Thermocouple is: what is it, principle of operation, types, circuit, with their own hands

It’s crucial to comprehend the technology underlying heating systems if we want to keep our homes toasty warm. The thermocouple is an essential part of many heating systems. However, what is a thermocouple exactly, and how does it operate?

A thermocouple is a straightforward yet brilliant tool for temperature measurement. It works on the basis of the Seebeck effect, which states that when two separate conductors have different temperatures, a voltage is produced. Put more simply, a small electrical current is produced when one end of the thermocouple is hotter than the other.

Thermocouples come in a variety of forms, each suitable for a particular range of temperatures and conditions. Among the more common types are Type J, Type T, and Type K. Different metal combinations, like nickel-chromium and nickel-aluminum, are used to create these thermocouples in order to accommodate a range of temperatures and applications.

For a thermocouple to function properly, its circuit must be understood. A thermocouple is usually attached to a meter or controller that senses the voltage produced by the change in temperature across the junction of the thermocouple. We can then precisely monitor and regulate the temperature of a heating system thanks to the conversion of this voltage into a temperature reading.

Although installing a thermocouple might seem like a task best left to the pros, with a little basic knowledge and supplies, you can make one yourself. A thermocouple is an easy-to-assemble and reasonably priced heating solution for a variety of applications. DIY enthusiasts can put one together with readily available components by following simple instructions.

Thermocouple A device that measures temperature by detecting the voltage generated when two different metals are joined and heated.
Principle of operation When two dissimilar metals are heated at one end, they produce a voltage at the other end, which is proportional to the temperature difference.
Types Common types include K, J, T, and E, each made of different metal combinations suited for various temperature ranges and environments.
Circuit Usually consists of the thermocouple wires connected to a measurement device like a thermometer or temperature controller.
With their own hands DIY thermocouples involve connecting the two different metal wires and insulating them properly for accurate temperature readings.

Where thermocouples are used

TP are used in equipment connected to high temperatures more frequently than other types of sensors. Examples of such equipment include fuel boilers and stoves, other burner-equipped equipment, boilers, soldering irons, pyrometers, furnaces, and metallurgy.

The nature of the sensor—a differential meter that measures by converting heat into electricity—is reflected in the term "thermoelectric transducer."

For high-precision thermoelectric thermometers operating in elevated temperature limits, thermocouples are a straightforward and efficient sensor.

Heating automation systems and fuel boilers are two excellent examples of applications. A TP-equipped sensor node sends out an electric signal that triggers the equipment.

The most widely used temperature meters for equipment are thermocouples, NTC thermistors, and PTC thermistors. Although the latter are thought to be more accurate in their ranges, they do not have the same range of temperatures as thermocouples.

Mounting methods

The mounting techniques mentioned above also work with thermocouples because they are made with the same dimensions as TCs. Refer to ο. in the RTD section above.

What is a thermocouple, its device

GOSTs 6616, R 8.585, and IEC 62460, 60584 govern TP. The latter’s item 2.2 describes a sensor as two multi-alloy conductors connected by a solder at one end to produce a thermoelectric effect that this segment will use to measure t°. The "hot solder," or junction point (head) of the electrodes is measured by the TP.

Though an unsightly piece of thin wires soldered together at one end may be all that a thermocouple device is, the sensor is still incredibly effective. frequently includes valuable metals.

  • two conductors, soldered at one end or, more rarely, twisted together. It is the hot junction, the sensitive segment that conducts the measurements;
  • the other ends – the place where there is no heat, connections to extension wires, the cold junction. They are connected at the receiver of the indicators.

When a galvanometer, microvoltmeter, or multimeter is inserted into the gap created by this closed circuit, the thermal EMF of a few millivolts or microvolts will be displayed. The amount is determined by the wire junction’s level of heating as well as the segment’s absence of a temperature indicator.

In other words, the EMF’s magnitude is determined by the alloys’ thermoelectric properties and the difference in t° between the cold and hot junctions.

A potential difference between their disconnected (cold) ends will manifest itself if the hot junction is heated.

Additionally, since the EMF force and temperature are related, the converter either independently or on the control unit of the serviced device calculates the temperature and converts the results into numbers or control commands.

What is a CHC?

The temperature on the cold segment should remain constant for special measurement accuracy, but this is challenging to accomplish under typical circumstances, so unique compensation schemes are employed. The difference in t° between the hot segment and the same segment determines the voltage recorded on the designated segment of the TP. For the purpose of calculating the same on the second, it is therefore necessary to know the heating level of the first. These computations are known as cold junction compensation, or CHS, and they are frequently employed in control of other pulse formation units or emergency shutdowns.

Because the extended wires of a thermocouple are susceptible to electrical interference, CHS always tends to measure (calculate) closer to the intended measurement point (signal deteriorates). When designing thermal sensors, manufacturers should take this into consideration.

Operating principle

To put it briefly, the TP is made up of wires made of two distinct alloys, each having unique electrical properties. When exposed to heat, the wires create a potential difference and a weak current, which are detected and recorded by the receiver.

However, if we dig further into the thermocouple’s operation, we have to discuss some important unique nuances.

Scientist T. Seebeck initially described thermoelectric response, which is used in the principle of thermocouple operation. There is a contact potential difference between the connected conductors. The sensor is made up structurally of two cores made of various alloys.

The ends come together to form a head, or contact, known as a hot junction (shown in red in the diagram below). This is most frequently formed by welding (seam, butt). The free ends are closed by compensation wires to the contacts of such devices, and at the points of connection with the TP, there is a cold junction (blue on Fig. below). The free ends go to the processing data and control units of the equipment being serviced.

The drawing above also depicts the electrodes of different metals, typically A and B, in various shades. A hermetic capsule (which could contain a liquid or inert gas), ceramic cylinders (shown in the figure below), a thermocouple, and how it works are all there to protect them.

Wikipedia’s explanation

The thermoelectric effect, which bears the name of the scientist T. Seebeck, serves as the basis for the action. An electromotive force, or thermo-EMF, is visible at the junction points of a circuit when it is closed, as with a millivoltmeter, for instance. The EMF would be mutually compensated, no current would flow, and electrodes made of the same alloys would be heated equally (equally).

In short, a thermocouple is a circuit that maintains a weak current by means of a potential difference between two conductors that are heated differently and have different junction temperatures. This process creates a thermo-EMF. a figure that corresponds to the junction’s t° difference. It is important to stress that only this value—and not any other—should be considered.

Another straightforward explanation of how a thermocouple functions is as follows: when two separate metal conductors are connected to form a closed electrical circuit and heated at this point, a tiny electric current and an electromotive force known as a thermo-EMF are produced. This data is sent by the TP to the measuring or serviced device’s chip, which processes it and determines the temperature (t°).

Thermocouple comparison table

We have already discussed the various kinds of thermoelectric transducers. It’s likely that the reader will wonder why there are so many different kinds of thermocouples.

The key point is that only a specific temperature range allows for the measurement accuracy that the manufacturer claims to be achievable. The linear characteristic of the product is guaranteed by the manufacturer within this range. The temperature dependence of the voltage may not be linear in other ranges, which will inevitably impact the accuracy. There is a maximum operating temperature for each material because different materials have varying degrees of fusibility.

We have created tables that highlight the key characteristics of the measuring transducers so that thermocouples can be compared. Here is an example of one of the table variations for comparing typical thermocouples.

Thermocouple type K J N R S B T E
Positive electrode material Cr-Ni Fe Ni-Cr-Si Pt-Rh (13 % Rh) Pt-Rh (10 % Rh) Pt-Rh (30 % Rh) Cu Cr-Ni
Negative electrode material Ni-Al Cu-Ni Ni-Si-Mg Pt Pt Pt-Rh (6 % Rh Cu-Ni Cu-Ni
Temperature coefficient 40…41 55.2 68
Operating temperature range, ºC 0 to +1100 0 to +700 0 to +1100 0 to +1600 0 to 1600 +200 to +1700 -185 to +300 0 to +800
Temperature limit values, ºC -180; +1300 -180; +800 -270; +1300 – 50; +1600 -50; +1750 0; +1820 -40; +900
Accuracy class 1, in the respective temperature range, (°C) ±1.5 from -40 °C to 375 °C ±1.5 from -40 °C to 375 °C ±1.5 from -40 °C to 375 °C ±1.0 from 0 °C to 1100 °C ±1.0 from 0 °C to 1100 °C ±0.5 -40 °C to 125 °C ±1.5 from -40 °C to 375 °C
±0.004×T from 375 °C to 1000 °C ±0.004×T from 375 °C to 750 °C ±0.004×T from 375 °C to 1000 °C ±[1 + 0.003×(T – 1100)] from 1100 °C to 1600 °C ±[1 + 0.003×(T – 1100)] from 1100 °C to 1600 ° ±0.004×T from 125 °C to 350 °C ±0.004×T from 375 °C to 800 °C
Accuracy class 2 in the respective temperature range, (°C) ±2.5 from -40 °C to 333 °C ±2.5 from -40 °C to 333 °C ±2.5 from -40 °C to 333 °C ±1.5 from 0 °C to 600 °C ±1.5 from 0 °C to 600 °C ±0.0025×T from 600 °C to 1700 °C ±1.0 from -40 °C to 133 °C ±2.5 from -40 °C to 333 °C
±0.0075×T from 333 °C to 1200 °C ±0, T from 333 °C to 750 °C ±0.0075×T from 333 °C to 1200 °C ±0.0025×T from 600 °C to 1600 °C ±0.0025×T from 600 °C to 1600 °C ±0.0075×T from 133 °C to 350 °C ±0.0075×T from 333 °C to 900 °C
IEC color coding of terminals Green to white Black – white Lilac – white Orange to white Orange – white None Brown – white Purple – white

See also: How to connect the Renault Sandero’s Volga signals

Seebeck phenomenon

Consists of. A current will flow in a closed circuit if two dissimilar conductors—preferably semiconductors, since the effect is more noticeable for semiconductors—keep their junctions, or the locations where these conductors connect, at different temperatures. Whichever junction has a higher temperature determines which way the current flows. With one variation in one direction and another in the opposite direction.

This device serves as a temperature sensor by being cut in one location. The junction 1—which we will heat or cool—and the other junction—which is inside the galvanometer and is at room temperature—are depicted in diagram 2 below. The galvanometer arrow will veer to one side or the other depending on whether the junction temperature T1 is higher or lower than the ambient temperature.

If both wires in the thermocouple circuit are of the same material, nothing will happen. To check this is very simple, take two copper wires with insulation, no one canceled the safety measures, connect them with one end to the galvanometer, and the other twist together (but it is better to solder), and start heating, as well as you can put it in water with pieces of ice. If you took the same wires, then the arrow of the device will remain at zero. But if you take different wires and connect them to the instrument in the same way and twist the other ends together. And after that you will heat or cool the bare ends of the wires, you can observe how and in what direction the galvanometer arrow will deviate.

Features, nuances on accuracy

The hot junction area’s t° determines the voltage at the cold tips in a proportionate manner. A linear thermoelectric property is observed in a specific temperature range, manifesting as a voltage dependence on the degree of the difference t° between the warm and cold points of the TP element. Because linearity is conditional, we can only discuss it when the last one’s t° is constant. If calibration is performed, this subtlety should be considered as there is a chance of substantial error if the heating varies at the cold ends.

The cold ends are placed in special capsules when high measurement accuracy is required. Within these capsules, special electronic devices process the resistance thermometer readings to maintain the stability of a single selected temperature level. With this method, accuracy can reach ±0.01. However, only a small number of technological processes need it. When working with thermocouples in water heaters, refrigerators, and other home appliances, for instance, the standards are typically less strict and allow deviations that are orders of magnitude smaller.

About thermocouples: what they are, principle of operation, connection, application

Taking temperature readings and loading them into control systems for additional processing are frequently required in the automation of technological processes. High-precision, low-inertia sensors capable of withstanding high temperature loads within a specific measurement range are needed for this. Thermocouples are differential devices that are frequently used as thermoelectric transducers. They work by converting thermal energy into electrical energy.

Additionally, the gadgets are a straightforward and practical thermoelectric thermometer temperature sensor that can measure temperatures accurately over a sizable range. In particular, an electrical signal from a thermocouple-based sensor initiates the control automation of gas boilers and other heating systems. The required level of measurement accuracy within the chosen temperature range is provided by sensor designs.

Differences between thermocouples and thermistors (NTC PTC)

Thermistors, or resistance sensors, and thermoelectric transducers differ in the following ways:

  • operating principle. A small current appears on the thermocouple, changing with different heating of its head, and the thermistor (semiconductor) reacts to such processes by changing its resistance;
  • constructive. Thermocouple design: two soldered conductors (current flows from them) of different alloys in a protective casing and with compensation wires, thermistor – a solid piece of semiconductor with conductors (current flows to it), the resistance of which is sensitive to temperature.

The following are benefits of thermocouples:

  • operating range. t° is much higher: a typical one reaches +600…+800° C, thermistors have a standard maximum plus limit of about +200…+600° C. There are thermocouples made of special alloys that operate at +2500 ° C, which for them can not be called something outstanding, it is, in general, the usual parameters. But also thermal sensors have special families of high-temperature models. But these are more special instruments, and still their range is smaller;
  • Thermistors are more accurate, but with some caveats. At high temperatures, their errors, as well as degradation, uncalibration can be higher than TPs. That is, thermocouples can be more accurate at particularly high temperatures. This disadvantage is also leveled for them if there is a transducer calculating the errors;
  • often requires a normalizing amplifier, which is needed for the thermocouple to increase sensitivity, so that its signal is stronger for better operation of the receiver that processes the information, so that it "sees" it;
  • thermistor is cheap because it does not require any of the additional components mentioned above. These devices are often required for TPs, so the cost of using them is ultimately higher;
  • resistance to mechanical influences, vibrations is better for thermocouples, they have reliable protective covers;
  • the reaction speed of TP is higher than that of thermistors;
  • Thermistors are more susceptible to wear and recalibration when operating at elevated temperatures. But this disadvantage is relative – such a sensor is often simply thrown away and a new one is bought, as the product is cheap;
  • thermistors degrade faster over time. Typically, manufacturers give a warranty of only 1000 hours for these detectors. Thermocouples are more survivable.

Therefore, while both thermistor and thermocouple temperature sensors rely on electrical parameters for their operation—the former measures resistance, the latter creates and modifies electromagnetic fields—they differ significantly from one another.

There’s a rule: use a thermocouple if t° is higher than +300° C. Thermistors are more prevalent in simpler, less expensive devices. When working with high temperatures, thermocouples—expensive and complicated equipment—are more frequently used. Thermistors may have the same errors as TPs in such circumstances, but they are more accurate in typical ranges (-50…+300° C).

It is more typical to use TPs in special, narrowly focused areas like laboratories, special research, and industry.

  • advantages of thermocouples: the operating temperature range is much wider, the response is faster, the service life is much longer than that of thermistors, TPs are less susceptible to uncalibration, degradation, mechanical damage. With a range of t° from +300° C, thermocouples are often indispensable;
  • disadvantages: the special features of TP applications increase costs (partially offset by survivability), and it is generally accepted that the accuracy of thermocouples is slightly worse than that of thermistors.

We will highlight separately an unqualified plus: the temperature sensors of the items under investigation (radio components, etc.) are exclusively thermocouples. in addition to the multimeter. It should be noted that although TPs are more resilient to such circumstances, inappropriate t° ranges always increase errors and the likelihood of failure.

Varieties of thermoelectric type converters

The variety of thermocouple types is enormous. The two primary factors of division are soldering variant and alloy variety. Multi-point TPs are another distinct type.

The type of electric pairs depending on the alloys of the conductors

Although the basic idea behind thermocouples is always the same, different alloys heat at different rates, so there can be variations in their operating ranges, speeds, and error rates.

The temperature range in which one is permitted to use a particular type of sensor is crucial, as it dictates the output voltage pulse for different combinations of metals.

Measurement resolution improves with increasing output voltage amplitude. Accuracy increases as repeatability does as well.

Certain types of TPs are suitable under specific conditions because they have different resolution and t° range ratios.

The composition of conductor alloys determines the nine types of thermocouples:

The letters stand for the varieties. (J, B, C, T, E, N, R, S, and so on).

Because it is the most widely used, appropriate for use in household appliances and other devices, and suitable for tasks lacking specific requirements, the K type thermocouple (also known as TCA) holds significant value for us.

Generally speaking, unless there is a valid reason to use another type, TXA is always advised. An explanation of a K-type thermocouple can be found on the following extremely specialized electronics website:

In the world of heating and insulation for homes, understanding the thermocouple is crucial. So, what exactly is a thermocouple? Well, it"s a small device that"s like a sensor, but it works differently. Its job? To measure temperature accurately. How does it do this? By using the principle of thermoelectricity, where temperature differences create tiny electric currents. There are various types of thermocouples, each suited for different temperature ranges and environments. When it comes to the circuit, it"s relatively simple: it consists of two different metal wires joined at one end. Want to make one yourself? It"s possible, with some knowledge and careful handling. Understanding the thermocouple helps in maintaining optimal heating systems, ensuring comfort and efficiency in your home.

Coupler options

Couplers are designed in various configurations to meet the unique needs of thermocouples. Versions with and without grounding to the protective capsule body are available in 1 and 2 element configurations.

The thermocouple’s inertia is decreased by grounding to the housing, which isn’t always possible, which enhances sensor performance and real-time accuracy. Certain models feature a hot junction outside the protective bulb (casing, housing) for added efficiency.

Analog and digital thermometers


These gadgets typically don’t cost much and don’t require a lot of upkeep. The scale is their primary issue. It either covers a large temperature range with approximative accuracy or indicates temperature with high accuracy but a very small measuring interval.


Although these devices cost more than analog ones, they are far more accurate. Wide measurement ranges are supported, and they find use in both home and professional settings.

Features of a digital thermometer’s construction:

  • Sensing element (usually a thermistor);
  • An analog-to-digital converter that transforms the electrical signal from the thermistor into a digital signal;
  • display;
  • power supply element;
  • Signal I/O required for interfacing with other devices.

Stability of metrological characteristics of resistance thermometer

Resistance thermocouples’ metrological properties invariably change while they are in use. Numerous variables affect the rate of change, including the operating temperature, the frequency and rate of temperature changes, the presence of chemically active substances in the medium being measured, and more. Resistance thermocouples’ metrological properties invariably change while they are in use. д. In this regard, the operating condition groups for sensors ΢ΡΛ΢, ΢ΡΜ΢, ΢ΡΜ΢ Ex, and ΢ΡΜ΢ Ex are introduced, and the acceptable drift values of resistance thermometers’ metrological characteristics are standardized based on these groups.

The period of time or operation of the measuring instrument during which the change in metrological characteristics does not exceed the modulus of the tolerance class of the measuring instrument, reduced by the systematic error of measurements during the measuring instrument’s testing, is defined as the interval between calibrations (IMP) according to RMG-74, "METHODS FOR DEFINING INTERVALUES AND INTERCALIBRATION INTERVALS OF MEASUREMENTS."

The resistance thermocouple’s drift is determined by the length of time the sensor operates at a high temperature. The impact of aging on RTD drift is hardly ever discussed in scientific literature. It is common knowledge that the temperature measurement value affects both the RTD’s value and drift rate. Thermocouples with copper resistance are known to be less stable than those with platinum resistance. When operating conditions are not too extreme, temperature changes in the physical properties of metals are the main cause of drift; the extent of these changes is determined by the maximum operating temperature and the length of exposure.

It is suggested that the intervals between calibrations be normalized by dividing them by the range of measured temperatures, taking into account the operating conditions. Give a different interval between verifications for each of the ranges, ranging from one to five years. Figure 4 depicts the suggested interval grading.

Multi-point thermoelectric transducers

There are instances when measuring t° simultaneously at several locations is necessary. This issue is resolved by the type of multi-point thermocouple. Data is recorded by these sensors along the transducer axis. These products are uncommon for everyday use; instead, they are employed in the petrochemical and chemical industries, where it is essential to understand the distribution of temperature in tanks, reactors, and other similar structures. There is a maximum of 60 points available. With this type of thermocouple, simple maintenance is sufficient—just one capsule and one installation are needed.

Other design variations

Various design options are shown below:

A few choices for thermal sensors with cable leads are listed below:

The role of extension (compensation) wires

The thermocouple requires an extension, also known as a compensation wire or cable, in order to connect to the equipment’s remote microcircuits, a secondary or serviced device, a receiver, data processing, and remote area research.

Except for the situations mentioned below, no elongation wires are used. This is yet another way that thermistors differ. The same material that goes into a thermocouple must be used. Consider the same wiring, marked CA, for a type K sensor with chromel-alumel conductors.

Only in cases where the TP is equipped with a converter that computes and eliminates error can compensation cable be skipped. The most prevalent type of these is a "tablet" with a 4–20 mA unified type signal inside the detector’s terminal segment.

A thermocouple is an essential part of heating systems because it gives precise temperature measurement. Anyone wishing to build or maintain a heating system must comprehend the basic principles of operation. A thermocouple essentially operates on the Seebeck effect principle, which states that when two distinct metals are connected at two junctions and their temperatures differ, a voltage is produced.

Thermocouples come in a variety of types to accommodate a range of applications. Thermocouples of the Type K, Type J, and Type T varieties are common; each has a unique temperature range and sensitivity. Selecting the appropriate kind is essential to guaranteeing precise temperature reading in particular settings.

Although a thermocouple system’s circuitry is rather straightforward, it is essential for precise temperature measurement. Usually, it entails attaching the thermocouple to a temperature controller or measuring device. Accurate temperature readings and correct installation are ensured by understanding the circuitry.

For those with some technical know-how, building a thermocouple system can be a fulfilling do-it-yourself undertaking. A basic understanding of electrical circuits and easily obtainable components allow enthusiasts to build their own thermocouple setups. To prevent any accidents, it is crucial to adhere to safety precautions and double-check connections.

In conclusion, anyone working with insulation and heating systems needs to have a basic understanding of thermocouples. Understanding thermocouples, from their basic function to the range of varieties available, enables people to make knowledgeable decisions about controlling and measuring temperature. Thermocouples are an essential component of heating technology due to their versatility and dependability, whether they are used for DIY projects or industrial applications.

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Sergey Ivanov

I like to help people create comfort and comfort in their homes. I share my experience and knowledge in articles so that you can make the right choice of a heating and insulation system for your home.

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