Thermocouple thermometer
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. 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.
The magnitude of the temperature difference potential: ,, where k is the Boltzmann constant, e is the electron charge is the density of electrons in the conductor, which is 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.
2. The fundamental 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 they are homogeneous conductors, the electron density is the same
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.
Standardized vs. non-standardized thermocouples
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. 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 | Base 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. |
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.
Construction of thermocouples
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.
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. 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.
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.
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.
i.e.: 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.
2. Cold end thermostat
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.
3. Display instrument mechanical zero 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. 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.
5. Compensating 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 |
Verification of thermocouples
l. 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 accuracy of thermocouple measurements, regular inspections must be carried out
Decide. 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 |
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 thermometers should be prepared to ensure the accuracy of temperature measurement in the installation and use.
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.
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.
RTD thermometer
We know that in the temperature range of 600~1300 °C, the thermocouple is ideal, but for the measurement of medium and low temperatures, the thermocouple has certain limitations, this is because the thermocouple in the low temperature area output thermoelectric potential is very small, the quality of the instrument is required to be high, such as platinum rhodium-platinum thermocouple at 10O°C temperature The thermoelectric potential is only 0.64mV, such a small thermoelectric potential has high requirements for the amplifier and anti-interference of the electronic potentiometer, and the maintenance of the instrument is also difficultOn the other hand, if the cold junction temperature can not be fully compensated, its influence is greater, and at low temperatures, the linearity of thermoelectric characteristics is poor, and certain measures must be taken when adjusting the temperature, which are the shortcomings of thermocouples in temperature measurement. For this reason, another type of measuring element is often used in the industry to measure low temperatures, i.e. thermal resistance. The measurement range of RTD thermometer is -20O°C~+850°C.
The biggest advantage of RTD thermometer is that it has high measurement accuracy, no cold junction compensation problem, and is especially suitable for low temperature measurement, so it is widely used in industry. The platinum resistance thermometer can measure -200°C, and the indium resistance temperature can measure the low temperature of 3.4 K.
The disadvantages are that it cannot measure too high temperatures, that it requires an external power supply, so its use is limited, and that the resistance of the connected wires is easily affected by the ambient temperature, which can lead to measurement errors.
The principle of temperature measurement of RTD
From physics, we know that the resistance value of a conductor (or semiconductor) changes with temperature, and generally speaking, there is a relationship between them as follows, namely
The temperature coefficient of resistance is usually used to describe the characteristic that the resistance value changes with temperature change α, which is defined as the relative change in resistance at a temperature change of 1°C in a certain temperature interval, in units of 1/°C. By definition, α can be expressed by:
The resistance of a metal conductor generally increases with increasing temperature, and α is a positive value, which is called a positive temperature coefficient of resistance. The semiconductor material used for temperature measurement has a negative α, i.e., it has a negative temperature coefficient of resistance. The α value of various materials is not the same, and for pure metals, it is generally about 0.38%~0.68%. Its size is related to the purity of the conductor itself, the larger the α, the higher the purity of the conductor material. The purity of the material is usually expressed in terms of resistance ratio, representing the resistance value at 1OO°C and the resistance value at 0°C. However, the resistance value of the semiconductor decreases with the increase of temperature, at about 2O °C, and the resistance value changes by -2~-6% for every 1 °C change in temperature. If the change in the resistance value can be measured, the change in temperature can be determined accordingly, and the purpose of temperature measurement can be achieved. Resistance thermometers measure temperature by taking advantage of the fact that the resistance value of a conductor (or semiconductor) changes with temperature. That is, the change in conductor resistance caused by temperature change is converted into a voltage signal through the measurement bridge, and then sent to the display instrument to indicate or record the measured temperature.
From the above, it can be seen that the measuring principle of RTD thermometer and thermocouple thermometer is different. The thermocouple thermometer measures the temperature change by converting the thermocouple of the thermometer element into the change of the thermoelectric potential, while the RTD thermometer measures the temperature by converting the temperature change into the change of the resistance value of the thermoresistance of the thermometer.
RTD thermometer is suitable for measuring the surface temperature of liquids, gases, vapors and solids in the low temperature range of -200~+850°C. In addition, its output signal is large and the measurement is accurate, so in 1990 the international temperature scale (ITS-90) stipulates: 13.8033K~961.78 °C temperature range with platinum resistance thermometer as the reference. Materials and requirements for RTD
The mechanism of RTD temperature measurement is to use the resistance value of a conductor or semiconductor to change with the change of temperature, but not all conductors or semiconductor materials can be used as measurement elements, and it is necessary to consider and select from other aspects of performance, and the requirements for RTD materials are:
1. Stable physical and chemical properties, high measurement accuracy, corrosion resistance and long service life.
2. The temperature coefficient of resistance should be large, that is, the sensitivity should be high.
3. The resistivity should be high, so that the volume of the RTD is smaller, and the time constant of temperature measurement is reduced.
4. The heat capacity should be small, so that the resistive body is less thermally inert and the response is more sensitive.
5. Good linearity, that is, the relationship between resistance and temperature is linear or smooth.
6. Easy to process, cheap price, reduce manufacturing cost.
7. Good reproducibility, easy to batch production and parts exchange. Commonly used RTDs
The most commonly used metal RTDs are platinum, copper, and nickel.
1.铂热电阻（-200~850℃）
Platinum RTD is characterized by high measurement accuracy, good stability and reliable performance, but in reducing media, especially at high temperatures, it is easy to be polluted by the vapor reduced from the oxide and become brittle, and change the relationship between resistance and temperature. In order to overcome the above shortcomings, the RTD core should be installed in a protective sleeve when used. The relationship between resistance value and temperature is as follows:
In the range of -200~0°C, the relationship between platinum resistance and temperature can be expressed by the following formula,
In the range of 0~850°C, the relationship between platinum resistance and temperature can be expressed by the following formula,
Where: resistance value of RTD at t°C;A=3.9083×10-3/°C, B=-5.775×10-7/°C2,C=-4.183×10-12/°C4.
The purity of platinum is currently available at the technical level, and the purity of the corresponding platinum is 99.9995%. The purity of industrial platinum resistance is that of standard platinum resistance.
2.铜热电阻（-50~150℃）
Copper RTD is commonly used in industry to measure the temperature in the range of -5O°C~+15O°C, copper is easy to purify, the price is much cheaper than platinum, the temperature coefficient of resistance is large and the relationship is linear, expressed by a formula, where. However, the resistivity (specific resistance) of copper is about 5/6 smaller than that of platinum, so when making a thermal resistance with a certain resistance value, compared with platinum, if the length of the resistance wire is the same, the copper resistance wire is very thin, and the mechanical strength is reduced, and if the wire diameter is the same, the length will increase many times, and the volume will increase. In addition, copper is easy to oxidize above 1OO°C, and the corrosion resistance is poor, so the operating temperature does not exceed 15O°C.
3.镍热电阻（-60~180℃）
The temperature coefficient of nickel RTD is large, and the sensitivity is higher than that of platinum and copper, and it is commonly used to measure the temperature in the range of -6O°C~+18O°C. The resistance ratio of a nickel RTD. The relationship between nickel resistance and temperature can be expressed by:
where A=0.5485/°C, B=0.665×10-3/°C2, C=2.805×10-9/°C4.
Due to the complexity of the manufacturing process of nickel RTD, it is difficult to obtain the same nickel wire α, so its measurement accuracy is lower than that of platinum RTD, and the standardized RTD currently specified by Continental has the graduation numbers of Ni100, Ni300, and Ni500.
4. Semiconductor thermistors
Semiconductor spot thermometer is a thermistor made of metal oxides such as manganese, nickel, copper and iron as a temperature measuring element, and its shapes are bead-shaped, round, gasket-shaped and sheet-shaped, and the commonly used ones are 61-type bead-shaped and miniature bead-shaped semiconductor thermistors. The difference with the general thermal resistance is that it is a negative resistance temperature coefficient, the temperature increases, the resistance decreases, and the change range is also large, and the temperature coefficient of resistance α up to -2~-7%, which is 10~100 times larger than that of metal thermal resistance, therefore, the display instrument with low accuracy can be used. The characteristic curve is shown in Fig. Because it has the advantages of good corrosion resistance, high sensitivity, small thermal inertia, simple structure, long life, and convenient long-distance measurement, it can be used for temperature measurement of corrosive medium temperature, surface temperature and body temperature, etc., and the disadvantages are small measurement range (-40-350 °C), poor interchangeability, and temperature-resistance characteristics are nonlinear.
The temperature coefficient of a thermistor α inversely proportional to the square of temperature, ie
α=-(β/T2)
Thermistors have high resistance values. Its resistance value is 1~4 orders of magnitude higher than that of platinum RTD, and the relationship with temperature is not linear, which can be expressed by the following empirical formula:
RT=AeB/T
where T—temperature, K;R—resistance at temperature T, Ω; e—the base of the natural logarithm; A, B—the constants of the thermistor material and structure, with the dimension of A being the resistance and the dimension of B being the temperature.
The diagram shows the resistance-temperature characteristics of a semiconductor thermistor, which is an exponential curve.
The thermistor is small in size, the thermal inertia is also small, the structure is simple, and it can be made into various shapes according to the needs, such as beads, sheets, rods, discs, films, etc., and the smallest bead-shaped thermistor can reach φ0.2mm.
Thermistors are abundant and inexpensive. It has good chemical stability, and the surface of the component is encapsulated with ceramic materials such as glass, which can be used in harsh environments. By making effective use of these characteristics, it is possible to develop a thermometer with high sensitivity, fast response speed and easy to use.
The materials commonly used in semiconductor thermistors are made of iron, nickel, manganese, cobalt, molybdenum, titanium, magnesium and other composite oxides sintered at high temperatures.
The main disadvantage of thermistors is that their resistance is nonlinear as a function of temperature. The stability and interchangeability of the components are poor. In addition, it cannot be used for high temperatures above 350°C except for high-temperature thermistors.
The structure, model, main specifications and technical characteristics of RTD
1. Ordinary thermal resistance
It is usually composed of four parts: resistor body, insulator, protective sleeve and junction box. With the exception of the resistive body, the rest of the parts have the same structure and shape as the corresponding part of the thermocouple.
Platinum resistors are made of very fine platinum wires wound around mica, quartz or ceramic supports, and are available in flat, cylindrical and spiral shapes. The commonly used WZB type platinum resistor is a platinum wire with a diameter of 0.03~0.07mm wound around a flat bracket made of mica sheet. The edge of the mica sheet has a zigzag notch, and the platinum wire is wound between the teeth to prevent short circuits. The windings made of platinum wire are covered on both sides with mica sheets. In order to improve the dynamic characteristics of RTD and increase mechanical strength. They are then riveted together on both sides with clamping pieces made of sheet metal. The wire end of the platinum wire winding is welded with the 1 silver wire lead-out wire, and the porcelain sleeve is worn to insulate and protect.
Micro platinum thermal resistance is also commonly used in industry, which has small size, small thermal inertia and good air tightness. Its structure is shown in the figure, it is wound with 0.04~0.05 annealed platinum wire (spiral platinum wire for quartz support) on a cylindrical glass rod engraved with threads (quartz bracket is used for high-temperature platinum resistance), 0.5mm platinum wire is used for the lead wire, and a 4.5mm special glass tube (or quartz tube) is sleeved as a protective sleeve.
In order to reduce the measurement error caused by the change of ambient temperature of the resistance of the lead wire and the connecting wire, it is hoped that the initial value of the platinum resistance is as large as possible, but if it is too large, the volume of the resistor body will increase and the thermal inertia will also increase. At the same time, the amount of heat generated on the RTD by the measurement current flowing through the RTD also increases, resulting in additional measurement errors. The industrial platinum resistance index number commonly used in mainland China is taken and taken. The indexing characteristics of platinum RTD are shown in the indexing table.
The copper resistor body is a copper wire winding (including the manganese-copper compensation part), which is made of a high-strength enameled copper wire with a diameter of 0.lmm wound on a cylindrical plastic bracket by a double-wire non-inductive winding method, as shown in the figure.
In order to prevent the copper wire from loosening, strengthen the mechanical fastening and improve its thermal conductivity, the whole element is impregnated with phenolic resin (or epoxy resin), and then must be dried (also plays an aging role), the drying temperature is 120 °C, keep it for 24 hours, and then cool to room temperature, and then weld the outlet terminal of the copper wire winding with the lead wire made of silver-plated copper wire, and wear an insulating sleeve, or directly solder it with an insulated wire.
The graduation number of the copper resistor is taken, taken. The indexing characteristics are shown in the indexing table.
In addition, there are also certain requirements for the resistor lead-out wire, which is generally required to have no great thermoelectric potential for the metal RTD wire and the connected copper wire, and has good chemical stability. Standard or specification instruments are made of gold or platinum. For the lead-out wires of industrial thermal resistance, silver is used at high temperatures and copper is used at low temperatures.
The first letter W represents the temperature, the second letter Z represents the thermal resistance, and the third letter indicates the graduation number of the thermal resistance, with P for platinum, C for copper, and N for nickel.
The main technical characteristics of thermal resistance
| code name | Graduation number | Resistance value at 0°C （Ω） | Allowable resistance error at 0°C （%） | Resistance Ratio(R100/R0) | Measuring range (°C) | The allowable value of the basic error （℃） |
| WZP | Pt10 | 10 （0~850℃） | A grade ± 0.006 Class B ± 0.012 | 1,385±0.001 | -200～+850 | Δt=±(0.15+2×10-3t) |
| Pt100 | 100 （-200~850℃） | A grade ± 0.006 Class B ± 0.012 | Δt=±(0.3+5×10-3t) | |||
| WZC | Cu50 | 50 | ±0.05 | 1,428±0.002 | -50～+150 | Δt=±(0.3+6×10-3t) |
| Cu100 | 100 | ±0.1 | ||||
| WZN | Ni100 | 100 | ±0.1 | 1.67±0.003 | -60～0 | Δt= (0.2+2×10-2t) |
| Ni300 | 300 | ±0.3 | ||||
| 0～+180 | Δt=±(0.2+1×10-2t) | |||||
| Ni500 | 500 | ±0.5 |
2. Special thermal resistance
l. Armored RTD
The armored thermal resistance is to put the temperature sensing element of the ceramic skeleton or glass skeleton into a thin stainless steel tube, and its surroundings are firmly filled with magnesium oxide to ensure that its 3 leads are well insulated from the protective tube, as well as between the leads. After it is fully dried, its end is sealed and then drawn into a solid whole through a mold, which is called an armored RTD.
Compared with ordinary RTD, armored RTD has the following advantages:
1) The outer diameter ruler is small, and the inner casing is solid, and the response speed is fast;
2) Earthquake-resistant, flexible, easy to use, suitable for installation in complex structures;
3) The temperature sensing element is not in contact with corrosive medium and has a long service life.
The outer diameter of the armored RTD is generally φ2~8mm, and individual ones can be made into φ0.2mm. The commonly used temperature is -200~600°C.
2. Thin film platinum thermal resistance
Thin-film platinum resistors are made by direct evaporation of pure platinum on an insulating substrate by vacuum coating. Its temperature range is -50-600°C. The accuracy of domestic components can reach class B in the German standard, and due to the small heat capacity and high thermal conductivity of the film, the thin film platinum thermal resistance can quickly and accurately measure the real temperature of the surface.
3. Thick film platinum thermal resistance
The thick film platinum resistance is mixed with high-purity platinum powder and glass powder, mixed with organic carrier to form a paste-like slurry, screen-printed on the corundum substrate, and then sintered and installed with a lead wire to adjust the resistance value. Finally, glass glaze is applied as an electrical insulating protective layer.
Thick film platinum resistors have basically the same range of applications as wirewound platinum resistors. It is significantly better than wirewound RTD in surface temperature measurement and in mechanical vibration environments.
RTD temperature measurement circuit
Both balanced and unbalanced bridges are instruments that measure the amount of change in resistance. When they are used in the field of "automatic detection", they can be scaled in a variety of ways depending on the type of resistive sensor (e.g., RTD, strain gauge, etc.) used. The figure shows the basic schematic diagram of the balanced bridge, in which Rt is the thermal resistance, the resistance value changes with temperature, R2 and R3 are fixed resistances, and R1 is the variable resistance, which is composed of these four resistors and the four bridge arms of the bridge. G is the galvanometer and E is the power supply. When the Rt value changes, the bridge balance is broken. The galvanometer G is deflected, and the R1 value is changed to rebalance the bridge, and the galvanometer G refers to zero, and there is at this time
Since R2 and R3 are known, Rt is proportional to R1, as long as the scale is laid along R1, the measured resistance value can be read out according to the position of the sliding contact, and the measured temperature can be known.
When using a bridge as a measuring instrument, the pinout of the industrial platinum resistor is a three-wire system to reduce the measurement error caused by the change of ambient temperature of the connected conductor resistance.
The lead wire of the standard or specification platinum RTD adopts a four-wire system, which can not only eliminate the influence of the resistance of the connected wires, but also eliminate the measurement errors caused by the parasitic potential in the line, as shown in the figure.
Checksum faults of RTDs
1. Calibration of RTD
Connect the thermal resistance and calibration equipment to be calibrated according to the circuit on the right, and measure the thermal resistance values of O°C and lO0°C to find the sum and meet the technical requirements.
The calibration principle is as follows: the thermal resistance 2 and the standard resistance 5 (generally 1Ω or 3Ω) that are calibrated are connected together in series to form a loop, the heating thermostat 1, the standard thermometer 3, the milliampere meter 4, the voltage divider 6, the double-pole double-throw switch 7, and the potentiometer 8 are connected in series to form a loop. Adjust the voltage divider 6 so that the loop current is about lmA. When the thermal resistance 2 is inserted into ice water or water boiler, the current produces a certain voltage drop on the standard resistance 5 that is calibrated on the thermal resistance 2, and the voltage drop can be transmitted to the potentiometer 8 by the transfer switch 7, and the reading is indicated by the potentiometer.
Changing the contact position of the transfer switch 7 can sequentially transmit the voltage drop on the standard resistance 5 and the calibrated thermal resistance 2 to the potentiometer. The resistance value of the resistance to be calibrated can also be calculated according to the formula, so that the value of the sum can be measured.
2. Failure of RTD
The common fault of thermal resistance is the open circuit, you can find the broken wire first and re-weld it, and then you can continue to use it. Platinum resistance can be broken with voltage 6~8V arc welding. The fault of short circuit also occurs from time to time, and it can be cemented with insulating liner or lacquer sheet, and the repaired thermal resistance should be re-calibrated if necessary.