What is a thermocouple?

What is a thermocouple?

It is a commonly used temperature sensing element in temperature measurement instruments. It directly measures the temperature and converts the temperature signal into a thermoelectric potential signal, which is then converted by electrical instruments (secondary instruments) into the temperature of the measured medium. Although the shapes of various thermocouples can vary greatly depending on their application, their basic structure is largely the same, typically consisting of a thermoelectric element, an insulating sleeve protective tube, and a junction box. These thermocouples are usually used in conjunction with display instruments, recording instruments, and electronic regulators. How a thermocouple works This relationship is widely used in practical temperature measurement. Since the cold junction t0 remains constant, the thermoelectric potential generated by the thermocouple varies only with changes in the temperature of the hot junction (the measuring end). This means that a specific thermoelectric potential corresponds to a specific temperature. By using the method of measuring the thermoelectric potential, we can achieve the purpose of temperature measurement The fundamental principle of thermocouple temperature measurement is that a closed circuit is formed by two conductors made of different materials. When there is a temperature gradient between the two ends, current flows through the circuit, generating an electromotive force (EMF) between the two ends. This phenomenon is known as the Seebeck effect. The two conductors, made of different materials, are the thermoelements, with the hotter end serving as the working end and the cooler end as the free end, which is typically maintained at a constant temperature. Based on the relationship between the EMF and temperature, a thermocouple calibration table is created. This table is based on the condition where the free end temperature is 0℃, and different thermocouples have their own calibration tables. When a third metal material is added to the thermocouple circuit, as long as the temperatures at both junctions of this material are the same, the thermoelectric potential generated by the thermocouple will remain unchanged, unaffected by the addition of the third metal. Therefore, when using a thermocouple for temperature measurement, a measuring instrument can be connected to measure the thermoelectric potential, which allows the temperature of the medium being measured to be determined. When measuring temperature with a thermocouple, it is essential that the temperature at the cold junction (the end connected to the measuring circuit through leads) remains constant, as this ensures that the thermoelectric potential is proportional to the measured temperature. If the temperature at the cold junction (the environment) changes during measurement, it can significantly affect the accuracy of the measurement. To compensate for the impact of changes in the cold junction temperature, measures are taken at the cold junction, which is referred to as cold junction compensation. Special compensating wires are used to connect to the measuring instrument.

Common types and characteristics of thermocouples

Common thermocouples can be categorized into two main types: standard and non-standard. Standard thermocouples are those for which the national standard specifies their thermoelectric potential-temperature relationship, allowable error, and a unified calibration table. They come with matching display instruments for selection. Non-standard thermocouples have a smaller range or quantity of applications compared to standard thermocouples and generally lack a unified calibration table, making them primarily used for measurements in special situations. Since January 1, 1988, China has standardized the production of thermocouples and resistance thermometers according to IEC international standards, designating seven types-S, B, E, K, R, J, T-as the unified standard thermocouples for China.

Theoretically, any two different conductors (or semiconductors) can be paired to form a thermocouple. However, as practical temperature measurement components, they must meet multiple requirements. To ensure reliability and sufficient accuracy in engineering applications, not all materials are suitable for thermocouples. Generally, the basic requirements for the electrode materials of thermocouples are:

1. Within the temperature measurement range, the thermoelectric properties are stable and do not change with time, and there is sufficient physical and chemical stability, which is not easy to be oxidized or corroded;

2, small temperature coefficient of resistance, high conductivity, small specific heat;

3. The thermoelectric potential generated in the temperature measurement should be large, and the thermoelectric potential is a linear or nearly linear single value function relationship with the temperature;

4. The material has good reproducibility,

How to install thermocouple?

In production, due to different objects under test, different environmental conditions, different measurement requirements, and different installation methods of thermal resistors and measures taken, there are many problems to be considered. However, in principle, it can be considered from three aspects: accuracy of temperature measurement, safety and convenience of maintenance. To prevent damage to the temperature sensing element, it should be ensured that it has sufficient mechanical strength. To protect the element from wear, a protective screen or tube should be added. To ensure safety and reliability, the installation method of the temperature sensing element should be determined based on specific conditions, such as the temperature and pressure of the medium to be measured, the length of the element, its installation position, and form. The following are a few examples to draw attention:

All temperature sensing elements installed to withstand pressure must ensure their sealing. For thermocouples operating at high temperatures, to prevent deformation of the protective tube, they should generally be installed vertically. If horizontal installation is necessary, it should not be too long, and a bracket should be used to protect the thermocouple. If the temperature sensing element is installed in a pipeline with high medium flow velocity, it should be installed at an angle. To prevent excessive erosion, it is best to install the temperature sensing element at the bends of the pipeline. When the medium pressure exceeds 10MPa, a protective sleeve must be added to the measuring element. The installation location of thermocouples and thermal resistors should also consider sufficient space for disassembly, maintenance, and calibration. Thermocouples and thermal resistors with longer protective tubes should be easy to disassemble and assemble

Thermocouple temperature measurement method

The thermal response time is complex, and different experimental conditions can lead to varying measurement results. This is because the thermal response time is influenced by the heat transfer rate between the thermocouple and its surrounding medium; a higher heat transfer rate results in a shorter thermal response time. To ensure the thermal response time of thermocouple products is comparable, national standards specify that the thermal response time should be measured using a specialized water flow test device. The water flow rate should be maintained at 0.4±0.05m/s, with an initial temperature ranging from 5-45℃ and a temperature step of 40-50℃. During the test, the water temperature should not change by more than ±1% of the temperature step. The thermocouple should be inserted to a depth of 150mm or the design insertion depth (whichever is smaller) and this should be noted in the test report.

Because the device is relatively complex, only a few units have this equipment at present, so the national standard stipulates that the manufacturer and the user can negotiate to adopt other test methods, but the data given must indicate the test conditions.

Because the thermoelectric potential of type B thermocouple is very small near room temperature, the thermal response time is not easy to measure. Therefore, the national standard stipulates that the thermoelectric electrode assembly of the same specification of type S thermocouple can be used to replace its own thermoelectric electrode assembly, and then the test can be carried out.

During the experiment, record the time T0.5 when the output of the thermocouple changes to 50% of the temperature step change. If necessary, also record the 10% thermal response time T0.1 and the 90% thermal response time T0.9. The recorded thermal response times should be the average of at least three tests, with each measurement deviating from the average by ±10%. Additionally, the time required for the temperature step change should not exceed one-tenth of the T0.5 of the tested thermocouple. The response time of the recording instrument or meter should also not exceed one-tenth of the T0.5 of the tested thermocouple.

Main types of thermocouples

1. Classification according to the type of fixing device As the main means of temperature measurement, thermocouple has a wide range of uses, so there are many requirements for fixing devices and technical performance. Therefore, the fixing devices of thermocouple are divided into six types: no fixing device type, threaded type, fixed flange type, movable flange type, movable flange angle ruler type, conical protective tube type.

2. Classification according to assembly and structure According to the performance and structure of thermocouples, they can be divided into: detachable thermocouples, explosion-proof thermocouples, armored thermocouples and special purpose thermocouples such as pressure spring fixed thermocouples.

What requirements should be paid attention to when installing thermocouple?

For the installation of thermocouples and resistance thermometers, attention should be paid to the accuracy of temperature measurement, safety and reliability, and convenient maintenance, and not affect the operation of equipment and production operations. To meet the above requirements, when selecting the installation parts and insertion depth of thermocouples and resistance thermometers, pay attention to the following points:

1. In order to ensure sufficient heat exchange between the measuring end of thermocouple and resistance thermometer and the measured medium, the measuring point should be reasonably selected, and thermocouple or resistance thermometer should be installed as far away as possible from valves, elbows and dead corners of pipelines and equipment.

2. Thermocouples and thermistors with protective sleeves have heat transfer and heat dissipation losses. In order to reduce measurement errors, thermocouples and thermistors should have sufficient insertion depth:

(1) For the thermocouple measuring the fluid temperature at the center of the pipeline, it should generally be inserted into the center of the pipeline (vertical installation or inclined installation). If the diameter of the pipeline is 200 mm, the insertion depth of the thermocouple or resistance should be selected to be 100 mm;

(2) For temperature measurements of high-temperature, high-pressure, and high-speed fluids (such as main steam temperature), to reduce the resistance of the protective sleeve to the fluid and prevent it from breaking under fluid pressure, a shallow insertion method can be used for the protective tube or a thermal sleeve thermocouple. The depth of the protective sleeve for the shallow insertion thermocouple should not be less than 75mm when inserted into the main steam pipe; the standard insertion depth for a thermal sleeve thermocouple is 100mm;

(3) If it is necessary to measure the temperature of flue gas in the flue, although the diameter of the flue is 4m, the insertion depth of thermocouple or resistance is 1 m;

(4) When the insertion depth of the measuring original exceeds 1m, it should be installed vertically as far as possible, or support frame and protective pipe should be added.

The following points should be paid attention to in order to correctly use the thermocouple to avoid errors

Correct use of thermocouple can not only accurately obtain the temperature value, ensure product qualification, but also save the material consumption of thermocouple, both save money and ensure product quality. Incorrect installation, thermal conductivity and time lag errors, they are the main errors in the use of thermocouple.

1. Errors introduced by improper installation If the installation position and insertion depth of the thermocouple do not accurately reflect the furnace's actual temperature, for example, the thermocouple should not be placed too close to the door or heating areas, and its insertion depth should be at least 8 to 10 times the diameter of the protective tube. The gap between the thermocouple's protective sleeve and the furnace wall is not filled with insulating material, which can cause heat to escape or cold air to invade the furnace. Therefore, the gap between the thermocouple's protective sleeve and the furnace wall should be sealed with refractory clay or asbestos rope to prevent the convection of hot and cold air, which could affect the accuracy of temperature measurement. If the cold end of the thermocouple is too close to the furnace body, the temperature may exceed 100℃. The installation of the thermocouple should avoid strong magnetic fields and electric fields as much as possible, so it should not be installed in the same conduit as power cables to prevent interference that could cause errors. The thermocouple should not be installed in areas where the measured medium flows very little. When measuring the temperature of gas inside the pipe with a thermocouple, the thermocouple must be installed in the direction opposite to the flow rate and must have sufficient contact with the gas.

2. Error introduced by insulation deterioration If the thermocouple is insulated, too much dirt or salt residue on the protective tube and pull plate causes poor insulation between the thermocouple poles and the furnace wall, which is more serious at high temperature. This will not only cause the loss of thermoelectric potential but also introduce interference, and the error caused by this can sometimes reach hundreds of degrees.

3. Error introduced by thermal inertia The thermal inertia of thermocouples causes the instrument's reading to lag behind the actual temperature changes, which is particularly noticeable during rapid measurements. Therefore, it is advisable to use thermocouples with finer thermoelements and smaller protective tube diameters. When the measurement environment allows, the protective tube can be removed. Due to the measurement lag, the amplitude of temperature fluctuations detected by thermocouples is smaller than those of furnace temperatures. The greater the measurement lag, the smaller the amplitude of the thermocouple's fluctuations, and the larger the difference from the actual furnace temperature. When using thermocouples with a large time constant for temperature measurement or control, the instrument may show minimal temperature fluctuations, but the actual furnace temperature could vary significantly. To ensure accurate temperature measurement, thermocouples with a small time constant should be chosen. The time constant is inversely proportional to the heat transfer coefficient and directly proportional to the diameter of the thermocouple's hot end, the material's density, and its specific heat. To reduce the time constant, in addition to increasing the heat transfer coefficient, the most effective method is to minimize the size of the hot end. In practice, materials with good thermal conductivity, thin tube walls, and small inner diameters are typically used for protective sleeves. For more precise temperature measurements, bare wire thermocouples without protective sleeves are used, but these can be easily damaged and require timely calibration or replacement.

4. Thermal resistance error At high temperature, if there is a layer of soot on the protective tube and dust is attached to it, the thermal resistance will increase and the heat conduction will be hindered. At this time, the temperature indication is lower than the true value of the measured temperature. Therefore, the external cleanliness of the thermocouple protective tube should be maintained to reduce the error.

The main advantages of thermocouples

1. High measurement accuracy. Because it is in contact with the measured object directly, it is not affected by the intermediate medium.

2. Wide measurement range. Common thermocouples can be measured continuously from-50 degrees --1600 degrees, and some special thermocouples can be measured as low as-269 degrees (such as gold iron nickel chromium) and as high as 2800 degrees (such as tungsten, rhenium).

3. Simple structure and easy to use. Thermocouples are usually composed of two different metal wires, and are not limited by size and beginning. They have a protective sleeve on the outside, which makes them very convenient to use.

What are the future trends and application fields of thermocouple?

I. Future development trend Material innovation and performance improvement New thermoelectric materials: develop materials with higher sensitivity and wider temperature range (such as oxide thermocouples, nanocomposites) to replace traditional metal alloys (such as K-type, J-type) Flexible thermocouples: The demand for wearable devices and curved temperature measurement scenarios is driving the development of flexible, thin-film thermocouples (such as printed electronics). High temperature superconducting materials: exploring stable temperature measurement schemes in extreme environments (such as aerospace and nuclear reactors). Intelligent and integrated Embedded signal processing: integrated miniature amplifier and digital compensation circuit, direct output of digital signal, reduce external interference. IoT fusion: Remote monitoring through wireless transmission (such as LoRa, NB-IoT) to support Industry 4.0 and smart city applications. Self-powered system: using the Seebeck effect of thermocouples to power low-power devices (such as wireless sensor nodes). Optimization of accuracy and reliability AI calibration technology: through machine learning to dynamically compensate for nonlinear error and aging drift, prolong the service life. Multi-sensor fusion: combined with infrared, RTD, etc., to improve the reliability of measurement in complex environment. Low cost and standardization MEMS process: large-scale production of microelectromechanical systems reduces the cost of micro thermocouples and expands consumer applications. International standard unification: adapt to the global supply chain, simplify the selection and maintenance process.

2, emerging application fields New energy and carbon neutrality Photovoltaic and energy storage: Monitor solar panel temperature (to prevent hot spot effect) and thermal management of energy storage systems. Hydrogen energy: high pressure hydrogen production and temperature monitoring of fuel cell stacks. Nuclear fusion: extreme high temperature measurements for future reactors (such as tungsten and rhenium thermocouples). High-end manufacturing and automation Semiconductor manufacturing: precision temperature control of wafer processing and etching equipment (millisecond response required). Additive manufacturing: real-time feedback of melt pool temperature in 3D printing process to optimize molding quality. Robot: Collaborative robot joint overheating protection. Biomedical and health Minimally invasive surgery: ultrafine thermocouples are integrated into a catheter or endoscope to monitor tissue temperature in real time. Wearable devices: continuous monitoring of body temperature changes (such as health management needs after the epidemic). Low temperature therapy: precise temperature control during liquid nitrogen cryotherapy. Aerospace and defense Supersonic aircraft: surface aerodynamic heating monitoring (materials resistant to more than 2000 C required). Satellite thermal control: reliability improvement in the extreme temperature environment of space. Engine health management: turbine blade temperature distribution monitoring. Smart home and consumer electronics Smart home appliances: precise temperature control of ovens, coffee machines and other home appliances. AR/VR devices: Prevent processor overheating from affecting user experience. Environment and agriculture Smart agriculture: greenhouse and soil temperature monitoring. Geothermal exploration: deep well temperature measurement to assist energy development.

summarize

The future of thermocouples will focus on three key areas: high-performance materials, intelligence, and cross-domain integration. They will continue to penetrate high-end sectors such as new energy, healthcare, and aerospace, and enter the consumer market as costs decrease. Their core advantages-simple structure, no power supply requirement, and heat resistance-ensure their irreplaceability, but they must also develop in tandem with emerging sensor technologies.