure surface temperatures, while needle probes are used for internal measurements and to monitor air temperatures in ovens.
Extruders, which operate under high temperature and pressure conditions, require precise temperature measurement. The thermocouple's sensor tip must be placed in the molten plastic, where it can accurately measure the temperature directly within the process. These thermocouples offer high accuracy and rapid response times and often utilize a Type K thermocouple probe to meet the demands of such challenging environments.
A pilot light ignites the furnace burner, and the thermocouple plays a crucial safety role by monitoring the flame. If the thermocouple does not detect a flame, it shuts off the gas supply, preventing gas from accumulating in the furnace and enhancing overall safety. This mechanism ensures that the furnace only receives gas when the pilot light is properly lit, reducing the risk of hazardous gas buildup.
A molten metal thermocouple is designed for use in non-ferrous metal environments and can measure temperatures up to 1250°C. These thermocouples are essential for monitoring and controlling the temperature of liquid metals throughout various stages, including melt preparation, holding, degassing, and casting operations. Their high-temperature capabilities and durability make them crucial for ensuring precision and quality in metal processing.
A thermocouple on a gas appliance plays a critical safety role by signaling the gas valve to remain open when the pilot light is lit. Positioned within the pilot flame, the thermocouple detects the heat and generates a voltage that keeps the gas flowing to the burner. If the pilot flame extinguishes, the voltage produced by the thermocouple drops, causing the gas valve to close and prevent the release of gas, thereby enhancing safety and preventing potential hazards.
Finding suitable instrumentation for high-pressure applications can be challenging due to the extreme temperatures and heavy vibrations involved. In these demanding environments, resistance thermometers (RTDs) and thermocouples are commonly used temperature sensors. However, thermocouples are often the preferred choice due to their robustness, wide temperature range, and ability to withstand high pressures and vibrations effectively.
There are two configurations of thermocouples for high pressure applications, which are pictured below:
Though thermocouples are very reliable and durable, they can fail over time and need to be regularly checked. Regardless of the wide variety of thermocouples available, they all operate on the same basic principle: two connected wires, where one wire serves as the reference junction and the other as the hot or measuring junction.
The testing of the efficiency of a thermocouple involves using a multimeter. Below is a description of a multimeter and instructions on how to test a thermocouple using it.
Multimeters come in various forms and styles. Despite these variations, they all display some basic symbols that indicate their different functions.
There are also prefixes that may be displayed as well.
Multimeters have settings for measuring AC and DC currents.
Some multimeters feature a continuity beeper that sounds when the meter detects a closed circuit. A continuity check is used to verify the presence of a complete path for current flow. The image below shows a multimeter equipped with a continuity beeper.
The multimeter should be able to read ohms, which measure the resistance to current flow in an electrical circuit. Conductors, such as silver, copper, gold, and aluminum, offer little resistance, while insulators have high resistance. These metals are commonly found in thermocouple wires. Since thermocouples generate millivolt signals, the multimeter used for testing must be highly sensitive.
For the resistance test, first remove the thermocouple from the application. Set the multimeter to the ohms option. Place one lead on the side of the thermocouple and the other lead at the end that was inserted into the application. If the thermocouple has proper continuity, the multimeter should display a small resistance reading.
For the open circuit test, first remove the thermocouple from the application. Set the multimeter to measure millivolts. Connect one lead to the side of the thermocouple and the other lead to the opposite end. Heat the end that was previously inserted into the application. The millivolt reading should fall within the acceptable range for the thermocouple type being tested.
The closed circuit test requires a thermocouple adapter. Insert the adapter into the application, then screw the thermocouple into the adapter. Connect one lead of the multimeter to the screw of the adapter and the other lead to the exposed end of the thermocouple. Activate the application to get a reading from the multimeter, which will be displayed in millivolts. If the thermocouple fails this test, it should be replaced.
Thermocouples are a cost-effective method for measuring a wide range of temperatures with accuracy. They are commonly used in boilers, water heaters, ovens, and airplane engines.
When preparing to read a thermocouple, it is necessary to understand a thermocouple reference table. Each type of thermocouple has its own reference table. Below is a portion of the reference table for a Type K thermocouple.
| Type K Thermocouple Reference Table | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| °C | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| Thermoelectric Voltage in mV | |||||||||||
| -270 | -6.458 | ||||||||||
| -260 | -6.411 | -6.444 | -6.446 | -6.448 | -6.450 | -6.452 | -6.453 | -6.455 | -6.456 | -6.457 | -6.458 |
| -250 | -6.404 | -6.408 | -6.413 | -6.417 | -6.421 | -6.425 | -6.429 | -6.432 | -6.435 | -6.438 | -6.441 |
| -240 | -6.344 | -6.351 | -6.358 | -6.364 | -6.370 | -6.377 | -6.382 | -6.388 | -6.393 | -6.399 | -6.404 |
| -230 | -6.262 | -6.271 | -6.280 | -6.289 | -6.297 | -6.306 | -6.314 | -6.322 | -6.329 | -6.337 | -6.344 |
| -220 | -6.158 | -6.170 | -6.181 | -6.192 | -6.202 | -6.213 | -6.223 | -6.233 | -6.243 | -6.252 | -6.262 |
| -210 | -6.035 | -6.048 | -6.061 | -6.074 | -6.087 | -6.099 | -6.111 | -6.123 | -6.135 | -6.147 | -6.158 |
| -200 | -5.891 | -5.907 | -5.922 | -5.936 | -5.951 | -5.965 | -5.980 | -5.994 | -6.007 | -6.021 | -6.035 |
| -190 | -5.730 | -5.747 | -5.763 | -5.780 | -5.797 | -5.813 | -5.829 | -5.845 | -5.861 | -5.876 | -5.891 |
| -180 | -5.550 | -5.569 | -5.588 | -5.606 | -5.624 | -5.642 | -5.660 | -5.678 | -5.695 | -5.713 | -5.730 |
| -170 | -5.354 | -5.374 | -5.395 | -5.415 | -5.435 | -5.454 | -5.474 | -5.493 | -5.512 | -5.531 | -5.550 |
| -160 | -5.141 | -5.163 | -5.185 | -5.207 | -5.228 | -5.250 | -5.271 | -5.292 | -5.313 | -5.333 | -5.354 |
| -150 | -4.913 | -4.936 | -4.960 | -4.983 | -5.006 | -5.029 | -5.052 | -5.074 | -5.097 | -5.119 | -5.141 |
| -140 | -4.669 | -4.694 | -4.719 | -4.744 | -4.768 | -4.793 | -4.817 | -4.841 | -4.865 | -4.889 | -4.913 |
| -130 | -4.411 | -4.437 | -4.463 | -4.490 | -4.516 | -4.542 | -4.567 | -4.593 | -4.618 | -4.644 | -4.669 |
| -120 | -4.138 | -4.166 | -4.194 | -4.221 | -4.249 | -4.276 | -4.303 | -4.330 | -4.357 | -4.384 | -4.411 |
| -110 | -3.852 | -3.882 | -3.911 | -3.939 | -3.968 | -3.997 | -4.025 | -4.054 | -4.082 | -4.110 | -4.138 |
| -100 | -3.554 | -3.584 | -3.614 | -3.645 | -3.675 | -3.705 | -3.734 | -3.764 | -3.794 | -3.823 | -3.852 |
| -90 | -3.243 | -3.274 | -3.306 | -3.337 | -3.368 | -3.400 | -3.431 | -3.462 | -3.492 | -3.523 | -3.554 |
| -80 | -2.920 | -2.953 | -2.986 | -3.018 | -3.050 | -3.083 | -3.115 | -3.147 | -3.179 | -3.211 | -3.243 |
| -70 | -2.587 | -2.620 | -2.654 | -2.688 | -2.721 | -2.755 | -2.788 | -2.821 | -2.854 | -2.887 | -2.920 |
| -60 | -2.243 | -2.278 | -2.312 | -2.347 | -2.382 | -2.416 | -2.450 | -2.485 | -2.519 | -2.553 | -2.587 |
| -50 | -1.889 | -1.925 | -1.961 | -1.996 | -2.032 | -2.067 | -2.103 | -2.138 | -2.173 | -2.208 | -2.243 |
| -40 | -1.527 | -1.564 | -1.600 | -1.637 | -1.673 | -1.709 | -1.745 | -1.782 | -1.818 | -1.854 | -1.889 |
| -30 | -1.156 | -1.194 | -1.231 | -1.268 | -1.305 | -1.343 | -1.380 | -1.417 | -1.453 | -1.490 | -1.527 |
| -20 | -0.778 | -0.816 | -0.854 | -0.892 | -0.930 | -0.968 | -1.006 | -1.043 | -1.081 | -1.119 | -1.156 |
| -10 | -0.392 | -0.431 | -0.470 | -0.508 | -0.547 | -0.586 | -0.624 | -0.663 | -0.701 | -0.739 | -0.778 |
| 0 | 0.000 | -0.039 | -0.079 | -0.118 | -0.157 | -0.197 | -0.236 | -0.275 | -0.314 | -0.353 | -0.392 |
The first column on the left of the table lists temperatures in increments of ten. The portion of the table to the right shows intermediate distances in increments of one, between the temperature ranges. For example, in the table above, -280°F is the third entry from the top. If the temperature reading on the thermocouple is -284°F, you would locate -280°F in the table and then move to the right to find the number under the column labeled 4. The numbers in this section of the table represent the millivolt readings corresponding to the temperature.
Reference junctions on a thermocouple may experience temperature fluctuations, which can lead to inaccurate readings. To ensure accurate measurements, the reference temperature can be stabilized by immersing the reference junction in water or by using a reference junction compensator. This compensator adjusts for any ambient temperature changes. The image below provides a simplified representation of a compensation calculator.
A homogeneous wire is physically and chemically uniform throughout its length. In a thermocouple circuit made from such a wire, no electromotive force (emf) will be generated, even with changes in temperature or thickness. For a thermocouple to function correctly and produce voltage, it must consist of two different metals joined together, as this is essential for generating an emf based on temperature differences.
The sum of the electromotive forces (emfs) in a thermocouple circuit will be zero if all junctions in the circuit are at the same temperature. Adding different metals to the circuit does not affect the voltage generated, as long as all junctions are at the same temperature. For instance, using copper leads to connect a thermocouple to measurement equipment or employing solder to join metals does not change the measured voltage. This is because the added junctions must be at the same temperature as the original junctions in the circuit, ensuring accurate temperature readings when using thermocouples with digital multimeters or other electrical components.
A thermocouple generates an electromotive force (emf) when two different metals are subjected to different temperatures. When calibrated with a reference temperature, a thermocouple can be connected to additional wires with the same thermoelectric characteristics without affecting the emf measurement. This means that extra wires, which maintain the same thermoelectric properties, can be added to the circuit without altering the emf produced by the thermocouple, as long as the temperature differences between the junctions remain constant.
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