Measurement of high temperatures by thermocouples. Thermoelectric converters - thermocouples
The main purpose of thermocouples is to measure temperature. Temperature is a physical quantity that quantitatively characterizes the measure of the average kinetic energy of the thermal motion of the molecules of a body or substance. From the analysis of the definition of temperature, we can conclude that this physical quantity cannot be measured directly. You can judge the change in the temperature of an object by changing other physical properties of this object (for example, volume, pressure, electrical resistance, thermo-EMF, radiation intensity, etc.).
To ensure the uniformity of temperature measurements, the International Practical Temperature Scale MPTS-68 was adopted as an international standard in 1968 (at present, the standard is the updated version of the scale in 1990 - ITS-90 (ITS-90), using as reference (reference) temperature points of change in the aggregate state of certain substances that can be reproduced.In addition, the standard defines the types of reference measuring instruments in the entire temperature range.A list of some fixed points of the ITS-90 is given in Table Table 1.
Depending on the range of measured temperatures, two main groups of measurement methods are distinguished: contact (thermometry proper) and non-contact (pyrometry or radiation thermometry). Non-contact methods are used, as a rule, to measure very high temperatures. Temperature measurement using thermocouples refers to the contact method of measurement.
The principle of operation of a thermocouple is based on the thermoelectric effect or the Seebeck effect. The advantages of thermocouples as a means of measuring temperature include high accuracy in measuring temperature values, a large temperature measurement range, their simplicity of design and reliability.
Thermocouples are classified according to the materials from which they are made, as well as the accuracy (tolerance) class (see Chapter 4, §3).
Chapter 1 Thermocouple Device
§1 The principle of operation of a thermocouple. Seebeck effect
The principle of operation of a thermocouple is based on the thermoelectric effect. The phenomenon of thermoelectricity was discovered by the German physicist T. Seebeck in 1821 and was also called the Seebeck effect.The Seebeck effect is as follows: if you connect two conductors (thermal electrodes) of dissimilar metals or alloys in such a way that they form a closed electrical circuit (Figure 1), and then maintain contact points (junctions) at different temperature, then the circuit will flow D.C.. A circuit that consists of only two different conductors (thermoelectrodes) is called a thermocouple or thermocouple.
The electromotive force that causes a current in the circuit is called the Seebeck thermo-EMF and, in the first approximation, depends only on the material of the thermoelectrodes and the temperature difference between the junctions.
The thermoelectrode, through which the current flows from the hot junction to the cold junction, was agreed to be considered positive, from cold to hot - negative. When designating a thermocouple, for example, ТХА (chromel-alumel thermocouple), the material of the positive electrode is indicated in the first place in the name, and the negative electrode is in the second.
Thus, knowing the temperature of one junction (usually it is kept constant, for example, equal to 0 ° C) and measuring the current or voltage in the circuit, one can unambiguously determine the unknown temperature of the other junction.
It is worth noting that the value of thermo-EMF is millivolts at a temperature difference of 100 K (173.15 ° C) and a cold junction temperature of 0 ° C (for example, a copper-constantan pair gives 4.25 mV, platinum-platinum-rhodium - 0.643 mV ).
§2 Thermoelectric thermometer. Thermocouple designs
It is more correct to say that the temperature is measured not with a thermocouple, but with a thermoelectric thermometer. The sensitive element of such a thermometer is a thermocouple; thermometric quantity - thermo-EMF that occurs in a thermocouple; thermometric property - change in thermo-emf with temperature change;The main factors on which the design of a thermocouple depends are its operating conditions. When designing one or another thermoelectric converter, factors such as the state of aggregation of the substance whose temperature is to be measured, the “aggressiveness” of the external environment, the range of measured temperatures, thermal inertia, and others are taken into account.
The following design features of thermocouples can be distinguished:
- The ends of two thermoelectrodes are connected to each other at one point, forming a working junction. The connection occurs, as a rule, using electric arc welding, and the thermoelectrodes are twisted together before welding. In special cases, brazing may be used instead of welding. Thermoelectrodes made of refractory metals, for example, in tungsten-rhenium or tungsten-molybdenum thermocouples, are often connected only by twisting without further welding.
- Thermoelectrodes must be interconnected only in the working junction. For the rest of the length, their electrical isolation from each other is required.
- The method of insulation of thermoelectrodes depends on the upper temperature limit of the use of a thermoelectric thermometer. If the specified limit does not exceed 100-120 ° C, then any insulation, including air insulation, can be used. At temperatures up to 1300 °C, insulation is performed using porcelain one- and two-channel tubes or beads. At higher temperatures, the electrical insulating properties of pyrometric porcelain deteriorate greatly, and it softens. In this regard, at higher temperatures, tubes made of aluminum oxide (up to 1950 ° C) and magnesium oxide, beryllium oxide, thorium dioxide and zirconium dioxide (above 2000 ° C) are used.
- Depending on the medium in which the temperature is measured, the thermocouple may have an outer protective tube with a closed end. This tube can be metal, ceramic or cermet. It must ensure the mechanical stability of the thermoelectric thermometer, the absence of mechanical stress on the thermoelectrodes, waterproofing, and in some cases the tightness of the thermometer. The material of the protective tube-case must withstand a long stay at the temperature of the upper limit of the use of this thermocouple design, and also be chemically resistant to the medium in which measurements are made, and have good thermal conductivity. The protective tube-sheath must be gas-tight and insensitive to sudden changes in temperature.
Figure 2 shows one of the thermocouple design options.
Classification of thermocouple construction types
According to the purpose and operating conditions:
- submersible;
- superficial.
- manufactured without a cover;
- with a steel case (up to t ≈ 600 °C);
- with a cover made of a special heat-resistant alloy (up to t ≈ 1000-1100 °C);
- with porcelain cover (up to t ≈ 1300 °С);
- with a cover made of refractory alloys (t ≈ 2000 °C and more).
- with a fixed fitting;
- with movable fitting;
- with movable flange.
- with an ordinary head;
- with waterproof head;
- with special termination of lead ends (without head).
- protected from the impact of non-aggressive and aggressive environments;
- unprotected (used when the measured medium does not have a harmful effect on thermoelectrodes).
- leaky;
- sealed, designed to operate at various conditional pressures and temperatures.
- vibration-resistant;
- impact resistant;
- ordinary.
- single zone;
- multizone.
- with high inertia - up to 3.5 minutes;
- with medium inertia - up to 1 minute;
- low inertia - up to 40 seconds;
- with unnormalized inertia.
The length of the working part of the thermocouple can be different: from 120 to 1580 mm for single-zone thermoelectric converters, up to 20000 mm for multi-zone ones.
§3 Extension (compensation) wires for thermocouples
According to the principle of operation of a thermocouple, described in Chapter 1, §1, the free ends of the thermocouple (cold junction) must be at a constant temperature, preferably close to 0 °C. Connecting wires are connected to these ends, which go to the measuring device. If the free ends are located in the head of a thermoelectric thermometer (see Figure 2), then this condition is practically impossible to fulfill. The thermometer head can be at very high temperatures, and these temperatures can also change due to changes in the state of the environment in which measurements are made. It is also not always possible to place the measuring instrument in close proximity to the thermocouple. Thus, there is a need to remove the connection points of the measuring device (free ends of the thermocouple) from the direct place of temperature measurement. This problem is solved with the help of compensation (extension) wires.A diagram of a thermoelectric circuit, which is obtained in the presence of extension wires, is shown in Figure 3.
Wires that satisfy the condition EAB (T 1 ; T 0) = ECD (T 1 ; T 0) are called extension (compensation). Such wires, connected to thermoelectrodes and connecting wires, develop at low temperatures (not more than 100-150 ° C) a thermo-EMF equal to the thermo-EMF of a thermocouple. The main purpose of compensating wires is to lead the free ends of the thermocouple to a zone with a known and constant temperature.
As an example, consider a platinum-rhodium-platinum (TPP) thermocouple. For this thermocouple, wires made of copper and copper-nickel alloy (0.6% Ni + 99.4% Cu) are used as extension wires. At T 1 \u003d 100 ° С and T 0 \u003d 0 ° С, they develop the same thermo-EMF as platinum-rhodium with platinum - 0.64 mV. In this case, the use of extension wires will allow the use of a smaller amount of expensive platinum-rhodium and platinum.
The structure of compensating wires is shown in Figure 4.
§4 The main sources of measurement errors using thermocouples
Any measurement is performed with a certain accuracy. The measurement accuracy depends on the method, external conditions, the condition of the measuring instruments and some other factors. The following are the main sources of error in temperature measurements using thermocouples.- Change in thermo-EMF during the operation of a thermocouple. This phenomenon is called thermoelectric instability of thermoelectrode alloys. It has been established that during operation, all thermoelectrode alloys change their thermo-EMF, which leads to a change in thermocouple readings. At relatively low temperatures or during short-term operation, changes in thermo-EMF may be insignificant and do not increase the measurement error. At high temperatures or long-term operation of thermocouples, instability can reach large values, which leads to a significant decrease in measurement accuracy. The main reasons causing thermoelectric instability are: interaction of thermoelectrodes with environment; interaction of electrodes with insulating and protective materials; interaction of thermoelectrodes with each other; internal processes occurring in thermoelectrode alloys under temperature changes, exposure to radiation, electromagnetic fields, high pressure.
- The measurement accuracy can be affected by the insulation resistance of thermoelectrodes. Under the influence of high temperatures, the electrical resistance of the insulation of thermoelectrodes can decrease, which, in turn, can lead to a significant distortion of the thermocouple readings.
- Incorrect selection of the measuring device can lead to the occurrence of measurement errors. With a decrease in the diameter of thermoelectrodes, the specific resistance (resistance per unit length) of the circuit increases. The same effect is observed with increasing temperature. If the input impedance of the meter does not match the resistance of the connected circuit, large measurement errors may occur.
- The reason for the occurrence of errors can be a change in the temperature of the free ends of the thermocouple. This temperature may change during the measurement or may differ from the temperature of the free ends during the calibration of the thermocouple.
- Measurement error can occur because thermocouple electrodes have different thermo-EMF values along their length. This phenomenon is called thermoelectric inhomogeneity of thermoelectrode alloys and occurs due to the inhomogeneity of the physical properties of metals and alloys from which thermocouple electrodes are made. The heterogeneity of physical properties is due to fluctuations in the composition and structure of materials. The reasons for such fluctuations can be radioactive irradiation, mechanical or electromagnetic effects on the electrodes themselves or the workpieces from which they are made, chemical reactions occurring during the manufacture or operation of thermocouple electrodes.
- Errors in determining the calibration characteristics of reference thermocouples.
- Deviation of the calibration characteristics of thermocouples from the standard calibration table.
Chapter 2 Types of Thermocouples and Their Parameters
§1 Thermocouple chromel-alumel (TXA)
One of the most common thermocouples used in industry and research. Allows you to measure temperatures up to 1100 °C for a long time and up to 1300 °C for a short time. Also used to measure low temperatures down to -200°C (70K). The chromel-alumel thermocouple is designed for operation in inert and oxidizing environments; it can be used for measurements in dry hydrogen and for short periods in vacuum. The thermoelectric characteristic of this thermocouple is almost linear, the sensitivity is about 40 µV/°C. The chromel-alumel thermocouple is the most stable among other types of thermocouples under reactor irradiation conditions.
The disadvantages of this thermocouple include high sensitivity to deformation of thermoelectrodes and reversible instability of thermo-EMF.
Thermocouple XA is produced in accordance with GOST 3044-84, thermoelectrode wire for this thermocouple - GOST 1790-77 and a number of specifications.
This thermocouple is used to measure temperature in industrial furnaces, heating devices, power equipment, as well as in a variety of scientific equipment and laboratory instruments.
Material of thermoelectrodes
In the XA thermocouple, the positive electrode is a wire made of nickel alloy chromel NH 9.5 (GOST 492-2006), the negative electrode is a wire made of nickel alloy alumel NMtsAK 2-2-1 (GOST 492-2006).
Recommended working environment
Thermocouple chromel-alumel is designed to measure temperature in oxidizing and inert media. The content of oxygen (O2) in the oxidizing environment must be at least 2-3% or its presence must be practically excluded. Otherwise, the selective oxidation of chromium sharply increases in chromel, its concentration decreases, which leads to a significant change in the decrease in the thermo-EMF of this alloy. The XA thermocouple can also be used in a reducing or alternating redox atmosphere if it has a reliable protective cover (see Chapter 1 § 2).
Isolation and protection
The following materials can be used as insulating materials for a chromel-alumel thermocouple: porcelain, asbestos, fiberglass, quartz, enamels, highly refractory oxides.
Operating recommendations
The most common reasons for the failure of a chromel-alumel thermocouple are: destruction of an alumel thermoelectrode due to its intercrystalline corrosion and embrittlement; destruction of the thermoelectrode from chromel due to its corrosion (corrosion of the "green rot" type).
Intercrystalline corrosion and embrittlement of the alumel alloy occurs as a result of heating the thermoelectrode to a temperature of 650-820 ° C in an atmosphere containing sulfur. Sources of sulfur can be: furnace fuel, residues of oils and emulsions in protective covers of thermocouples, some grades of asbestos, cement and other materials from which protective covers can be made. It is possible to prevent intercrystalline corrosion of alumel only by completely eliminating the ingress of sulfur into the atmosphere surrounding the thermoelectrodes.
Chromel alloy corrosion can be caused by selective internal oxidation of chromium (part of this alloy) due to the operation of the thermoelectrode in an atmosphere containing water vapor or CO (weakly oxidizing atmosphere). Chromel corrosion can be prevented by using large diameter ventilated protective sheaths or sheaths with getters placed inside.
§2 Chromel-Kopel thermocouple (TKK)
Main properties and applicationsOne of the most common thermocouples used in industry and research. Chromel-Copel thermocouple allows temperature measurements in inert and oxidizing media up to 800 °C for a long time and up to 1100 °C for a short time. The lower limit of measured temperatures is limited to -253 °C. Due to the presence in the industry of the thermocouple chromel-alumel, the thermocouple chromel-kopel is used, as a rule, for long-term measurements up to 600 °C. Thermocouples of this type are the most sensitive of all industrial thermocouples. The sensitivity of the XK thermocouple exceeds 81 µV/°C at temperatures above 200°C. Also, this thermocouple has an almost linear calibration characteristic. THC is characterized by exceptionally high thermoelectric stability at temperatures up to 600 °C. The disadvantages of thermocouples of this type include high sensitivity to deformation of the thermoelectrode.
Chromel-Copel thermocouples are calibrated according to calibration tables in accordance with GOST 3044-77. Wire for thermoelectrodes is supplied in accordance with GOST 1790-77 and a number of specifications.
Chromel-Kopel thermocouples are widely used in various fields of industry and in scientific research; often used to measure small temperature differences.
Material of thermoelectrodes
In the HK thermocouple, the positive electrode is a wire made of nickel alloy chromel NH 9.5 (GOST 492-2006), the negative electrode is a wire made of copper-nickel alloy Kopel MNMts 43-0.5 (GOST 492-2006).
Recommended working environment
The main working medium of the XK thermocouple is an oxidizing medium or one containing inert gases. The thermocouple can also be used in vacuum at high temperature, but for a short time. Continuous use of the Chromel-Copel thermocouple in this environment can result in the selective evaporation of chromium from the positive electrode.
Use of this thermocouple in sulphurous, reducing, variable redox, and slightly acidic atmospheres requires good (gas-tight) shielding. In an atmosphere containing chlorine or fluorine, the Chromel-Kopel thermocouple can operate at temperatures up to 200 °C.
§3 Iron-constantan thermocouple (TJK)
Main properties and applicationsThermocouples of this type are widely used in industry and scientific research. The iron-constantan thermocouple allows measurements in reducing, oxidizing, as well as inert media and vacuum. Thermocouple ZhKn allows you to measure both positive temperatures (up to 1100 °C) and negative temperatures (up to -203 °C). It should be noted separately that it is the measurement of positive temperatures together with negative temperatures that is the recommended use of this type of thermocouple. The use of these thermocouples for measuring exclusively negative temperatures is not recommended, since there are analogues with the best performance. With prolonged use, the maximum working temperature is 750 °С, for short-term - 1100 °С.
Thermocouples of this type have a high sensitivity, which is 50-65 µV/°C. It is also worth noting their relatively low cost. The disadvantages of thermocouples of this type include high sensitivity to deformation of thermoelectrodes, as well as low corrosion resistance of an iron thermoelectrode.
Material of thermoelectrodes
In the LCD thermocouple, the positive electrode is made of commercially pure iron (low-carbon steel), the negative electrode is made of the copper-nickel alloy Konstantan MNMts 40-1.5 (GOST 492-2006). It is worth noting that iron wire is not made specifically for thermometry, a wire designed for other purposes is used.
Recommended working environment
The iron-constantan thermocouple works stably in oxidizing and reducing atmospheres. At temperatures of about 769 °C and 910 °C, iron, from which the positive electrode of the thermocouple is made, undergoes magnetic and α↔γ transformations, which affect the thermoelectric properties. In connection with the above, a thermocouple that has been at temperatures above 760 ° C even for a short time interval cannot be used for further accurate measurements at temperatures below 760 ° C, since its readings may not correspond to the calibration table.
The service life of a thermocouple depends on the cross section of the thermocouples. The diameter of the thermocouple electrodes should be chosen in direct proportion to the measured temperature. Some sources give the following recommendations for choosing the diameter of thermocouple electrodes in cases for long-term temperature measurement: 760 °C - 3.2 mm; 590 ° С - 1.6 mm; 480 °С - 0.8 mm; 370 ° С - 0.3-0.5 mm.
At temperatures above 500 °C, the use of the ZhKN thermocouple in an atmosphere containing sulfur is possible only if there is a reliable gas-tight protection.
§4 Tungsten-rhenium thermocouple (TVR)
Main properties and applicationsThe tungsten-rhenium thermocouple is one of the best industrial thermocouples for measuring temperatures above 1800 °C. Thermocouple BP is used to measure temperatures up to 3000 °C. The lower limit of measured temperatures is usually limited to 1300 °C. The working atmosphere is argon, nitrogen, helium, dry hydrogen or vacuum. Thermo-EMF at 2500 °C is 34 mV for thermocouples made of VR5/20 and VAR5/VR20 alloys and 22 mV for thermocouples made of VR10/20 alloy, the sensitivity of thermocouples is 7–10 and 4–7 μV/°C, respectively.
Tungsten-rhenium thermocouples have good mechanical properties at high temperatures, can operate under the influence of large alternating loads, as well as with frequent and abrupt heat cycles. Thermocouples of this type are unpretentious in manufacturing and installation, as they are relatively insensitive to contamination.
Among the shortcomings of VR thermocouples, one can single out poor reproducibility of thermo-EMF; instability of thermo-EMF under irradiation conditions; a significant drop in sensitivity at temperatures above 2400 °C.
It is worth noting that a thermocouple made of VAR5/BP20 alloys gives a more accurate result in long-term measurements than a thermocouple made of BP5/20 alloys.
Graduation of tungsten-rhenium thermocouples is carried out according to calibration tables in accordance with GOST 3044-77. Wire for thermoelectrodes made of alloys VR5, VAR5 and VR20 is manufactured according to specifications. Thermoelectrode wire made of VR10 alloy is not mass-produced.
BP thermocouples are used in industries associated with high temperatures. For example, a tungsten-rhenium thermocouple is used to measure temperature in the production of refractory metals, hard alloys and ceramics, in the smelting and casting of steels and alloys, to measure the temperature of gas flows and low-temperature plasma in gas turbine engines, MHD generators, and also in nuclear power engineering.
Material of thermoelectrodes
In tungsten-rhenium thermocouples, the materials for the electrodes are alloys VR5 - positive thermoelectrode and VR20 - negative; VAR5 - positive thermoelectrode and VR20 - negative or VR10 - positive thermoelectrode and VR20 - negative.
Recommended working environment
Tungsten-rhenium thermocouples are designed for long-term temperature measurement in pure inert media, dry hydrogen and vacuum. Even a small amount of oxygen significantly reduces the life of the thermocouple. In oxidizing environments, thermocouples of this type can only be used to measure temperature in fast processes. At temperatures above the values at which catastrophic oxidation begins, the service life of the thermocouple is calculated in minutes.
The use of BP thermocouples is not recommended in humid hydrogen and carbonaceous reducing environments. The reaction of tungsten-rhenium alloys with hydrocarbon vapors begins already at 1000 °C. Interaction with carbon can lead to embrittlement of thermoelectrodes and a significant increase in thermocouple instability. The appearance of brittleness is already observed at 1700°C. Contact with carbon lowers the limiting measured temperature to 2500 °C. However, there are cases of using a tungsten-rhenium thermocouple in high-temperature furnaces with graphite heaters. The general conclusion can be formulated as follows: the service life of a thermocouple depends to a large extent on the nature of the atmosphere, the insulation material and the operating temperature.
Isolation and protection
To isolate thermoelectrodes, ceramics from BeO, HfO2, ThO2, Y2O3 are used. Beryllium oxide can be used at temperatures not exceeding the melting point of this material (~2570 °C). BeO is the most commonly used insulator for BP thermocouples. It should be noted that it is necessary to use BeO with a purity of at least 99.9%.
To measure temperatures below 1600 °C, thermocouple electrodes are insulated with Al2O3 oxide of 99.5% purity or MgO. In this case, ceramics must be calcined to remove organic and inorganic impurities.
At very high temperatures, thermocouples with bare thermoelectrodes are used. In oxidizing environments, metal sheaths made of Nb, Ta, Mo and Mo-Re, W-Re alloys with coatings are mainly used to protect thermocouples. A thermocouple with iridium-coated thermoelectrodes can be operated in air for a short time (30-40 hours at a temperature of 2000-2400 °C).
§5 Thermocouple tungsten-molybdenum (VM)
Main properties and applicationsThe thermocouple is designed to measure high temperatures. Measurements using a tungsten-molybdenum (VM) thermocouple can be carried out in inert media, hydrogen or vacuum. The range of measured temperatures is 1400-1800 °C, the maximum operating temperature is ~2400 °C. The BM thermocouple has a sensitivity of 6.5 µV/°C in the indicated temperature range. Thermoelectrodes have high mechanical strength. In the manufacture, installation and operation of the thermocouple, there are no strict requirements for maintaining chemical purity. The tungsten-molybdenum thermocouple is the cheapest to manufacture among other thermocouples suitable for measuring high temperatures.
Among the shortcomings of the VM thermocouple, one can single out the poor reproducibility of thermo-EMF; a small amount of thermo-emf and sensitivity; polarity inversion; brittleness after heating at high temperatures.
The main field of application of the BM thermocouple is short-term temperature measurements of liquid steels, alloys and slags in various types of furnaces, converters and ladles. It is worth noting that with the advent of tungsten-rhenium thermocouples (see Chapter 2 § 4) and platinum-rhodium-platinum-rhodium (see Chapter 2 § 7), the tungsten-molybdenum thermocouple began to be used to measure temperatures in the smelting and pouring processes of only irresponsible alloys.
Wire for the manufacture of thermoelectrodes from tungsten and molybdenum is supplied according to specifications.
Material of thermoelectrodes
For the manufacture of thermoelectrodes of thermocouples VM, technical purity metals are used. High-purity metals are generally not used, as they significantly increase the cost of the thermocouple and impose increased requirements on the absence of contamination. The positive electrode in a tungsten-molybdenum thermocouple is made of tungsten, the negative electrode is made of molybdenum (due to polarity inversion, this statement is true for temperatures above 1400 ° C). For the manufacture of tungsten wire, rods of the VRN brand are used, for the manufacture of molybdenum wire, rods of the MCH brand are used.
Recommended working environment The tungsten-molybdenum thermocouple is used to measure temperature in hydrogen, inert gases or vacuum. Tungsten and molybdenum begin to oxidize in air at about 400°C. As the temperature rises, the oxidation process intensifies. These metals do not react with hydrogen up to the melting point and with inert gases. In this case, neither hydrogen nor inert gases should contain oxidizing impurities. The usual operating temperature range of the BM thermocouple in industrial operation is 1400-1800 °C. In special cases, this range can be extended up to 2100 °C. In this case, it is recommended to use a thermocouple without insulation, since at temperatures above 2000 ° C, molybdenum and tungsten begin to interact with many oxides, from which insulation is usually made.
If the electrodes are protected by ceramics and the thermocouple has a protective cap, it can be used to take short-term temperature measurements in oxidizing media and molten metals.
Isolation and protection
The thermoelectrodes of tungsten-molybdenum thermocouples for single measurements of the temperature of liquid steel are insulated with alumina ceramics (Al2O3) and protected with quartz tips.
§6 Thermocouples platinum-rhodium-platinum (TPP)
Main properties and applicationsThermocouples platinum-rhodium-platinum are among the most common for measuring temperatures up to 1600 °C. This type includes thermocouples made from platinum and platinum-rhodium alloy (10% Rh), and from platinum and platinum-rhodium alloy (13% Rh). Thermocouples PP are designed to perform temperature measurements in oxidizing and inert media. The maximum operating temperature for long-term measurements is 1400 °C, for short-term measurements - 1600 °C. Platinum-rhodium-platinum thermocouples have an almost linear thermoelectric characteristic in the temperature range of 600-1600 °C, sensitivity 10-12 μV/°C (10% Rh) and 11-14 μV/°C (13% Rh). Other advantages of these thermocouples are high measurement accuracy, good reproducibility and thermo-EMF stability. It is worth noting that thermocouples of this type act as reference instruments for reproducing the International Practical Temperature Scale (IPTS) in the temperature range from 630.74 to 1064.43 °C.
The disadvantages of PP thermocouples include high cost, instability of operation under irradiation conditions, high sensitivity to contamination by metallic and non-metallic impurities during manufacture, installation and operation.
Platinum-rhodium-platinum thermocouples are used in various industries and sciences where high accuracy and reliability of measurements are required.
The thermocouple PR (10% Rh) is calibrated in accordance with GOST 3044-77, the thermoelectrode wire is manufactured in accordance with GOST 10821-75. Thermoelectrode wire for thermocouples PR (13% Rh) is manufactured according to specifications.
Material of thermoelectrodes
For the manufacture of PP thermocouples, platinum-rhodium alloys PR10 or PR13 are used, containing 10% and 13% rhodium (Rh), respectively, and pure platinum.
A thermoelectrode made of platinum-rhodium is positive, and platinum is negative.
Recommended working environment
The platinum-rhodium-platinum thermocouple is designed to measure temperature in oxidizing and inert media. In the presence of protection, thermocouples of this type can be used for measurements in reducing media and media containing vapors of arsenic, sulfur, lead, zinc, and phosphorus.
In practice, PP thermocouples are rarely used to measure temperatures below 0 °C. The fact is that the sensitivity of this type of thermocouple decreases with decreasing temperature and becomes equal to zero at -138 ° C. However, in some thermo-EMF standards, thermocouples are rated at temperatures down to -50 °C. Thermocouples platinum-rhodium-platinum are not used to measure temperatures in the range of 0-300 °C, and for temperatures of 300-600 °C they are used only to obtain comparative data.
The upper temperature limit for short-term use of the PP thermocouple is limited to 1600 °C, for long-term use - 1400 °C. At temperatures above 1400 °C, the grains of the platinum thermoelectrode rapidly grow. With good protection, the thermocouple can be used for long-term measurements at temperatures up to 1500 °C.
Isolation and protection
Quartz, porcelain, mullite, sillimanite, and refractory porcelain can serve as insulation for thermoelectrodes of working thermocouples up to a temperature of 1200 °C. Thermoelectrodes of exemplary thermocouples are insulated with fused quartz. If the thermocouple is used to measure temperatures up to 1400 °C, then ceramics with a high content of Al2O3 are used as insulation. In weakly oxidizing and reducing atmospheres above 1200°C, and in all cases where thermocouples are used above 1400°C, high-purity alumina ceramics should be used. When working in a reducing atmosphere, magnesium oxide is sometimes used as insulation.
Inner sheaths for thermocouples are usually made from the same materials as insulating ceramics. A prerequisite is the gas tightness of such materials.
To protect the working junctions of thermocouples intended for single temperature measurements of liquid steels and alloys, quartz tips are used.
Operating recommendations
PP thermocouples are very sensitive to various kinds of chemical contamination, which can cause embrittlement and strength reduction, as well as the occurrence of a strong thermocouple drift. The platinum electrode is especially sensitive to contamination. Sources of pollution can be the materials from which the insulation and the protective cover are made, the heating device and its atmosphere, objects that are in close proximity to the thermocouple.
Recommendations for preventing contamination of thermoelectrodes. Thermal electrodes must be insulated with one two-channel ceramic tube along the entire working length. Between the insulating tube and the ceramic protective sheath, as well as between the thermoelectrodes and the tube, there must be sufficient, well-ventilated gaps. The thermoelectrodes must be thoroughly cleaned of traces of grease and grease before they are placed in insulating and protective ceramics. Metal sheaths must also be cleaned of dirt, grease residues, chips, etc. Before mounting, all thermocouple components - electrodes, insulating and protective ceramics and sheaths - must be annealed at high temperature. The design of the thermocouple must be such that the thermoelectrodes do not support the insulating ceramic. This recommendation is especially important for vertically mounted thermocouples.
§7 Thermocouples platinum-rhodium-platinum-narodium (TPR)
Main properties and applications
Thermocouple PR is designed to measure temperature in oxidizing and neutral media. It can also be used in a vacuum. The maximum operating temperature for long-term measurements is 1600 °C, for short-term measurements it is 1800 °C. At temperatures above 1200 °C, a platinum-rhodium-platinum-rhodium thermocouple has a linear thermoelectric characteristic, a sensitivity of 10.5–11.5 μV/°C, and good thermo-emf stability. The PR thermocouple can be used without extension wires due to its low sensitivity in the temperature range of 0-100 °C.
Compared to platinum-rhodium-platinum thermocouples, the platinum-rhodium-platinum-rhodium thermocouple has a slightly lower thermal EMF, while it can measure higher temperatures. The PR thermocouple has greater mechanical strength, greater stability at high temperatures, less tendency to grain growth and embrittlement, and less sensitivity to contamination.
The PR thermocouple is actively used in areas where long-term temperature measurement above 1400 °C is required. Such areas include metallurgy, glass-smelting, cement industry, production of refractories. Also, thermocouples of this type are used in exemplary thermometers.
The platinum-rhodium-platinum-rhodium thermocouple is calibrated in accordance with GOST 3044-77, the thermoelectrode wire is manufactured in accordance with GOST 10821-75.
Material of thermoelectrodes
To manufacture the PR thermocouple, platinum-rhodium alloys PR30 and PR6 are used, containing 30% and 6% rhodium (Rh), respectively. The purity of platinum and rhodium, which are used in the production of alloys, must be greater than or equal to 99.95%.
A thermoelectrode made of platinum-rhodium PR30 is positive, and a thermoelectrode made of platinum-rhodium PR6 is negative.
Recommended working environment
Thermocouples platinum-rhodium-platinum-rhodium are used in oxidizing and neutral environments, as well as in vacuum. The maximum operating temperature of the PR thermocouple is determined by the melting point of the negative thermoelectrode made of the PR6 alloy (1820 °C) and is 1800 °C (according to GOST 3044-77 and GOST 6616-74 for short-term measurements). For long-term measurements, the operating temperature is limited to 1600 °C.
Without reliable protection thermocouples of this type cannot be used in reducing atmospheres and atmospheres containing vapors of metals and non-metals.
Isolation and protection
High-purity Al2O3 ceramics are used to isolate and protect PR thermocouples.
Operating recommendations
The causes of failure of platinum-rhodium-platinum-rhodium thermocouples due to embrittlement, reduced mechanical strength, or extremely large thermo-EMF drift, as a rule, coincide with the causes of similar problems that occur with platinum-rhodium-platinum thermocouples. But the failure of PR thermocouples occurs much less frequently compared to PP thermocouples, since platinum-rhodium alloys are less susceptible to chemical contamination and grain growth than pure platinum, from which the negative electrode of PP thermocouples is made.
§8 Summary table of thermocouple types
The summary table contains the main parameters of thermocouples with standard calibrations. This table does not include thermocouples with individual graduations, such as the tungsten-molybdenum thermocouple (see Chapter 4, §3).Thermocouple type | Materials of thermoelectrodes | Operating temperature range, °С | color coding | |
---|---|---|---|---|
positive | negative | |||
CCI (S) Chapter 2, §6 |
Platinum Rhodium (10% Rh) | Platinum | 0 – 1300 (1600) | |
CCI (R) Chapter 2, §6 |
Platinum Rhodium (13% Rh) | Platinum | 0 – 1300 (1600) | |
TPR (B) Chapter 2, §7 |
Platinum Rhodium (30% Rh) | Platinum Rhodium (6% Rh) | 600 – 1700 | |
THK (L) Chapter 2, §2 |
Chromel | Kopel | -200 – 700 (900) | |
THA (K) Chapter 2, §1 |
Chromel | Alumel | -200 – 1200 (1300) | |
TFA (J) Chapter 2, §3 |
Iron | Constantan | -200 – 750 (900) | |
TVR (A) Chapter 2, §4 |
Tungsten-rhenium (5% Re) | Tungsten-rhenium (20% Re) | 0 – 2200 (2500) |
Notes:
- in the column "Range of operating temperatures" in brackets is the maximum operating temperature for short-term use;
- the Color Coding column describes the color coding adopted by the International Electrotechnical Commission (IEC).
Chapter 3 Thermocouple Materials
§1 Requirements for thermoelectrode alloys
Thermocouples are used to measure a wide range of temperatures in various media. At the same time, measuring instruments must provide adequate accuracy and have acceptable service life. In connection with the features listed above, special requirements are imposed on the materials used for the production of thermocouples.- The thermo-EMF of thermoelectrode alloys forming a thermocouple must be large enough to be measured with the required accuracy. It is desirable that the thermo-EMF value linearly depend on the temperature value.
- The melting temperature of thermoelectrode alloys must be above the maximum operating temperature of the thermocouple. The difference between the indicated temperatures must be at least 50 °C.
- Thermoelectrode alloys must have corrosion resistance in the working environment of a thermocouple. This requirement cannot always be met, therefore, in such cases, thermoelectrodes are protected from the effects of the environment with the help of a protective cover.
- Thermoelectrode alloys should be distinguished by reproducible and uniform properties when they are produced on an industrial scale.
- Alloys for thermocouples must keep their thermoelectric characteristic unchanged during calibration and operation.
- Alloys for thermocouples should have good ductility and strength.
§2 Nickel and copper-nickel alloys
Nickel and copper-nickel alloys are widely used in the manufacture of thermocouple thermoelectrodes and compensating wires. The most popular in the production of thermocouples are nickel alloys alumel and chromel, copper-nickel alloys - kopel and constantan.Alumel
Nickel alloy intended for the production of thermocouple thermoelectrodes and compensating wires. Used in XA thermocouples (chromel-alumel) as a negative electrode (see Chapter 2§1). The brand of this alloy has the following designation: NMtsAK 2-2-1.
Chemical composition
The main chemical element that is part of the alumel alloy is Nickel (Ni). In addition to nickel, the NMtsAK 2-2-1 alloy contains 0.6-1.2% cobalt (Co); 1.6-2.4% aluminum (Al); 1.8-2.7% manganese (Mn); 0.85-1.50% silicon (Si). The sum of impurities, which include arsenic (As), carbon (C), iron (Fe), phosphorus (P), lead (Pb), sulfur (S) and some other substances, is 0.7%.
Physical properties
Mechanical properties
Chromel
Nickel alloy intended for the production of thermocouple thermoelectrodes and compensating wires. Used in thermocouples XA (chromel-alumel), XK (chromel-kopel) as a positive electrode material (see Chapter 2§2). The brand of this alloy has the following designation: HX 9.5.
Chemical composition
The main chemical element that is part of the chromel alloy is Nickel (Ni). In addition to nickel, the HX 9.5 alloy contains 0.6-1.2% cobalt (Co) and 9.0-10.0% chromium (Cr). The sum of impurities, which include arsenic (As), carbon (C), iron (Fe), phosphorus (P), lead (Pb), sulfur (S) and some other substances, is 1.4%.
Physical properties
Mechanical properties
Kopel
Copper-nickel alloy intended for the production of thermocouple thermoelectrodes and compensating wires. Used in XK (chromel-kopel) thermocouples as negative electrode material (see Chapter 2§2). The brand of this alloy has the following designation: МНМЦ 43-0.5.
Chemical composition
The main chemical elements that make up the kopel alloy are nickel (Ni), cobalt (Co) and copper (Cu). The content of nickel + cobalt (Ni + Co) is 42.5-44.0%, the rest is copper (Cu). In addition to nickel, the MNMts 43-0.5 alloy contains 0.1-1.0% manganese (Mn). The sum of impurities, which include arsenic (As), carbon (C), iron (Fe), phosphorus (P), lead (Pb), sulfur (S) and some other substances, is 0.6%.
Physical properties
Mechanical properties
Constantan
Copper-nickel alloy intended for the production of thermocouple thermoelectrodes and compensating wires. It is used in LCD thermocouples (iron-constantan) as a negative thermoelectrode material (see Chapter 2§3). The brand of this alloy has the following designation: МНМЦ 40-1.5.
Chemical composition
The main chemical elements that make up the constantan alloy are nickel (Ni), cobalt (Co) and copper (Cu). The content of nickel + cobalt (Ni + Co) is 39.0-41.0%, the rest is copper (Cu). In addition to nickel, the MNMts 40-1.5 alloy contains 1.0-2.0% manganese (Mn). The sum of impurities, which include arsenic (As), carbon (C), iron (Fe), phosphorus (P), lead (Pb), sulfur (S) and some other substances, is 0.9%.
Physical properties
Mechanical properties
§3 Refractory metals and alloys
Refractory metals and alloys are widely used in the manufacture of thermocouple thermoelectrodes for measuring high temperatures. The most popular in the production of thermocouples are tungsten-rhenium alloys VR, refractory metals tungsten and molybdenum.Tungsten-rhenium alloys
The most common tungsten-rhenium alloys for the production of thermocouples are alloys BP5 and BP20. These alloys are used for the manufacture of thermoelectrodes for VR thermocouples (tungsten-rhenium - tungsten-rhenium) (see Chapter 2 § 4).
Chemical composition
The main component of BP alloys is tungsten (W). Depending on the grade, each alloy contains a different amount of rhenium (Re). So alloy VR5 contains 5±0.5% rhenium (Re), VR20 - 20±0.5% rhenium (Re). The content of impurities and additives in these alloys should not exceed 0.1%.
Tungsten
The refractory metal tungsten has found application in the production of high-temperature thermocouples. For these purposes, tungsten of technical purity grade VRN is used. This refractory metal is used for the manufacture of positive electrodes of the BM (tungsten-molybdenum) thermocouple (see Chapter 2 § 5).
Chemical composition
VRN grade tungsten contains not less than 99.85% tungsten (W) and not more than 0.040% molybdenum (Mo), 0.005% silicon (Si), 0.011% calcium (Ca), 0.005% nickel (Ni), 0.013% iron + aluminum (Fe + Al).
Molybdenum
The refractory metal molybdenum has found application in the production of high-temperature thermocouples. For these purposes, molybdenum of technical purity grade MCH is used. This refractory metal is used for the manufacture of negative electrodes of the BM (tungsten-molybdenum) thermocouple (see Chapter 2 § 5).
Chemical composition
Molybdenum grade MCH contains not less than 99.85% tungsten (W) and not more than 0.040% molybdenum (Mo), 0.005% silicon (Si), 0.011% calcium (Ca), 0.005% nickel (Ni), 0.013% iron + aluminum (Fe + Al).
§4 Precious metals and alloys
Platinum
The noble metal platinum is used to produce thermocouples with high measurement accuracy. For these purposes, pure platinum grade PLT is used. Negative thermoelectrodes are made from platinum in PP (platinum-rhodium-platinum) thermocouples.
Chemical composition
For the manufacture of thermocouple thermoelectrodes, pure platinum (100% Pt) is used, in which the R100/R0 value must be at least 1.3910.
Platinum Rhodium
An alloy of platinum and rhodium used to make thermocouple electrodes. The most widely used in this area are alloys of platinum with rhodium grades PR10, PR13, PR6, PR30. Alloys PR10, PR13 are used in thermocouples PP (platinum-rhodium-platinum). Positive thermoelectrodes are made from these alloys. Alloys PR30 and PR6 are used in thermocouples PR (platinum-rhodium-platinum-rhodium). These alloys are used to produce positive and negative thermoelectrodes, respectively.
Chemical composition
The main chemical element in platinum-rhodium alloys is platinum. The percentage of platinum and rhodium varies depending on the grade of the alloy. The content of impurities is not standardized, but is limited to the use of platinum and rhodium for the manufacture of alloys with a purity greater than or equal to 99.95%. Alloy PR6 - 94% platinum (Pt), 6% rhodium (Rh); PR10 - 90% platinum (Pt), 10% rhodium (Rh); PR13 - 87% platinum (Pt), 13% rhodium (Rh); PR30 - 70% platinum (Pt), 30% rhodium (Rh).
Chapter 4 Thermocouple Manufacturing
§1 Production of thermoelectrode wire
Thermoelectrode wire is used for the manufacture of thermocouple electrodes. This wire is manufactured in accordance with the requirements of national standards or specifications, depending on the type of thermocouple. Standards and specifications govern chemical composition, physical properties alloys from which the wire is made, as well as its mechanical properties, dimensions and limit deviations for them.For example, wire for chromel-alumel thermocouples must comply with the requirements of GOST 1790-77. In accordance with the specified standard for the manufacture of thermoelectrodes, wire of the following diameters is used 0.2; 0.3; 0.5; 0.7; 1.2; 1.5; 3.2; 5 mm. Also, this standard regulates the diameters of the wire for thermoelectrodes of thermocouples chromel-kopel, chromel-constantan. GOST 1791-67 defines the diameters of the wire from which extension wires for thermocouples chromel-kopel, chromel-alumel and platinum-rhodium-platinum are made. According to the specified standard, the wire can have a diameter of 0.20; 0.30; 0.40; … 1.00; … 2.50 mm. TU 11-75 regulates the dimensions of the wire for the manufacture of tungsten-rhenium thermocouple electrodes. A wire with a diameter of 0.10 is produced; 0.20; 0.35 and 0.50 mm.
A thermoelectrode wire of a given diameter is obtained during the technological operation of broach. Depending on the required wire diameter, either a bar or a wire of a larger diameter than the one you want to make is used as a workpiece. Broach can be carried out in several stages. Depending on the material from which the wire is made, the drawing process can be carried out in conjunction with heating, as well as in the presence of lubrication. After drawing, the wire may be subjected to additional heat or chemical treatment to remove grease and improve properties. For example, a thermocouple wire for electrodes and extension wires for XA, XK thermocouples is annealed. You can learn more about the process of manufacturing thermocouple wire from tungsten and molybdenum in the articles and.
§2 Matching
In the process of manufacturing a thermocouple, it becomes necessary to select a pair of thermoelectrodes made of different alloys in such a way as to minimize the deviations of the real thermoelectric power developed by the thermocouple at given temperatures from standard values. Currently, there are a number of techniques that allow such a selection.To ensure the selection of electrodes that form a pair, it is necessary to know their thermoelectric properties when working with the same reference thermoelectrode. An electrode made of pure platinum is used as a reference thermoelectrode.
The choice of platinum as the material of the reference thermoelectrode is due to the following reasons:
- this metal has a high chemical inertness;
- this metal has well-studied physical properties;
- This metal has a fairly high melting point.
§3 Graduation and verification of thermocouples
The main purpose of a thermocouple is to measure temperature. The change in temperature leads to the emergence of thermo-EMF in the electrical circuit, which includes the thermocouple electrodes. Thus, the measuring device, also included in the electrical circuit, determines the change in thermo-EMF (see Chapter 1 § 1). But the ultimate goal is to determine the temperature. Accordingly, it is necessary to compare specific thermo-EMF values with specific temperature values. The thermoelectric thermometer scale should display degrees.Thermocouples can be roughly divided into two groups:
- with nominal static conversion characteristics (standard graduations);
- with individual graduations (non-standard graduations).
For thermocouples with individual calibrations, there is no dependence of thermo-EMF on temperature, determined by state standards. For each thermocouple from this group, it is necessary to calibrate. The calibration methods for such thermocouples coincide with the calibration methods for standard thermocouples. Examples of such thermoelectric converters are tungsten-molybdenum, tungsten-tantalum, titanium carbide-graphite thermocouples and some others.
Due to various factors, the readings of a particular thermocouple may differ from the readings regulated by the standard (the causes of measurement errors are described in Chapter 1 § 4). In this regard, it is necessary to carry out verification of thermocouples. This operation is performed for new thermocouples of standard types in order to determine their accuracy class and with a given frequency for all thermocouples during operation to control the accuracy of measurements. For thermocouples of standard types, verification is carried out in accordance with the requirements of GOST 8.338-2002 “Thermoelectric converters. Verification Methods.
There are four main methods for checking thermocouples:
- direct comparison method;
- difference (differential) method;
- electrode comparison method;
- by fiducial points.
In accordance with the direct comparison method, the temperature in the heating device, in which the working junctions of the reference and verified thermocouples are located, is determined using the reference thermocouple, after which the thermo-EMF developed by the verified thermocouples is measured. The furnace must be heated to the specified temperature with a tolerance of no more than ±10 °C. During the measurement of thermo-EMF of verified thermocouples, the temperature of the working junction (in the furnace) should not change by more than 0.4 °C / min. This method is used for verification of working (technical) thermocouples.
Difference (differential) method
The difference method gives a higher accuracy than the direct comparison method. In this method, the thermo-EMF difference between the reference and verified thermocouples is measured. The thermo-EMF of the calibrated thermocouple is obtained by calculation on the basis of the measured difference between the thermo-EMF and the thermo-EMF of the reference thermocouple. This method is also used for verification of reference thermocouples.
Electrode comparison method
The electrode-by-electrode comparison method consists in the fact that at certain temperatures set in the heater according to the indications of a reference thermocouple, thermo-EMF is measured between the same electrodes of the reference and verified thermocouples. Based on the obtained thermo-EMF values, the thermo-EMF of the calibrated thermocouple is calculated. This method is also used for verification of reference thermocouples.
Verification method at fiducial points
This method provides for the verification of thermocouples at the melting (solidification) points of pure metals and is used for verification of reference thermocouples of higher discharges.
The following points were chosen as reference points:
- solidification point of copper (1084.620 °C);
- aluminum solidification point (660.323 °C);
- the hardening point of zinc (419.527 ° C).
Reference values of thermo-EMF of thermal converters (thermocouples) at the corresponding reference points:
- the solidification point of copper is 10574 ± 30 μV;
- solidification point of aluminum - 5860 ± 17 μV;
- the solidification point of zinc is 3447 ± 14 μV.
If the verification requirements are not met, the thermocouple is rejected or transferred to a lower accuracy class. Calibration intervals(frequency of verification) are regulated normative documents(standards, specifications, and others) for the respective types of thermocouples.
Conclusion
This article discusses various aspects related to thermocouples - purpose, principle of operation, types, production.Thermoelectric thermometers, which are based on thermocouples, are currently one of the most common means of measuring temperature. This is evidenced by a large number of types of thermocouples, as well as designs of thermoelectric thermometers described in this article.
The presence of local and international standards that regulate the requirements for thermocouples greatly simplifies their selection and operation.
Description of the principle of operation of the thermocouple and the process of its production allows you to get basic set knowledge, useful for direct work with thermoelectric thermometers.
Bibliography
- https://en.wikipedia.org/wiki/Thermocouple - Thermocouple
- Garcia V. – Temperature measurement: theory and practice
- https://slovari.yandex.ru/~books/TSB/Thermometry/ - Thermometry
- Preobrazhensky V.P. – Thermotechnical measurements and devices…
- Zimin G.F. – Verification and calibration of thermoelectric converters…
- https://ru.wikipedia.org/wiki/Getter_(getter) - Getter (getter)
- http://metallurgicheskiy.academic.ru/2094/Gas tightness - Gas tightness
- Nikonov N.V. - Wolfram. Properties, application, production, products (http://www..pdf)
- Nikonov N.V. – Thermocouples. Types, characteristics, designs, production (http://www..pdf)
- https://en.wikipedia.org/wiki/Graduation - Graduation
- http://temperatures.ru/pages/graduirovochnye_tablicy - Calibration tables for thermocouples (NSH)
- GOST R 8.585-2001 “Thermocouples. Nominal static conversion characteristics"
- GOST 8.338-2002 “Thermoelectric converters. Verification Methods»
- GOST R 8.611-2005 “Platinum-rhodium-platinum reference thermoelectric converters of the 1st, 2nd and 3rd category. Verification Method"
- https://ru.wikipedia.org/wiki/Temperature_electric_resistance_coefficient - Temperature coefficient of electrical resistance
- https://ru.wikipedia.org/wiki/Coefficient_of_thermal_expansion - Coefficient of thermal expansion
- https://en.wikipedia.org/wiki/Ferromagnetism - Ferromagnetism
- https://en.wikipedia.org/wiki/Paramagnets - Paramagnets
- Used to measure temperatures in the range from -200 °C to +1000 °C (recommended limit depending on the diameter of the thermoelectrode wire).
- In the temperature range from 200 to 500 °C, a hysteresis effect may occur, when the readings during heating and cooling may differ. In some cases, the difference reaches 5 °C.
- Works in a neutral atmosphere or an atmosphere with excess oxygen.
- After thermal aging, the readings decrease.
- A change in thermo-emf may occur when used in a rarefied atmosphere, as chromium can be released from the Ni-Cr output (so-called migration). In this case, the thermocouple shows an underestimated temperature.
- The sulfur atmosphere is detrimental to the thermocouple, as affects both electrodes.
- Used to measure temperatures in the range from -200 °C to +800 °C (recommended limit depending on the diameter of the thermoelectrode wire).
- Used to measure temperatures in the range from -40 °C to +900 °C.
- It has high sensitivity, which is a plus.
- Electrode materials have thermoelectric homogeneity.
- Used to measure temperatures in the range from -250 °C to +300 °C.
- It can work in an atmosphere with a slight excess or lack of oxygen.
- The use of thermocouples of this type at temperatures above 400 °C is not recommended.
- Not sensitive to high humidity.
- Both leads can be annealed to remove materials causing thermoelectric discontinuity.
- Rust may form on the iron terminal due to moisture condensation.
- Works well in a discharged atmosphere.
- The maximum application temperature is 500 °C, because above this temperature, rapid oxidation of the leads occurs. Both leads are rapidly destroyed in a sulfur atmosphere.
- Readings increase after thermal aging.
- Low cost, because The thermocouple contains iron.
- Used to measure temperatures from 0 to 760 °C.
- Used to measure high temperatures from 0 to 2500 °C in an inert environment.
- This is a relatively new type of thermocouple developed from the K-type thermocouple. The K-type thermocouple can easily become contaminated with impurities at high temperatures. By fusing both electrodes with silicon, it is possible to contaminate the thermocouple beforehand, and thus reduce the risk of further contamination during operation.
- Recommended operating temperature up to 1200°C (depending on wire diameter), short-term operation at 1250°C is possible.
- High stability at temperatures from 200 to 500 °C (significantly smaller hysteresis than for type K thermocouple).
- It is considered the most accurate base metal thermocouple.
Types of noble metal thermocouples and their features
1. Type B (platinum-rhodium-platinum-rhodium)- The maximum temperature at which the thermocouple can operate is 1500 °C (depending on the wire diameter).
- Short-term use is possible up to 1750 °C.
- There is an effect of pollution by hydrogen, silicon, copper and iron vapors at temperatures above 900 °C. But this effect is less than for type S and R thermocouples.
- Can work in an oxidizing environment.
- It is not recommended to use at temperatures below 600 °C, where the thermo-EMF is very small and non-linear.
- The maximum temperature at which the thermocouple can operate is 1350 °C.
- Short-term use is possible up to 1600 °C.
- There is an effect of pollution by hydrogen, carbon, copper and iron vapors at temperatures above 900 °C. When the content of iron in the platinum electrode is 0.1%, the thermal EMF changes by more than 1 mV (100°C) at 1200°C and 1.5 mV (160°C) at 1600°C. The same picture is observed with copper contamination. Conclusion: thermocouples of this type cannot be reinforced with a steel tube or the electrodes should be insulated from the tube with gas-tight ceramics.
- Can work in an oxidizing atmosphere.
- At temperatures above 1000°C, the thermocouple may become contaminated with silicon, which is present in some types of protective ceramic materials. It is important to use ceramic tubes made of high purity alumina.
- It is not recommended to use below 400 °C, because the thermo-EMF in this area is small and extremely non-linear.
- It has the same properties as type S thermocouples.
Thermal converter (converter, temperature sensor)- this is a measuring instrument (device) that converts the measured temperature into a signal (NSH) for subsequent transmission, processing or registration by means of TP automation.
This section presents the following types and brands of supplied thermoelectric converters (thermocouples) - TP (TXA, TKhK, CCI, TPR, TGK, TVR, etc.):
TP (TXA, THC) -2088, -2388, -2187, -2188, -1085, -2488, -0195(0295, 0395), -0188(0198, 0199);
TP008(THA, THK-008);
TPK, TPL-005, -004, -001,
ТХА(ТХК) -1…18;
Chamber of Commerce and Industry (TPR) -023, -178,
TTPP, TTPR-53.
Types and brands of thermoelectric converters (thermocouples) - TP.
NSH of thermal converters - TP: ТХК(L), ТХА(K), ТПП(S,R), ТPR(B), ТЖК(J), ТНН(N), TVR(A-1,2,3), ТМК( T).
THC- Thermoelectric converter (thermocouple ХК - chromel-kopel (L)), temperature measurement range -200…+600С (max 800С)).
THA- Thermoelectric converter (XA thermocouple - chromel-alumel (K)), temperature measurement range -200…+1100С (max 1300С)).
CCI- Thermoelectric converter (thermocouple PP - platinum-platinum (S,R)), temperature measurement range 0…+1300С (max 1700С)).
TPR- Thermoelectric converter. (thermocouple PR - platinum-rhodium (B)), temperature measurement range +600…+1600С (max +200…1800С)).
TGK- Thermoelectric converter. (thermocouple LCD - iron-constantan (J)), temperature measurement range -40 ... + 800С (max -200 ... + 1200С)).
TVR- Thermoelectric converter (thermocouple VR - tungsten-rhodium (A)), temperature measurement range - A1(max 0...+2500C), A2(max 0...+1800C), A3(max 0...+1600C)).
TNN- Thermoelectric converter (thermocouple – НН(N)), range -200…+1100С (max 1300С).
TMK- Thermoelectric converter (thermocouple - MK(T)), range -200…+400С.
TP(TXA, THC)-2088, TP-2388, TP-2187, TP-1085, TP-2488, TP-0195(0295, 0395), TP-0188(0198, 0199).
TP-008(type A,B,C,D,E,F,G,I,K,L,M,N,P) - thermoelectric converters ТХА-008, ТХК-008.
TPK, TPL-005- thermoelectric converters (thermocouples) with switching head 195, 205, 215, 265, 275, 285, 295, also 2TPK, 2TPL).
TPL-004- thermoelectric converters with thermocouple cable (TPL-054, 064, 074, 084, 094, 104, 114, 124, 134, 144, 154, 164, 174, 184, 194, 204, also 2TPL).
TPK, TPL-001- thermoelectric converters (thermocouples) surface in soft insulation(TPK, TPL-011, 021, 031, 041).
ТХА, ТХК-1…18- Thermoelectric temperature converters (thermocouples).
TTPP-53, TTPR-53
– high-temperature thermal converters (TTPP-53 up to 1300C, TTPR-53 from 600 to 1600C).
CCI-023, TPR-023
– high-temperature thermal converters in corundum straw (similar to CCI/R-1888).
TPP-178, TPR-178
- high-temperature thermal converters in a corundum case (similar to CCI/R-1788).
Mounting and mounting fittings for thermocouples
Bosses BP, BS, BP
(bosses are intended for installation at the place of operation of thermal converters and protective sleeves).
Protective sleeves GZ-015, GZ-016, GT-015, GZ-6.3/25/50
(protective sleeve is designed for installation of thermal converters on objects and provides their protection from the pressure of the working environment).
Movable fitting ShP
(movable fittings are designed for fixing and regulating the immersion depth of thermocouples in the zone of measured temperatures).
Wires and cables for thermocouples
Thermoelectrode cables
(compensating thermocouple cable KTK, KTL);
Mounting and thermoelectrode wires.
ADDITIONAL INFORMATION:
Thermoelectric converters (TP) - thermocouples (TKhK(L), TXA(K), TPP(S,R), TPR(B), TGK(J), TNN(N), TVR(A-1,2,3) , TMK(T)).
A thermocouple (thermoelectric converter) of the TXA, TKhK, TPP, TPR, etc. type consists of two conductors soldered at one end, made of metals with different thermoelectric properties. The soldered end, called the "working junction", is immersed in the measured medium, and the free ends ("cold junction") of the thermocouple are connected to the input of the secondary device (temperature meter-regulator). The principle of operation of thermocouples is based on the fact that at a temperature difference between the "working" ("hot") and "cold junctions" in the circuit of the thermoelectric converter (thermocouple), thermo-EMF begins to self-generate (generate), which has a certain temperature dependence for each type of thermocouples (TXA, TKhK, TPP, TPR, etc.) - NSH (nominal static characteristic- ХА, ХК, etc.), which is the output signal of the thermal converter and is perceived by the recording devices as an input signal.
Since thermo-EMF depends on the temperature difference between the two junctions of the thermocouple, in order to obtain correct readings, it is necessary to know the temperature of the "cold junction" in order to compensate for this difference in further calculations.
In modifications of inputs intended for operation with thermocouples, a circuit for automatic temperature compensation of the free ends of the thermocouple is provided. The cold junction temperature sensor is a semiconductor diode installed next to the connecting terminal block.
Thermocouples must be connected to the device using special compensating (thermoelectrode) wires made from the same materials as the thermocouple. It is allowed to use metal wires with thermoelectric characteristics similar to those of thermocouple electrode materials in the operating temperature range. When connecting the compensating wires to the thermocouple and the device, the polarity must be strictly observed.
In order to avoid the influence of noise on the measuring part of the device, it is recommended to shield the communication line between the device and the sensor. As a screen, it can be used, incl. grounded steel pipe.
If these conditions are violated, significant measurement errors may occur.
Recommended parameters of the sensor connection line (thermoelectric converter - thermocouple) with the secondary device (temperature meter - temperature controller):
Design lines - thermoelectric compensating cable.
The maximum line length is up to 20 meters.
The maximum line resistance is up to 100 ohms.
Secondary devices: temperature meters-regulators.
Thermal converters (resistance thermometers, thermocouples, sensors with a unified output signal (mA, V), being primary devices (sensors) for measuring temperature, emit a signal (НХ, mA, V) perceived by secondary measuring and control devices - measuring regulators and temperature recorders.
Simple temperature meters-regulators consist of the following functional blocks:
inputs - are used to connect various types of sensors to the device; input signal processing block - includes correction of sensor readings, digital filters, calculators of additional quantities (differences, ratios, etc.);
logical devices (LU) - form control signals for output devices;
output devices (VU) - are used to transmit recording or control signals to actuators.
How to choose and order (buy) a thermocouple.
1. Clearly determine for what purposes you need a thermal converter, in what conditions it will be operated; how and with what accuracy and frequency it is really necessary to carry out measurements.
2. Choose what type and modification of the temperature sensor really suits you, and what functionality is really needed (because all sorts of "excesses" may be unreasonably expensive).
3. Check if there is enough specifications and parameters for correct ordering (see order forms).
4. What optional equipment it is also necessary (installation and assembly fittings and elements (lugs, protective sleeves, etc.) auxiliary blocks, assemblies, devices, thermoelectrode compensating cable (for thermocouples), mounting wire, etc.).
5. What amount of equipment and additional costs (including packaging and delivery) are you willing to pay.
6. Are you competent to make decisions on making changes to the project, and whether you may be interested in the proposals of modern analogues that have a better PRICE-QUALITY ratio (according to our engineers).
7. What form of payment and delivery time are acceptable for you (please note that partial prepayment or urgent execution of an order ("out of turn") can sometimes lead to a slight increase in the cost of products).
8. In what way is it more convenient for you to receive the products (self-pickup, delivery, shipment through a transport campaign or otherwise).
Copyright © 2008 TeploKIP. Instrumentation and control - Thermocouples - thermoelectric converters (TP): TKhK (L), TXA (K), Chamber of Commerce and Industry (S, R), TPR (B), TGK (J), TNN (N), TVR (A), TMK (T ) and other thermal converters.
Thermocouple- two conductors of dissimilar materials connected at one end and forming part of a device that uses the thermoelectric effect to measure temperature.
There are following types of thermocouples:
R- CCI (Platinum - 13% rhodium/platinum)
S- CCI (Platinum - 10% rhodium / platinum)
B- TPR (Platinum - 30% Rhodium / Platinum - 6% Rhodium)
J- TFA ((Iron/copper - nickel (iron/constantan))
T- TMK (Copper/copper - nickel (copper/constantan))
E- THKn (Nickel - chromium / copper - nickel (chromel / constantan))
K- THA (Nickel - chrome / nickel - aluminum (chromel / alumel))
N- ТНН (Nickel - Chromium - Silicon / Nickel - Silicon (Nichrosil / Nisil))
A (A-1, A-2, A-3)- TVR (Tungsten - rhenium / tungsten - rhenium)
L- THC (Chromel/kopel)
M- TMK (Copper/copel)
Thermocouples do not require an auxiliary power supply and have a wide range of measured temperatures. However, they have a noticeable non-linearity in the conversion characteristic. Some problems are created by the need to take into account (or compensate) the influence of the temperature of the free ends of the thermocouple on the measurement result. In addition, low output voltage (and relatively low sensitivity) requires rather sensitive secondary converters (amplifiers) and/or output devices.
Instruments and converters based on thermocouples are widely used. Compact digital thermometers based on thermocouples are currently the main and most popular instrument in temperature measurements.
Thermocouple output - constant pressure- can be quite easily converted into a digital code or measured by simple means (for example, with a small-sized digital multimeter). Thermocouples can be connected for further conversion to various secondary measuring transducers (instruments), both analog and digital, for static and dynamic measurements.
The temperature range measured by thermocouples is quite wide: from -200 to +2000 °С. Thermocouple-based meters are characterized by high accuracy and sensitivity, good repeatability of the conversion characteristics. The typical output voltage range is 0...50 mV (depending on the materials used in the thermocouple), the typical temperature conversion coefficient (thermocouple sensitivity) is in the range 10...50 µV/°C.
The main characteristics of some types of TP
In the practice of typical temperature measurements, thermocouples of three types are most often used: J, K, T.
Thermocouples Type J have a minimum cost, high sensitivity, moderate accuracy, but cannot (should not) be used for a long time at extreme temperatures (above 1000 °C), since their calibration characteristic is violated.
Thermocouples type K are characterized by average cost, average accuracy, good sensitivity and a wide temperature range (up to 1300 °C). This type of thermocouple is the most common.
Thermocouples type T have an average cost, average sensitivity, high accuracy. They are suitable for working at low temperatures.
Thermocouples of types R and S.