Source: Thermal Thermal Control Learning and Sharing Thermal Power Thermal Automation
1 Thermoelectric phenomena and fundamental laws about thermocouples
A thermocouple thermometer consists of a thermocouple, an electrical measuring instrument, and connecting wires. It is widely used to measure temperatures in the range of -200~1300°C. In special cases, high temperatures of 2800°C or low temperatures of 4K can be measured. The thermocouple can convert the temperature signal into an electrical signal, which is convenient for the remote transmission and multi-point switching measurement of the signal, and has the advantages of simple structure, convenient production, high accuracy and small thermal inertia.
1.1 Thermocouple temperature measurement principle
In a closed circuit consisting of two different conductors or semiconductors A or B, if the two contacts are at different temperatures t0 and t, there will be an electromotive force in the circuit, which is called thermoelectric potential, and this phenomenon is called the thermoelectric effect. The thermoelectric potential is a function of the temperature t0 and t, and the constant junction temperature t0 is a single-valued function of the temperature t.
The thermoelectric potential is composed of the temperature difference potential and the contact potential.
Thermoelectric potential: refers to the thermal electromotive force generated by the difference in temperature at both ends of a conductor. When the temperature of the two ends of the same conductor is different, the movement speed of electrons at the high temperature end (measurement end, working end, and hot end) is greater than that of the electrons at the low temperature end (reference end, free end, cold end), and the electrons at the high temperature end are positively charged per unit time, and the electrons at the low temperature end are negatively charged, and an electrostatic field is formed between the high and low temperature ends pointing from the high temperature end to the low temperature end. The electric field prevents the movement of electrons from the high temperature end to the low temperature end, and increases the movement speed of the electrons from the low temperature end to the high temperature end, and when the motion reaches dynamic equilibrium, the corresponding potential difference between the two ends of the conductor is generated, which is called the temperature difference potential. The direction of the temperature difference potential: from the low temperature end to the high temperature end.
Magnitude of the thermoelectric potential:
where k is the Boltzmann constant, e is the electron charge is the electron density in the conductor as a function of temperature, and t and to are the temperatures at both ends of the conductor. It can be seen that the magnitude of the thermoelectric potential is related to the properties of the conductor and the temperature at both ends of the conductor, but not to the length of the conductor, the size of the cross-section, and the temperature distribution along the length of the conductor.
Contact potential: is generated at the point of contact between two different materials, A and B. A and B materials have different electron densities, if the electron density nA of conductor A is greater than the electron density nB of conductor B, the number of electrons diffused from A to B is more than that diffused from B to A, A has a positive charge due to the loss of electrons, and B has a negative charge due to the gain of electrons, so an electrostatic field from A to B is formed on the contact surface of A and B. This electrostatic field will hinder the diffusion motion of electrons, induce the drift movement of the electrons, and when the diffusion and drift reach dynamic equilibrium, a potential difference will be formed on the contact surface of A and B, that is, the contact potential. The direction of the contact potential: from a conductor with low electron density to a conductor with high electron density;
The magnitude of the contact potential: or, where: k is the Boltzmann constant, and e is the electron charge. The higher the temperature, the greater the contact potential, and the greater the electron density ratio of the two conductors, the greater the contact potential. It can be seen that the contact potential is related to the properties of the two conductors, the temperature of the contact point, but not the length of the conductor, the size of the cross-section, and the temperature distribution along the length of the conductor.
The total potential of the thermocouple loop is:
That is, the thermoelectric potential is a function of the temperature at the high end and the temperature at the low end, and if the temperature at the low end is constant, the thermoelectric potential is a single function of the temperature at the high end. By measuring the magnitude of the thermoelectric potential, the value of the temperature to be measured (at the high end) can be obtained.
1.2 The basic laws of thermocouple loops
1) The law of homogeneous conductors
In a closed loop consisting of a homogeneous conductor or semiconductor, it is impossible to generate a thermoelectric potential in the loop, regardless of the length of the conductor, the cross-sectional area, and the temperature distribution along the length.
Proof: Known:
Because it is a homogeneous conductor with the same electron density,
And because , the total electric potential of the loop is equal to 0.
Conclusions: (1) Thermocouples must be composed of two materials with different properties, and (2) When there is a temperature difference in a closed loop composed of one material, if there is a thermoelectric potential in the loop, the material is inhomogeneous. ——It is used for the uniformity detection of electrode materials.
2) The law of intermediate conductors
Insertion of the third and fourth 、...... in the thermocouple loop Homogeneous conductors, as long as the temperature of the two access points of each conductor is the same, the access of these conductors does not affect the thermoelectric potential in the loop.
Proof: Take the example of inserting a third homogeneous conductor C into a thermocouple loop. Ensure that the temperature of both access points is t0, as shown in the figure: the loop potential is:
Thereinto:
Old:
That is, the addition of the C conductor does not affect the thermoelectric potential in the circuit.
Conclusions: (1) Connecting wires and measuring instruments can be connected to the thermocouple circuit, (2) It can facilitate the selection of thermocouple electrodes, and (3) The surface temperature and liquid medium temperature can be measured in an open circuit.
3) The law of intermediate temperature
The thermocouple with contact temperature t1 and t3 is equal to the algebraic sum of the thermoelectric potential of two thermocouples of the same nature with contact temperatures t1, t2 and t2 and t3 respectively, that is, the thermoelectric potential of the thermocouple is only related to the contact temperature at the high temperature end and the low temperature end, but not to the intermediate temperature.
Conclusion: (1) The cold junction temperature of the thermocouple can be calculated and corrected, and (2) the compensation wire can be connected to the thermocouple circuit.
2. Standardized and non-standardized thermocouples
2.1 Thermoelectrode materials and their properties
The thermoelectrode material should meet the following requirements: 1) the thermoelectric potential and thermoelectric potential rate (sensitivity) are large, and the relationship between the thermoelectric potential and temperature is linear, 2) the conductivity is high, and the temperature coefficient of resistance is small, 3) the physical and chemical properties are stable (when used for a long time, the thermoelectric properties can be guaranteed), 4) the replicability is good (can be mass-produced), easy to interchange, 5) the machinability is good, easy to install, and 6) the price is cheap.
2.2 Standardized thermocouples
Standardized thermocouple: It is a thermocouple with mature manufacturing process, wide application, mass production, excellent and stable performance, and has been included in professional or national industrial standardization documents. The standardization document specifies a uniform thermoelectrode material and its chemical composition, thermoelectric properties and allowable deviations for the same model of standardized thermocouples, i.e., standardized thermocouples have a uniform indexing table. The indexing table reflects the relationship between the potential temperature in the form of a table, and it should be noted that the potential temperature relationship is obtained when the temperature of the cold junction is 0, and special attention should be paid to its use. Standardized thermocouples of the same model are interchangeable and easy to use.
At present, there are 8 kinds of standardized thermocouples in the world, and the model of these thermocouples (sometimes called graduation number), electrode material, measurable temperature range and use characteristics are shown in the following table.
Note: The former of the electrode material is the positive electrode, the latter is the negative electrode, and the following number is the percentage content of the material. The temperature measurement range is the limit value of the thermocouple in a good use environment, and the upper limit of the allowable temperature measurement is 60%~80% of the limit value when actually used, especially for long-term use.
| Graduation number | material | Temperature Range (°C) | Features of use: |
| S | Platinum rhodium 10-platinum | -50~1768 | The metal is easy to purify, the replication accuracy and temperature measurement accuracy are high, the physical and chemical properties are stable, and the oxidized or neutral medium below 1300 °C can be used for a long time. It is expensive, the thermoelectric potential is small, and the thermoelectric characteristics are nonlinear, so it cannot be used in reducing atmospheres and atmospheres containing metal or non-metallic vapors. The most accurate thermocouple above 300°C. |
| R | Platinum rhodium-13-platinum | -50~1768 | The basic performance and use conditions are the same as those of the S index thermocouple, but the thermoelectric potential is slightly larger, which is more used in European and American countries. |
| B | PtRhium-30-PtRhium-6 | 0~1820 | It can be used for a long time in an oxidizing and neutral environment below 1600°C, and cannot be used in a reducing atmosphere or an atmosphere containing metal or non-metal vapors. The thermoelectric potential and thermoelectric potential rate are smaller than those of the S-graduation thermocouple, and the cold junction temperature is not compensated when the cold junction temperature is lower than 50°C. |
| Towards | Nickel-chromium-nickel-silicon | -270~1372 | Metal thermocouples, 3.2mm diameter thermocouples can be used for a long time at high temperatures of 1200°C. Reliable operation in reducing, neutral and oxidizing atmospheres below 500°C. Above 500°C, it can only work in a reductive, neutral atmosphere. The thermoelectric potential rate is 4~5 times larger than that of the S-scale thermocouple, and the temperature-electric potential relationship is close to linear. |
| N | Nichrome Silicon - NiSilicon | -270~1300 | |
| And | Nickel-chromium-copper-nickel alloy (constantan) | -270~1000 | Metal thermocouples, 3.2mm diameter thermocouples can be used at a high temperature of 750°C for a long time, and are also suitable for temperature measurement in low temperature (below 0°C) and humid environments. It is a standardized thermocouple with the highest thermopotential rate. |
| J | Iron-copper-nickel alloy (constantan) | -210~1200 | It is suitable for oxidizing and reducing atmosphere, and can also be used in vacuum and neutral atmosphere, and cannot be used in sulfur-containing atmosphere above 538 °C. Good stability, high sensitivity, low price. Cathode iron is susceptible to corrosion. |
| T | Copper-copper-nickel alloy (constantan) | -270~400 | It is suitable for use in oxidation, reduction, vacuum, neutral atmosphere, with corrosion resistance in humid atmosphere, especially suitable for temperature measurement below 0°C. Main features: good stability, high low temperature sensitivity, low price, 100~200°C temperature measurement accuracy is the highest. |
2.3 Non-standardized thermocouples
Non-standardized thermocouples are inferior to standardized thermocouples in terms of scope and quantity. However, in some special occasions, such as high temperatures, low temperatures, ultra-low temperatures, high vacuums, and subjects with nuclear radiation, these thermocouples have some particularly good properties. There is no uniform indexing table for non-standardized thermocouples. Non-standardized thermocouples include tungsten-rhenium thermocouples (tungsten has a melting point of 3387 °C and rhenium has a melting point of 3180 °C, which is used to measure temperatures up to 2760 °C), iridium-rhodium thermocouples, which can measure high temperatures of 2000 °C in weakly reducing media, which are suitable for aerospace technology, double platinum-molybdenum thermocouples have a low neutron capture area and are specially used for temperature measurement in nuclear reactors, and non-metallic thermocouples such as carbide, boride, and nitride enable high temperature measurement in oxidizing atmospheres without precious metals. Due to the poor reproducibility and poor mechanical strength of non-metallic thermocouples, they are greatly limited in use.
3. Structure of thermocouples
3.1 Thermocouples for general industrial use
Thermocouples for common industrial use are usually composed of a thermode, an insulating tube, a protective sleeve, and a junction box, as shown in the figure.
(1) Thermal electrode The diameter of the thermoelectrode is determined by the price of the material, mechanical strength, conductivity, the use of the thermocouple and the temperature measurement range. The diameter of the precious metal electrode is 0.3~0.65mm, and the diameter of the ordinary metal electrode is 0.3~3.2mm. The length of the thermoelectrode is available in a variety of specifications, mainly determined by the installation conditions and insertion depth, generally 300~2000mm. The hot ends of the thermocouples are connected by welding, and the joint shape is spot welded, butt welded and twisted spot welded. The diameter of the solder joint should not exceed twice the diameter of the thermode.
(2) Insulating tube In order to prevent the electric potential between the thermal electrodes from being short-circuited, an insulating tube is set on the thermal electrode. There are various forms of insulating pipes, such as single hole, double hole, and four hole. The selection of insulating pipe materials is carried out according to the allowable working temperature of the material, rubber, plastic, polyethylene and other materials can be used at low temperature, ordinary ceramics (below 1000 °C), high-purity alumina (below 1300 °C), corundum (below 1600 °C), etc.
Commonly used insulator materials and their service temperature range
| The name of the material | Operating temperature range (°C) | The name of the material | Operating temperature range (°C) |
| Eraser, plastic | 60～80 | Quartz tubes | 0～1300 |
| Silk, dry paint | 0～130 | Porcelain tubes | 1400 |
| Fluoroplastics | 0～250 | Recrystallized alumina tubes | 1500 |
| Glass filament, glass tube | 500 or less | Pure alumina tubes | 1600～1700 |
(3) Protective sleeve In order to prevent the thermal electrode from mechanical damage and chemical corrosion, the thermal electrode and insulating tube are usually packed into an impermeable protective sleeve. The material and form of the casing are determined by the characteristics of the medium to be measured, the installation method and the time constant. Common materials are brass, #20钢, stainless steel, high-temperature heat-resistant steel, pure alumina, corundum, cermet, etc., and beryllium oxide and thorium oxide can also be used when measuring higher temperatures, up to 2200°C. The installation can be in two forms: threaded connection and flange connection.
Commonly used protective tube materials and their applicable temperature ranges
| The name of the material | Long-term use (°C) | Short-term use (°C) | The name of the material | Long-term use (°C) | Short-term use (°C) |
| Copper or copper alloys | 400 | High-grade refractory porcelain tubes | 1400 | 1600 | |
| 20# carbon steel pipe | 600 | Recrystallized alumina tubes | 1500 | 1700 | |
| 1Cr18Ni9Ti stainless steel | 900~1000 | 1250 | High-purity alumina tubes | 1600 | 1800 |
| 28Cr Iron (High Chromium Cast Iron) | 1100 | Zirconium boride | 1800 | 2100 | |
| Quartz tubes | 1300 | 1600 |
The temperature measurement time constant of ordinary industrial thermocouples varies with the material and diameter of the protective sleeve (generally 10~240s), when the metal protective sleeve is used, the outer diameter is 12mm, the time constant is 45s, when the outer diameter is 16mm, the time constant is 90s, and the time constant of the metal thermocouple resistant to high voltage is 2.5min.
(4) Junction box There is a binding post in the junction box as a connecting device for the thermoelectrode and the compensation wire or wire. According to different uses, there are ordinary type, splash-proof type, waterproof type, flameproof type and socket type.
3.2 Armoured thermocouples
Armoured thermocouples are solid combinations of thermoelectrodes, insulating materials and metal sleeves that are stretched and processed. It can be made very thin, very long, and can be bent as needed in use. Casing materials include copper, stainless steel and nickel-based superalloys. The bushing and the thermode are filled with insulating powder, and the commonly used insulating materials are magnesium oxide, alumina, etc. The thermodes in the sleeve are single-core, double-core, and four-core, which are insulated from each other. At present, the armored thermocouple produced has a wall thickness of 0.12~0.6mm, a thermoelectrode diameter of 0.025~1.3mm, an outer diameter of 1~6mm, a length of 1~20m, the thinnest outer diameter of 0.2mm, and the longest length of more than 100m. The measuring end of the armored thermocouple has open end shape (0.01~0.1s), shell shape (0.01~2.5s), insulated shape (0.2~8.0s), flat variable section shape and circular variable section shape.
The main characteristics of armored thermocouples are small heat capacity at the measuring end, fast dynamic response (time constant less than 10s), high mechanical strength, good flexibility, high pressure, strong vibration and impact, and can be installed on devices with complex structures.
3.3 Fast-reacting thin-film thermocouples
Thin-film thermocouples are evaporated into an insulating substrate by vacuum evaporation, and the two are firmly combined to form a thin-film measuring end, on which a layer of silica film is evaporated as an insulating and protective layer.
The characteristics of thin-film thermocouples are that the measuring end is a very thin film (can be as thin as 0.01~0.1μm), and the size is also very small, so the heat capacity of the measuring end is small, and the time constant is very small (up to a few milliseconds), which is used to measure the temperature that changes quickly. Due to the heat resistance limitation of the adhesive, it can only be used in the range of -200~300°C. If the electrode material is evaporated directly onto the surface of the object to be measured, the time constant can reach the microsecond level.
Thermal electrodes are: nickel-chromium-nickel-silicon, copper-constantan, iron-nickel, etc. The picture on the right is a schematic diagram of an iron-nickel thin film thermocouple, its size is 60mm, 6mm, 0.2mm, the thickness of the metal film is between 3~6um, the time constant is less than 0.01s, and the temperature measurement range is 0~300°C.
4. Thermocouple cold junction temperature compensation
According to the thermocouple temperature measurement principle, the thermoelectric potential is a function of the temperature of the hot end and the temperature of the cold end, and the thermoelectric potential is a function of the temperature of the hot end under the condition that the temperature of the cold end is constant. In practical application, the cold end of the thermocouple is placed in the atmosphere very close to the hot end, which is greatly affected by the fluctuation of high temperature equipment and ambient temperature, so the cold end temperature is not constant. In order to eliminate the influence of cold junction temperature fluctuations on temperature measurement, cold junction temperature compensation is necessary. The commonly used cold junction temperature compensation methods include: calculation correction method, cold junction constant temperature method, display instrument mechanical zero point adjustment method, compensation bridge (cold junction temperature compensator) method, compensation wire method, auxiliary thermocouple method, PN junction compensation method, etc.
4.1 Calculation Correction Method
The indexing relationship of the thermocouple is obtained under the condition that the temperature of the cold end is 0 °C, if the temperature of the cold junction of the thermocouple is t0, not 0 °C, then the thermoelectric potential of the thermocouple cannot be measured to check the indexing table, the thermoelectric potential correction must be carried out, and then, the indexing table obtains the measured hot end temperature, and the correction potential is.
Namely:
Total potential = measured thermocouple output potential + corrected potential
Applicable occasions: laboratory temperature measurement, temperature measurement with direct reading instruments used in the field. The prerequisite is that the cold junction temperature is measurable and almost constant. Disadvantages: It is not convenient for continuous temperature measurement.
4.2 Cold end constant temperature method
The temperature of the cold junction of the thermocouple is constant, which makes it easy to compensate and correct. Generally, the freezing point tank (0 °C) or the industrial incubator (50 °C) is selected for constant temperature.
(1) Freezing point groove method
Put the cold end of the thermocouple in the ice-water mixture, and the output potential of the thermocouple is the total potential of the cold junction temperature at 0°C, which can be directly checked or sent to the display instrument to display the temperature of the hot end.
(2) Incubator method
The incubator method places the cold end of the thermocouple in an automatic incubator. Automatic incubators often use steam or electricity as a heat source. Here is a brief explanation of the industrial incubator as an example. The principle of the industrial incubator is shown in the figure.
It should be noted that the electric potential e(t,50) sent by the thermocouple of this method cannot be used for the final temperature display, and the mechanical zero position of the instrument should usually be adjusted for correction.
4.3 Display instrument mechanical zero point adjustment method
When the electric potential fed into the display meter is e(t,t0) and t0 is known and constant, the mechanical zero point of the meter is adjusted to the scale corresponding to the temperature of t0 with the thermocouple disconnected. This is equivalent to applying the electric potential e(t0,0) inside the display instrument in advance, and after connecting the thermocouple, the total electric potential used for temperature display is e(t,0), because the scale of all display instruments is scaled according to the indexing table, so the meter correctly displays the measured hot end temperature value.
4.4 Compensation bridge (cold junction temperature compensator) method
If an additional electric potential that changes with temperature can be obtained, and the electric potential is connected in series in the thermocouple circuit to compensate for the change of thermocouple thermoelectric potential due to the temperature change of the cold junction, the electric potential in the display instrument can be guaranteed to be affected by the temperature change of the cold junction, and the purpose of automatic compensation can be achieved. Commonly used cold-junction temperature compensators operate on the principle of unbalanced bridges shown in the diagram. As can be seen from the figure, the thermoelectric potential output of the thermocouple (and the compensation wire) is added to the unbalanced voltage of the unbalanced bridge and sent to the temperature display instrument.
The structure and working principle of the cold-junction temperature compensator are briefly described as follows: R1, R2 and R3 are 1Ω constant value resistors wound by three manganese copper wires, Rs is the current limiting resistance, Rcu is 1Ω at 20°C, and the power supply voltage of the bridge is 4V. When the temperature of the cold junction of the thermocouple (compensation wire) is 20°C, the compensation bridge is in the initial equilibrium state, the unbalanced voltage Uab=0, and the thermocouple sends the electric potential e(t,20) to the display instrument. When the cold junction temperature of the thermocouple increases and is higher than 20°C, the thermoelectric potential will decrease due to the increase of the cold junction temperature, and the resistance of Rcu will increase, and the output voltage of the unbalanced bridge will increase, i.e., Uab>0; when the cold junction temperature of the thermocouple decreases and is lower than 20°C, the thermoelectric potential will increase due to the decrease of the cold junction temperature, and the resistance of Rcu will decrease, and the output voltage of the unbalanced bridge will decrease, i.e., Uab<0, it can be seen that the change direction of the unbalanced voltage of the compensation bridge is exactly the opposite direction of the change direction of the thermoelectric potential, which can play a compensating role. If the increase in the unbalanced voltage is exactly equal to the decrease in the thermoelectric potential, the compensation is fully realized, and the potential of the display meter is not affected by the temperature change at the cold junction. Since the thermoelectric characteristics of the thermocouple are not exactly the same as the temperature-output characteristics of the bridge, the cold junction temperature compensator does not fully compensate at various points in the compensation range. Generally speaking, the full compensation point is the initial equilibrium temperature and the upper limit temperature of the compensation range.
In addition, the thermoelectric characteristics of thermocouples with different indexing numbers are different, and the required compensation voltage is different, that is, the compensator signal is different, and the difference between the compensators is usually only the resistance value of the current limiting resistor.
It should be noted that if the initial equilibrium temperature of the compensation bridge is not 0°C, the electric potential sent to the display instrument needs to be corrected, and the method of mechanical zero adjustment of the display instrument is usually adopted.
4.5 Compensation wire method
According to the intermediate temperature law, when the contact temperature is lower than 100°C, a pair of wires with the same thermal-electrical characteristics can be used instead of a thermocouple for measurement, i.e., a compensation wire is used. Although the compensation wire cannot change the temperature of the cold junction, it can move the position of the cold junction of the thermocouple, that is, the cold junction can be moved from a place with severe temperature fluctuations to a relatively stable location, which is convenient for the correct indication of temperature in conjunction with other temperature compensation methods. For example, the cold end of the thermocouple that measures the temperature of the furnace is usually not far from the outside of the furnace, and the temperature at this place is affected by the change of the temperature of the high-temperature equipment and the environment, and the fluctuation is more violent, and the temperature at the place is generally higher than the compensation temperature of the cold end temperature compensator, so the above-mentioned temperature compensation method cannot be adopted. When the temperature is stable, the preset potential method such as mechanical zeroing of the display instrument can be used, and when the temperature is not very stable, the cold junction temperature compensator can be used to compensate because the temperature is in the compensation range of the cold junction temperature compensator.
| Model | Thermocouples are used | Temperature at bridge equilibrium (°C) | Compensation Range (°C) | Power Supply (V) | Internal Resistance (Ω) | Compensate for errors |
| WBC-01 | Platinum rhodium 10-platinum | 20 | 0～50 | ～220 | 1 | ±0.045mV |
| WBC-02 | Nickel-chromium-nickel-silicon | ±0.16mV | ||||
| WBC-03 | Nickel-chromium-copper | ±0.18mV | ||||
| WBC-57-S | Platinum rhodium 10-platinum | 20 | 0～40 | 24 | 1 | ±(0.015±0.0015t) |
| WBC-57-K | Nickel-chromium-nickel-silicon | ±(0.04±0.004t) | ||||
| WBC-57-EA | Nickel-chromium-copper | ±(0.005±0.0065t) |
Commonly used compensation leads
| Compensation wire type | Thermocouple graduation number is used | Compensating wire alloy wire | Insulation coloring | Tolerance at 100°C (°C) | Tolerance at 200°C (°C) | ||||
| cathode | negative electrode | cathode | negative electrode | Normal | Precision | Normal | Precision | ||
| SC | S | SPC (Copper) | SNC (Copper-Nickel) | red | green | 5 | 3 | 5 | — |
| KC | Towards | KPC (Copper) | KNC（镍硅） | red | blue | 2.5 | 1.5 | — | — |
| KX | Towards | KPX（镍铬） | KNX（铜镍） | red | black | 2.5 | 1.5 | 2.5 | 1.5 |
| EX | And | EPX (Nickel Chromium) | ENX (Copper-Nickel) | red | palm tree | 2.5 | 1.5 | 2.5 | 1.5 |
| JX | J | JPX (iron) | JNX (Copper-Nickel) | red | purple | 2.5 | 1.5 | 2.5 | 1.5 |
| TX | T | TPX（铜） | TNX（铜镍） | red | white | 2.5 | 1.5 | 2.5 | 1.5 |
5. Verification of thermocouples
5.1 Verification of thermocouples
Thermocouples should be calibrated or verified in advance before use, and standard thermocouples must be individually indexed. After a period of use, the thermoelectric characteristics of the thermocouple will gradually change due to the high temperature volatilization, oxidation, external corrosion and pollution, grain structure changes and other reasons of the thermocouple, and the measurement error will occur during use, and sometimes the measurement error will exceed the allowable range. In order to ensure the measurement accuracy of thermocouples, they must be checked periodically. There are two verification methods for thermocouples, the comparative method and the fixed-point method. The comparative method is mostly used in industry, so only the comparative method is introduced here.
The method of measuring the temperature of the same object at the same time with a calibrated thermocouple and a standard thermocouple, and then comparing the two indications to determine the basic error and other quality indicators of the thermocouple being tested, is called the comparative method. The basic requirement for the comparative method of thermocouple verification is to create a uniform temperature field so that the measurement end of the standard thermocouple and the thermocouple under test feel the same temperature. A uniform temperature field must be long enough along the thermal electrode so that the thermal conductivity error along the thermal electrode is negligible. Industrial and laboratory thermocouples use tubular furnaces as the basic equipment for verification. In order to ensure that there is a sufficiently long isothermal zone in the tubular furnace. The ratio of the length to the diameter of the tubular furnace chamber is required to be at least 20:1. In order to keep the hot end of the tested thermocouple and the standard thermocouple in the same temperature environment, a nickel block can be placed in the constant temperature area of the tubular furnace, and a hole can be drilled in the nickel block so that the hot end of each thermocouple can be inserted into it for comparison and measurement. A system for verifying thermocouples in a tubular furnace by a comparative method, as shown in the figure, the main devices are a tubular furnace, a freezing point tank, a transfer switch, a manual DC potentiometer and a standard thermocouple.
Take the same time interval during the inspection, according to the standard, the inspected l, the inspected 2、......、 the inspected n, the inspected n、......、 the inspected 2, the inspected 1, the standard after a cycle each has two readings, generally two cycles of measurement, get four readings. Finally, data processing and error analysis were carried out to obtain their arithmetic mean, and the measurement results of the standard and the test were compared. Thermocouples are considered to be qualified if the allowable error of the thermocouple under inspection at each verification point is within the specified range.
Allowable error for a variety of commonly used thermocouples
| Thermocouple materials | Calibration Temperature (°C) | Thermocouples allow deviations | |||
| Temperature (°C) | Deviation (°C) | Temperature (°C) | Deviation (°C) | ||
| Platinum rhodium-platinum | 600; 800; 1000; 1200 | 0～600 | ±2.4 | ＞600 | 0.4% of the measured thermoelectric potential ± |
| Nickel-chromium-nickel-silicon | 400; 600; 800; 1000 | 0～400 | ±4 | ＞400 | accounts for the measured thermoelectric potential ±0.75% |
| Nickel-chromium-copper | 200; 400; 600 | 0～300 | ±4 | ＞300 | ±1% of the measured thermoelectric potential |
6. Use and installation of thermocouples
Thermocouples are indexed and verified without a protective sleeve and are inserted to a sufficient depth in the furnace chamber with a uniform temperature field. In order to avoid large errors, various measures should be taken to ensure the accuracy of temperature measurement in the installation and use.
6.1 Precautions for the use of thermocouples
1) In order to reduce the measurement error, the thermocouple should be in full contact with the measured object, so that the two are at the same temperature.
2) The protective tube should have sufficient mechanical strength and can withstand the corrosion of the measured medium, the coarser the outer diameter of the protective tube, the better the heat resistance and corrosion resistance, but the greater the thermal inertness.
3) When dust and other substances are attached to the surface of the protective tube, the error will be caused by the increase of thermal resistance, so that the indicated temperature is lower than the real temperature.
4) If it works for a long time at the highest operating temperature, it will cause errors due to changes in the material of the thermocouple.
5) Error caused by the decrease in insulation resistance of the measurement line. Try to increase the insulation resistance, or ground the thermocouple's housing.
6) Compensation and correction of cold junction temperature. The cold junction of the thermocouple should ideally be kept at 0°C, which is difficult to achieve with instruments used in field conditions and must be accurately corrected by compensation methods.
7) The influence of electromagnetic induction. The signal transmission line of the thermocouple should be avoided as much as possible in the strong current area (such as high-power motors, transformers, etc.) when wiring, and it should not be laid in close parallel to the power line. If you really can't avoid it, you should also take shielding measures.
6.2 Installation principles of thermocouples
When installing thermocouples, the following principles should be followed;
1) The thermocouple should form a countercurrent with the measured medium, that is, the thermocouple should be inserted in the direction of the measured medium during installation. At the very least, it must be orthogonal to the side medium. As shown in Fig.
2) The working end of the thermocouple should be in the place where the flow velocity is the largest in the pipeline, and the end of the thermocouple protection pipe should be about 5-10mm higher than the center line of the pipeline.
3) The thermocouple should have sufficient insertion depth. It has been proved in practice that under the condition of the maximum allowable insertion depth, with the increase of the insertion depth, the temperature measurement error decreases, and the temperature measuring element can be installed obliquely or along the axis of the pipeline.
4) The diameter of the pipe is too small, such as the diameter is less than 80mm. Often, the measurement error is caused by the insufficient insertion depth, and the expansion tube should be connected when installing the thermocouple, and the appropriate part can be selected to reduce or eliminate this error.
5) Temperature measurement with a large amount of dust gas. Due to the large amount of dust contained in the gas, the wear of the protective tube is severe, so the protective tube with the end cut can be used, or an armored thermocouple can be used. The use of armored thermocouples not only has a fast response, but also has a long life.
6) The thermocouple is installed in the negative pressure pipeline, and its tightness must be ensured to prevent the cold air from being sucked in from the outside, so that the measured value is low.
7) The lid of the thermocouple press the wire box should be facing upwards to avoid the immersion of rain or other liquids, which will affect the accuracy of the measurement.