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Temperature Measurement I. Fill in the blanks 1. Temperature measurement can be divided into two main categories, namely contact temperature measurement and non-contact temperature measurement. 2. Expansion thermometers utilize the principle that an object’s volume expands when heated; these include gas, liquid, and solid expansion thermometers. 3. The symbol for thermodynamic temperature is T, and its unit is K (Kelvin). 4. In the International Temperature Scale, both Kelvin temperature and Celsius temperature are used, with the relationship _t = T – 273.15. 5. Glass tube thermometers belong to the category of expansion thermometers; they are primarily composed of a liquid ； Memory ； capillary ； and a scale to make up. 6. The nickel-chromium/nickel-silicon thermocouples have a K-type calibration, while the iron-constantan thermocouples have a J-type calibration. 7. Compensation wires are divided into two types: compensation type and extension type, where X denotes the extension type and C denotes the compensation type. 8. The principle of a resistive thermometer is based on the property that the resistance of a conductor or semiconductor changes with temperature. 9. In industrial measurements, there are two wiring methods for thermal resistors: two-wire and three-wire. The three-wire method reduces the impact of changes in lead resistance on the reading significantly. 10. The function of the thermocouple compensation wire is to extend the cold end of the thermocouple. 11. When calibrating or repairing moving-coil meters, adjusting the meter’s measurement range can be achieved by changing the tension of the wire and altering the resistance of the R series circuit. 12. The compensation wire for K-type thermocouples is made of copper-constantan material. 13. The methods for thermocouple compensation include the freezing-point cell method, calculation method, zero-setting method, and bridge compensation method. 14. The transfer of temperature scales involves the calibration of various temperature measuring instruments during production, in order to transfer the standard to these instruments ； Calibration of temperature measuring instruments in use or after repair – two aspects. 15. Solid expansion thermometers are made of two materials with different coefficients of expansion, and they are divided into two main categories: rod type and bimetallic type. 16. A pressure thermometer is made based on the principle that the pressure of a substance in a sealed container increases as the temperature rises. 17. The common sensing fluids for glass liquid thermometers are mercury and organic liquids. 18. When using the same series of thermocouples to measure the same temperature, the corresponding measured temperature is calculated as 1/N of the thermocouple value. 19. The welding methods for the working end of thermocouples include acetylene welding, arc welding, butt welding, and salt bath welding; among these, acetylene welding and salt bath welding can only be used to weld thermocouples made of non-precious metals. 20. Instruments that use the properties of light radiation for temperature measurement include, among those widely used today, total radiation thermometers, partial radiation thermometers, luminance thermometers, and colorimetric thermometers. 21. Optical radiation characteristic temperature measuring instruments; their physical basis relies entirely on the two laws of Planck’s law of radiation and Stefan-Boltzmann law. 22. A total radiation thermometer is mainly composed of an optical system, a radiation receiver, and auxiliary measuring instruments, and it is generally used in high-temperature environments ranging from 700 to 2000°C. 23. A luminosity thermometer operates on the principle that different objects emit different amounts of monochromatic brightness at various temperatures. 24. When installing the temperature sensing element, there are two fixing methods: threaded connection fixation and flange fixation. 25. The maximum operating value of a thermometer is generally 90% of its full scale, while the normal indication value should be between 5% and 75% of the full scale. 26. When multiple temperature sensing elements share one display instrument, the indicated value is generally between 20% and 75% of the full scale. 27. Among glass thermometers, pressure thermometers, and bimetallic thermometers, the bimetallic thermometer has the best vibration resistance; the glass thermometer offers the highest precision; and the pressure thermometer enables remote display. 28. The model for industrial glass mercury thermometers is WNG, the model for industrial glass liquid thermometers is WNY, and the model for bimetallic thermometers is WSS. 29. The model for gas-pressure thermometers is WTO, while the model for steam-pressure thermometers is WTZ. 30. The condition for a thermocouple to generate a thermoelectric potential is that the materials of the two thermal electrodes are different, and the temperatures at the two junctions are different. 31. The thermocouple potential is related to the thermocouple material and the junction temperature, and is independent of the length and thickness of the wire. II. Short Answer Questions 1. Armored thermocouples are widely used in factories; please explain their advantages. ①It has a fast dynamic response, with a time constant as low as 0.01 seconds. ②It has good flexibility; after annealing, the bending radius of the sleeve can be twice its diameter. ③It can be made in different lengths as needed, and can be connected directly to the instrument without the need for a compensation wire. ④It has excellent shock and impact resistance, as well as good strength. ⑤The working end has a low heat capacity, resulting in little impact on the object being measured. 2. Briefly describe the principle of temperature measurement in glass thermometers. Temperature is measured by utilizing the difference between the volume change caused by the thermal expansion of the temperature-sensitive substance (mercury, alcohol, kerosene, etc.) inside the glass thermometer and the volume change of the glass itself when heated; the reading shown is the difference between the volume change of the temperature-sensitive substance and that of the glass. 3. Explain the principle of temperature measurement in bimetallic thermometers. A temperature sensing sensor is created by welding two bimetallic strips with different coefficients of expansion together. When the temperature changes, the bimetallic strips bend, and the degree of bending is proportional to the temperature. In practical applications, one end of the strip is fixed while the other end deforms; this deformation is then used, through mechanisms such as transmission and amplification, to move a pointer that indicates the temperature value. 4. Briefly describe the properties and functions of thermocouple compensation wires. A compensation wire is a wire made of a material with thermoelectric properties similar to those of the working thermocouple; it is used to extend the reference terminal of the thermocouple to the desired location, without introducing any additional temperature measurement errors beyond what is permissible in the working thermocouple circuit. 5. Why are thermocouples calibrated? During operation, thermocouples are affected by factors such as the environment, temperature, and protective sleeves. After being used for a period of time, their thermoelectric output characteristics change, and this effect is more pronounced in high-temperature, highly corrosive environments or during special types of measurements. When these output characteristics deviate beyond acceptable limits, that is, when they fall outside the specified error range, it leads to increased inaccuracies in the readings, which in turn results in a decline in the quality of the products being controlled. Therefore, regular calibration is necessary to prevent such issues from occurring. 6. Explain the temperature measurement principle of thermal resistance thermometers. The principle of operation for a thermistor thermometer is to measure temperature by utilizing the property that the resistance of a conductor or semiconductor changes with temperature. 7. Describe the main advantages of resistance thermometers. ①High measurement accuracy ； ②It has a large measurement range, especially at low temperatures ； ③Easy to use in automatic measurement, facilitating remote measurements ； ④Compared to thermocouples, it has no reference terminal error. 8. Why are platinum resistors made of expensive platinum widely used? ①Its physical and chemical properties are extremely stable, with strong oxidation resistance ； ②It can be used over a wide temperature range (below 1200°C) ； ③Easy to purify, has good reproducibility, excellent processability, and can be used to produce various types of filaments and platinum foil ； ④It has a higher resistivity compared to other thermal resistance materials. 9. Briefly describe the characteristics of thermocouples. ①High measurement accuracy ； ②It has a simple structure and is easy to manufacture; it can meet the requirements of various measurement targets, such as fast measurement, small size, and point temperature measurement ； ③Fast dynamic response ； ④Remote measurement is possible ； ⑤Wide temperature measurement range. 10. Explain the advantages, disadvantages, and precautions of mercury thermometers. The advantages are that mercury thermometers have a simple structure, are intuitive to use, offer high sensitivity and accuracy, are inexpensive, have a wide temperature measurement range, and are widely used. The disadvantages are fragility, difficulty in automatic recording, and limited long-distance signal transmission. Precautions: Mercury vapor is harmful to the human body in case of damage during manufacturing and use. 11. Thermocouples are made based on the thermoelectric effect. Please explain what the thermoelectric effect is When conductors made of two different materials form a closed circuit, and if the temperatures at the two ends of the circuit are different, an electric current of certain magnitude will flow through the circuit. The magnitude of this current depends on the properties of the conductor materials as well as the temperatures at the ends; this phenomenon is known as the thermoelectric effect. 12. Explain in detail the model numbers, calibration numbers, and measurement ranges of the 7 types of industrial thermocouples. ①Platinum-rhodium 10-platinum thermocouple, with calibration number S; it can operate for extended periods at temperatures up to 0–1300°C, and temporarily up to 1600°C. ②The Platinirhodium 30-Platinirhodium 6 thermocouple, with calibration class B, can operate continuously at temperatures up to 0–1600°C, and temporarily up to 1800°C. ③Nickel-chromium-nickel-silicon (nickel-chromium-nickel-aluminum) thermocouples, with a calibration type of K; their measurement range is 0–1300°C. Their calibration curve is more linear, and they exhibit a high thermoelectromotive force. ④Copper-constantan thermocouples with a temperature scale of T can operate in the range of -200 to 400°C. They feature high precision, good stability, and high sensitivity at low temperatures. ⑤Nickel-chromium-copper-nickel thermocouples with an E calibration type, having a measurement range of -200 to 900°C. They feature good stability, high sensitivity, and low cost. ⑥Iron-copper-nickel thermocouple, classification number J, operating temperature range -40～750°C. It exhibits good linearity and high sensitivity below 700°C. ⑦Platinum-rhodium 13-platinum thermocouple, classification number R; operating temperature range of 0–1300°C, with temporary operation possible up to 1600°C. No. 300.15 13. What are the requirements for the structure of a thermocouple in order for it to function properly? ①The working end of the thermocouple must be welded firmly and reliably, with no pores, slag, or impurities at the weld site. ②In addition to the working contacts, the electrodes must have good insulation to prevent short circuits and leakage between them; the filament should also be free from any mechanical damage. ③The leads connect easily and firmly to the external wires. ④When operating under hazardous conditions, the protective tube should ensure that the medium is isolated from the thermocouple, allowing the thermocouple to function properly. ⑤The temperature probe should have sufficient mechanical strength. 14. When manufacturing thermocouples, the two electrodes can be wound together before welding; why can’t several more turns be wound before welding? Generally, the number of hinge weld turns should not exceed 2–3; otherwise, the working end will move upward, causing errors during measurement. 15. The wires inside a dynamic coil are mostly made of copper; why then does this not cause any measurement errors when using it together with thermocouples for temperature measurement? According to the intermediate conductor law of thermocouples, for a thermocouple composed of conductors A and B, when a third conductor is introduced, as long as the temperatures at both ends of this third conductor C remain the same, the total potential of the circuit remains unchanged after conductor C is connected. When this third conductor is replaced by a moving-coil meter, the temperatures at its two junctions are equal, so no measurement error occurs. 16. Briefly describe the advantages of armored thermoresistors. ①Its characteristics are similar to those of armored thermocouples; it can have a very small outer diameter and features a fast response time. ②It has good mechanical properties, as well as resistance to vibration and impact. ③The lead and protective tube are integrated, offering good flexibility for easy installation. ④The resistor is enclosed in a metal tube, making it resistant to erosion by harmful substances. 17. Briefly describe the sources of error in contact temperature measurement. Temperature measurement by contact method relies on the heat exchange between a thermosensitive element and the medium whose temperature is to be measured; once equilibrium is reached, the temperature of the thermosensitive element itself reflects the temperature of the medium in question. This method is simple and reliable, but it can cause measurement errors for the following reasons: ① Errors resulting from heat conduction along the temperature-sensing element ； ②Errors caused by the thermal radiation of the temperature sensing element ； ③The error induced in the field to be measured due to the insertion or contact of the temperature sensing element ； ④Errors caused by poor contact during contact method measurement ； ⑤Due to the thermal inertia of the temperature sensing element, dynamic errors will also occur when the temperature of the object being measured changes rapidly. 18. Briefly describe the advantages of temperature measurement using optical radiation. During the measurement process, no contact is made with the object being measured, and the introduction of measuring elements does not affect the distribution of the temperature field under examination. Due to this non-contact nature, the measuring elements do not need to reach the same temperature as the medium being measured, which allows for a higher measurement range. Additionally, fast measurements can be achieved. 19. When using a glass thermometer, can it be inserted as deep as desired? Typical glass thermometers have a immersion line; when in use, it should be inserted up to that line. If there is no such line, the point where the upper part of the thermometer starts to widen serves as the immersion mark. When using a fully immersed thermometer, it must be completely submerged in the medium; if full immersion is not possible, adjustments must be made using a formula. 20. Why is it necessary to adjust the wire resistance of thermoresistors? Thermal resistors are generally measured using an unbalanced bridge circuit. The two wires of the thermal resistor are connected to the adjacent arms at each end of the bridge. When designing the bridge, so that it reaches equilibrium at the temperature corresponding to the starting point on the instrument scale, the resistance of these wires is set to a specific value (usually 5Ω). In practical use, if the wire resistance is lower than this specified value, adjustments must be made; otherwise, measurement errors will occur. 21. What are the faults of thermal resistors? What is the reason? ①The indicated value is lower than the actual value or the indication is unstable, due to metal shavings and dust inside the protection tube, dust accumulation between the terminals, and a short circuit in the thermal resistor. ②The gauge indicates infinity due to an open circuit in the thermal resistor and an open circuit in the lead wires. ③The gauge shows a negative value; the reason is an incorrect connection between the gauge and the thermal resistor, resulting in a short circuit in the thermal resistor. ④The relationship between resistance and temperature changes due to the corrosion and deterioration of the resistance wire. 22. Briefly describe the temperature measurement principle of a radiometric pyrometer. When an object is heated, it emits radiation energy of various wavelengths, and the intensity of this radiation energy is proportional to the fourth power of the temperature of the object. Radiation pyrometers operate on this principle. 23. For the display instrument used with a radiation temperature sensor, is its reading the true temperature of the object being measured? At the same temperature, the radiation intensity of different objects varies. The scale of a radiometric pyrometer is based on the use of a black body as a standard; the instrument is calibrated according to the temperature of this black body. Therefore, the reading on the instrument corresponds to the temperature of the black body, rather than the actual temperature of the object being measured. 24. What are the types of in-situ temperature measuring instruments? Which is suitable for which occasion? Local temperature instruments include bimetallic thermometers, glass thermometers, and pressure-type thermometers. In most cases, a bimetallic thermometer is the suitable choice; if it is installed at a high position or at a distance from where it needs to be viewed, one with a dial of 150 mm can be used. Glass thermometers can be used in applications that require high measurement accuracy, low vibration, and easy reading. Pressure-type thermometers can be used in the following situations: ① Measuring temperatures below -80°C ； ②The temperature measurement point is located at a distance, with an observation distance of over 6 meters, or the temperature measurement pipeline is under the floor and blocks the line of sight ； ③Places with vibration. 25. How to select thermocouples and thermal resistors? ①Thermocouples and thermal resistors are generally selected based on the temperature measurement range. In environments with vibration, thermocouples are preferable; whereas in situations where high accuracy in temperature measurement is required and there is no severe vibration that could cause measurement errors, thermal resistors are more suitable. ②When measuring reducing gases with a hydrogen content of over 5% at temperatures above 870°C, an air-blown thermocouple or a tungsten-rhenium thermocouple should be used. ③When measuring the temperature of the outer wall of pipes and equipment, surface thermocouples or surface thermal resistors are used. ④When a temperature measurement point needs to be displayed at two locations or a backup is required, a dual-element temperature sensor should be used. ⑤When a temperature measurement port requires multiple temperatures, a multi-point (multiple) special thermocouple should be used. ⑥When measuring flowing media containing solid, hard particles, wear-resistant thermocouples should be used. ⑦In areas with explosion hazards, explosion-proof thermocouples and thermal resistors should be used. ⑧When a bent installation or fast response is required for the temperature sensing element, armored thermocouples or thermal resistors can be used. 26. In the installation of thermocouples, what is the difference between grounding and not grounding the hot end of the thermocouple? The hot end of the thermocouple makes contact with the wall of the protective sleeve rather than with the protective tube, resulting in a much smaller measurement lag. This also helps to reduce interference and improves the reliability and accuracy of the measurements. However, whether the hot end can be grounded depends on the structure of the input circuit of the secondary meter; for the hot end of the thermocouple to be grounded, the input circuit of the secondary meter must not share a common grounding point with it. 27. How many fixing methods are there when installing temperature sensing elements on pipes and equipment? What occasions is it suitable for? Generally, there are two methods of fixation: threaded connection and flange fixation. The threaded connection fixing method is generally suitable for installing temperature sensors in pipes that are not exposed to corrosive media, offering the advantages of a small size and compact installation. The flange mounting method is suitable for installing temperature sensing elements on equipment. In high-temperature and highly corrosive media, coking slurry media, highly toxic media, powdered materials, as well as when measuring the temperature at multiple points in a catalyst layer, flange fixation should also be used to facilitate maintenance. 28. The temperature-sensing element of a pressure-type thermometer is the bulb, but why is it required that the temperature of the environment in which the capillary is located remain constant during installation and use? A pressure-type thermometer measures temperature based on the expansion of volume or increase in pressure of the medium contained within the bulb when heated; therefore, during installation, the bulb must be completely immersed in the medium being measured, otherwise its volume will decrease upon heating, resulting in an underreported reading ； If the capillary is heated or cooled, the medium inside it will expand and contract as well, resulting in additional errors; therefore, the ambient temperature around the capillary must be kept constant, and insulation measures should be taken if necessary. 29. What should be noted when installing and using pressure-type thermometers? During installation, the capillaries should be laid straight with fixed spacing, and their minimum bending radius should be greater than 50 mm. The thermowell must be fully submerged; otherwise, it will cause measurement errors. It should be calibrated before installation; since pressure is transmitted through capillaries, there is a certain delay, so readings should be taken once stability is achieved. 30. What is a semiconductor thermometer? What are its features? A semiconductor thermometer uses a semiconductor thermistor as the temperature-sensing element, along with a highly sensitive microammeter, and is commonly used to measure the rapidly changing temperature of a specific point or surface. It is characterized by a relatively simple manufacturing process and low cost. It has a large negative temperature coefficient, high sensitivity, and a small size; the diameter of the bead-shaped resistors is sometimes as low as 0.2–0.5 mm, which enables rapid response. Due to the high resistance value, the internal resistance of the wires and contact resistance can be ignored during measurement. However, its temperature measurement range is narrow, the relationship between temperature and resistance is nonlinear, the manufacturing process is difficult to control, and interchangeability is poor, so it is not easy to be widely adopted.