General information
A thermocouple, or thermoelectric transducer, is a device for measuring temperature, the operation of which is based on the thermoelectric effect.
Principle of operation of a thermocouple
The operating principle of a thermocouple is based on the occurrence of a potential difference in conductors, the so-called thermoelectric effect (or Seebeck effect). It consists in the fact that in a closed circuit consisting of dissimilar conductors, a thermo-EMF arises if the contact points have different temperatures (cold and hot junctions). The proportionality factor in this dependence is called the thermo-EMF coefficient. A circuit that consists only of two dissimilar conductors is called a thermoelement or thermocouple. The accuracy of readings depends on the type of construction, the wiring scheme of conductors, and some other parameters.
Design of a thermocouple
A thermocouple consists of two conductors (thermoelectrodes), each made from different alloys. Various alloys of non-ferrous and noble metals are used in thermocouples. Noble metals significantly increase measurement accuracy due to lower thermoelectric inhomogeneity and resistance to oxidation. They are used for measurements up to 1900 °C; for higher temperatures, special heat-resistant alloys are required. Base metals are applied up to 1400 °C.
The ends of the thermoelectrodes form a junction (hot junction), created by twisting, a narrow weld, or butt welding. The hot junction is placed in the medium with the measured temperature. The free ends of the thermocouple are connected with compensating wires to the contacts of a measuring device or linked with an automatic control unit. At these connection points, another junction is formed, the so-called cold junction.
Thermo-EMF arises due to the potential difference between the junctions when the hot junction is heated or cooled. The voltage at the cold junction is proportional to the temperature at the hot junction.
To protect thermoelectrodes from aggressive hot environments, they are placed in a sealed capsule filled with inert gas or liquid. Sometimes ceramic beads are placed over the electrodes. To reduce inertia, in some models of thermoelectric converters the hot junction is left outside the protective tube.
The type of insulation is selected depending on the maximum operating temperature: up to 100–120 °C – any insulation; up to 1300 °C – porcelain tubes or beads; up to 1950 °C – Al2O3 tubes; above 2000 °C – MgO, BeO, ThO2, or ZrO2 tubes.
.Types of thermocouples
Technical requirements for thermocouples are defined in GOST 6616-94. Standard tables for thermoelectric thermometers — nominal static conversion characteristics, tolerance classes, and measurement ranges are provided in IEC 60584-1,2 and in GOST R 8.585-2001.
Platinum-rhodium/platinum — TPP13 — Type R;
Platinum-rhodium/platinum — TPP10 — Type S;
Platinum-rhodium/platinum-rhodium — TPR — Type B;
Iron-constantan (iron-copper-nickel) — Type J;
Copper-constantan (copper-copper-nickel) — Type T;
Nicrosil-nisil (nickel-chromium-silicon/nickel-silicon) — Type N;
Chromel-alumel — Type K;
Chromel-constantan — Type E;
Chromel-copel — Type L;
Copper-copel — Type M;
Silhos-silin — Type I;
Tungsten and rhenium — tungsten-rhenium — Type A-1, A-2, A-3.
Type K thermocouples operate in a neutral atmosphere or in oxygen-rich conditions. They are not recommended for use in sulfur-containing environments, as sulfur affects both electrodes and damages the thermocouple.
In Russian practice, the following alloys are most commonly used for thermocouples:
Copel (56% Cu and 44% Ni)
Alumel (95% Ni, the rest Al, Si, Mn)
Chromel (90% Ni and 10% Cr)
Constantan (40% Ni, 1.5% Mn, balance Cu)
Platinum-rhodium (90% Pt and 10% Rh)
The design of a thermocouple and the materials of the conductors depend on its intended use: different combinations of metals are designed for different environments and temperature ranges.
Comparison table of 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, °C | 0 to +1100 | 0 to +700 | 0 to +1100 | 0 to +1600 | 0 to +1600 | +200 to +1700 | -185 to +300 | 0 to +800 |
| Limit temperatures, °C | -180; +1300 | -180; +800 | -270; +1300 | –50; +1600 | -50; +1750 | 0; +1820 | -250; +400 | -40; +900 |
| Accuracy class 1, within the specified 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 from –40 °C to 125 °C | ±1.5 from –40 °C to 375 °C | |
| ±0.004T from 375 °C to 1000 °C | ±0.004T from 375 °C to 750 °C | ±0.004T 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 °C | ±0.004T from 125 °C to 350 °C | ±0.004T from 375 °C to 800 °C | ||
| Accuracy class 2, within the specified 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 | ±0.0025T from 600 °C to 1700 °C | ±1.0 from –40 °C to 133 °C | ±2.5 from –40 °C to 333 °C |
| ±0.0075T from 333 °C to 1200 °C | ±0.0075T from 333 °C to 750 °C | ±0.0075T from 333 °C to 1200 °C | ±0.0025T from 600 °C to 1600 °C | ±0.0025T from 600 °C to 1600 °C | ±0.0075T from 133 °C to 350 °C | ±0.0075T from 333 °C to 900 °C | ||
| Lead color coding according to IEC | Green – White | Black – White | Purple – White | Orange – White | Orange – White | None | Brown – White | Violet – White |
Types of thermocouple junctions
There are three types of thermocouple junctions: grounded, ungrounded (insulated), and exposed.
At the end of a sensor with a grounded junction, the thermocouple wires are attached to the inner wall of the sheath. This provides good heat transfer from the outside through the sheath wall to the thermocouple junction. In the insulated type, the junction is separated from the sheath wall. The response time is slower than that of the grounded type, but the insulated type provides galvanic isolation.
An exposed junction protrudes beyond the sheath and is directly affected by the surrounding medium. This type ensures the fastest response time but can only be used in non-corrosive and non-sealed environments.
Grounded junctions are used for measuring temperatures in aggressive environments or in applications with high pressure. The grounded junction is welded to the protective sheath, providing a faster response than insulated types.
Insulated junctions perform well in aggressive environments where it is recommended to have a thermocouple electrically isolated from the sheath and shielded by it. A welded-wire thermocouple is physically insulated from the sheath by MgO (magnesium oxide) powder.
An exposed junction is recommended for measuring static or dynamic temperatures of non-corrosive gases where a quick response time is needed. The junction extends beyond the protective metal sheath, resulting in more accurate and faster readings. The sheath insulation is sealed at the connection points, preventing any penetration of moisture or gas that could cause errors.
Applications of thermocouples
Thermocouples are high-precision, low-inertia sensors capable of withstanding significant thermal loads within a defined measurement range. Their scope of application is vast, primarily due to their wide measurement range: from extremely low to extremely high temperatures. These devices are also widely used because of their stability and measurement accuracy.
In everyday applications, thermocouples are used in various appliances, from simple to technologically advanced: from irons, soldering irons, and refrigerators to automobiles and heating boilers. Thanks to their wide temperature measurement range (from –250 °C to +2500 °C), thermocouples are extensively used in industry, utilities, science, and medicine. Thermoelectric converters also function as part of automation and control systems, collecting and transmitting temperature change data. These sensors are reliable, inexpensive, sufficiently accurate, and have low inertia. Their use is determined by their technical characteristics and features, and for some systems, thermocouples are the only viable option.
Advantages and disadvantages of thermocouples
Advantages
Wide temperature measurement range: –250 °C to +2500 °C
Operation in aggressive environments
High accuracy. The error is up to 1–2 °C in standard devices, which is generally sufficient for industrial and household needs. More precise instruments achieve 0.01 °C.
Simplicity of manufacturing and maintenance and high reliability
Low cost for most models
Disadvantages
• Continuous monitoring of the cold junction, calibration, and verification of control equipment
• Structural changes in metals during manufacturing
• Readings are affected by the temperature of the free ends, requiring correction
• Dependence on atmospheric composition, sealing costs
• Measurement errors due to electromagnetic interference. Small current values require shielding of wires to reduce noise. On long thermocouple and extension wires, an “antenna” effect may occur for existing electromagnetic fields.
• Need for high-sensitivity instruments to obtain measurements
• Performance degradation during prolonged use under temperature fluctuations
• Non-linear dependence of thermo-EMF if operating temperatures are exceeded
• Achieving accuracy better than 1 °C is difficult; resistance thermometers or thermistors must be used
• Peltier effect: when taking readings, current flow through the thermocouple must be excluded, since current cools the hot junction and heats the cold junction
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