When should a thermocouple be chosen? When should a thermal resistor be chosen? Which one is more suitable?

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In daily work, temperature measurement instruments are often used. Both thermal resistors and thermocouples are types of temperature measurement instruments; so when measuring temperature at the same location, should we choose a thermal resistor or a thermocouple? Today, let’s take a comprehensive look at it. Structure of thermocouples: There are 3 types of shapes for the front-end junction of thermocouples, as shown in the figure below. Joining can be carried out using methods such as gas welding, butt welding, resistance welding, arc welding, and silver welding, depending on the type of thermocouple, its wire diameter, and the operating temperature. In industrial applications, an external sleeve is commonly used to facilitate installation and extend the service life of thermocouples. Casing is generally divided into protective type and armored type. A thermocouple with a protective tube is a type of thermocouple in which the core wire and insulating tube of the thermocouple are inserted into a protective tube. The protective tube prevents the core wire from oxidizing and corroding, while also maintaining the mechanical strength of the thermocouple. There are various types of protective tubes, and the commonly used ones are shown in the table below. Encased thermocouples: The measurement principle of encased thermocouples is the same as that of thermocouples with protective tubes. It uses thin metal tubes (referred to as sleeves) as a substitute for the insulating tube (ceramic) in the above image, and employs powders such as magnesium oxide (MgO) as the insulating material. Due to its thin outer diameter and ease of bending, it is most suitable for measuring temperatures on the back side of objects and in narrow spaces. Furthermore, compared to thermocouples with protective tubes, it has a more sensitive response speed. The outer diameter range of the casing for armored thermocouples is wide; it can be extended and processed into various sizes ranging from 8.0 mm in diameter to 0.5 mm in diameter. The finer the core wire is drawn, the lower the usual upper temperature limit. For example, for K-type thermocouples, the typical upper temperature limit is 600°C for those with an outer diameter of 0.5 mm, and 1050°C for those with an outer diameter of 8.0 mm. The structure of the thermoresistor is shown in the figure below. There are 3 shapes for the components of thermoresistors, and currently, those in ceramic packaging are the most common. Ceramic-packaged types are used for thermoresistors with protective tubes as well as armored thermoresistors. The diameter of the bare platinum wires in ceramic and glass packages is around several dozen micrometers, while that of those in mica plate types is approximately 0.05 mm. The leads use platinum alloy wires that are much thicker than the component wires. Types of thermal resistance elements. Illustrations of thermal resistances with protective tubes. Armored thermal resistances. Differences: 1. Although they are all contact-type temperature measuring instruments, their temperature measurement ranges differ. Thermocouples are used in environments with high temperatures, as their thermoelectrical potential is very low in medium and low temperature ranges (this can be checked using tables). When the potential is low, stringent measures against interference and high-quality secondary instruments are required; otherwise, the measurements will be inaccurate. Moreover, in lower temperature ranges, the relative errors caused by changes in the cold junction temperature and ambient temperature become quite significant, making it difficult to achieve full compensation. At medium and low temperatures, thermistors are generally used for temperature measurement; their range is 200–500, and they can even measure lower temperatures (for example, carbon resistors can measure temperatures around 1 K). Nowadays, platinum thermistors of the Pt100 type are commonly used (there are also Pt50, 100, and 50 types, with these values representing the resistance of the thermistor at 0 degrees Celsius; in older classification systems, BA1 and BA2 were used to denote this, with BA1 having a resistance of 46 ohms at 0 degrees Celsius. In industry, copper resistors are also used, with CU50 and CU100 being the relevant classifications, though their temperature measurement range is smaller, ranging from –50 to 150 degrees Celsius. In some special applications, indium resistors and manganese resistors are also utilized.) 2. The basic principle behind using thermocouples for temperature measurement is the thermoelectric effect; an auxiliary instrument used for this purpose is a voltmeter, or an electronic potentiometer when higher precision is required. Resistance works based on the property that the resistance values of conductors and semiconductors change with temperature; a secondary meter is an unbalanced bridge. 3. According to the principle of thermocouple temperature measurement, the measured temperature has a one-to-one functional relationship with the thermoelectric potential only when the temperature of its cold end remains constant. In practical applications, a cheap connecting wire (also known as a compensation wire) with thermoelectric properties similar to those of the corresponding thermocouple is used to extend the cold end of the thermocouple to a location where the temperature is relatively constant (preferably 0 degrees). For example, copper-constantan wires are used as compensation wires to extend nickel-chromium/nickel-silicon thermoresistors. Therefore, there are two extension wires from the thermocouple to the secondary meter. The thermistor is connected to the secondary meter via copper wires. To reduce measurement errors caused by environmental changes, a three-wire connection method is generally used: two wires connect the thermistor in series across adjacent bridge arms, while the third wire provides power supply. It is required that the resistance value of each wire, together with that of the adjustment resistor, total 5 ohms (±0.01).

For on-site diagnosis during operation:
1. Thermocouples: Thermocouples have positive and negative poles, and the compensation wires also have positive and negative ends. First, ensure proper connections and correct configuration. Common issues during operation include short circuits, open circuits, poor contacts (which can be detected using a multimeter), and deterioration (which can be identified by the color of the surface). When checking, it is necessary to separate the thermocouple from the secondary meter. Here’s a method I use in practice for reference: Use a tool to short-circuit the compensation wire on the secondary meter; if the meter indicates room temperature, then the meter is functioning properly. Next, short-circuit the terminals of the thermocouple – if the meter shows the temperature of the environment where the thermocouple is located, then there is a problem with the compensation wire. Then, use the millivolt range of a multimeter to estimate the thermoelectric potential of the thermocouple; if it is normal, check the manufacturing process.
2. Thermistors: The main issues are short circuits and open circuits, which can be detected using a multimeter. If a short circuit is suspected, simply disconnect one of the wires at the resistor terminal and check the meter reading: if it reaches its maximum value, the thermistor has a short circuit; if it returns to zero, there is a short circuit in the wires. If the meter readings are low or unstable despite proper connections and configuration, it is possible that water has entered the protective casing. If the reading is at its maximum, the thermistor has an open circuit; if it is at its minimum, there is a short circuit. Generally, thermistors are used for temperatures below 300 degrees, while thermocouples are used for temperatures above 300 degrees. As the temperature changes, the resistance value of the thermistor changes, and the electromotive force of the thermocouple also changes. Thermal resistors currently use copper thermal resistors and platinum thermal resistors. Based on the value of the thermal resistor at 0 degrees, they are classified into different grades, such as PT100, PT1000, CU50, etc. Taking PT100 as an example, the ‘PT’ denotes platinum, while ‘100’ indicates that the resistance value of the thermal resistor at 0 degrees is 100 ohms. Thermocouples currently come in various types designated by scales such as K, B, and S, each representing different materials for use in different temperature ranges. For example, type K is made of nickel-chromium-nickel-silicon material and is generally used for measuring temperatures from 0 to 800 degrees; type B is made of platinum-rhodium 30-platinum-rhodium 6 and is used for measuring temperatures from 800 to 1600 degrees. What is the principle of operation of a thermocouple? The working principle of a thermocouple is based on the Seebeck effect, which is the physical phenomenon in which a thermal current is generated within a circuit when two conductors of different compositions are connected at their ends, and if the temperatures at those two ends differ. A thermocouple consists of two different wires (thermoelectrodes), one end of which is welded together to form the measuring end of the thermocouple (also known as the working end). Insert it into the medium at the temperature to be measured ； The other end of the thermocouple (the reference end or free end) is connected to the display instrument. If there is a temperature difference between the measuring end and the reference end of the thermocouple, the display instrument will indicate the thermoelectromotive force generated by the thermocouple. What is the measurement principle of a thermal resistor? Thermistors are used to measure temperature by taking advantage of the property that the resistance of metal conductors or semiconductors changes as their temperature changes. The part of the thermistor that is exposed to heat (the temperature-sensing element) is formed by evenly winding fine metal wires around a framework made of insulating material, or by using laser sputtering techniques on a substrate. When the medium under test has a temperature gradient, the temperature measured is the average temperature of the medium layer within the range where the temperature-sensing element is located. What is an armored thermocouple and what are its advantages? In the IEC1515 standard, it is named “mineral insulated thermocouple cable”, that is, a thermocouple cable with inorganic mineral insulation. It is formed by integrally drawing a thermoelectric element, an insulator, and a sheath; its outer surface appears to be covered with a layer of \"armoring,\" which is why it is called an armored thermocouple. Compared with ordinary assembled thermocouples, it has advantages such as high pressure resistance, good bendability, excellent oxidation resistance, and a long service life. What are the different types of thermocouple calibration numbers? What are their characteristics? The calibration types of thermocouples mainly include S, R, B, N, K, E, J, T, etc. Among them, S, R, and B belong to noble-metal thermocouples, while N, K, E, J, and T belong to base-metal thermocouples. The S grade is characterized by its strong oxidation resistance; it is suitable for continuous use in oxidative and inert atmospheres. The maximum operating temperature for long-term use is 1400°C, and 1600°C for short-term use. Among all thermocouples, the S type has the highest accuracy grade and is typically used as a standard thermocouple ； Compared to the S grade, the R grade has a thermoelectromotive force that is about 15% higher; otherwise, their properties are almost identical ； }The B scale has an extremely low thermoelectromotive force at room temperature; therefore, a compensation wire is generally not used during measurements. Its long-term operating temperature is 1600°C, and the short-term temperature is 1800°C. It can be used in oxidative or neutral atmospheres, as well as for a short period of time under vacuum conditions. The N series thermocouples are characterized by strong high-temperature oxidation resistance at 1300°C, good long-term stability of the thermoelectromotive force as well as reproducibility under short-term thermal cycling; they also exhibit good resistance to nuclear radiation and low temperatures, allowing them to partially replace S series thermocouples ； The K grade is characterized by its strong oxidation resistance; it is suitable for continuous use in oxidative and inert atmospheres. The maximum operating temperature is 1000°C for long-term use, and 1200°C for short-term use. Most widely used in all thermocouples ； The characteristic of type E thermocouples is that, among commonly used thermocouples, they have the highest electromotive force, meaning the highest sensitivity. It should be used continuously in an oxidizing or inert atmosphere, at a usage temperature of 0-800℃ ； The J classification is characterized by its ability to be used in both oxidizing atmospheres (with a maximum operating temperature of 750°C) and reducing atmospheres (with a maximum operating temperature of 950°C); it is also resistant to corrosion by H2 and CO gases, and is widely used in the oil refining and chemical industries ； The characteristic of T-type thermocouples is that they offer the highest accuracy level among all base-metal thermocouples, and they are typically used to measure temperatures below 300°C. What are the advantages and disadvantages of N-type thermocouples compared to K-type thermocouples? Advantages of N-type thermocouples: strong high-temperature oxidation resistance and high long-term stability. In K-type thermocouples, the preferential oxidation of Cr and Si elements in the positive nickel-chromium electrode leads to uneven alloy composition and drift in the thermoelectromotive force. In N-type thermocouples, increasing the content of Cr and Si causes the oxidation mode of the nickel-chromium alloy to change from internal oxidation to external oxidation, resulting in the oxidation reaction occurring only at the surface ； -It exhibits good stability under low-temperature short-term thermal cycling, and the magnetic transition is suppressed ； -Strong resistance to nuclear radiation. The N-type thermocouple eliminates the prone-to-degradation elements Mn and Co found in the K-type, further enhancing its resistance to neutron irradiation ； -In the range of 400–1300°C, the thermoelectric properties of N-type thermocouples are more linear than those of K-type thermocouples. Disadvantages of N-type thermocouples: - The material of N-type thermocouples is harder than that of K-type thermocouples, making them more difficult to process ； -The price is relatively high. The thermal expansion coefficient of N-type thermocouples is 15% lower than that of stainless steel; therefore, the outer sleeve of N-type armored thermocouples should be made of NiCrSi/NiSi alloy ； The nonlinear error is relatively large in the range of -200 to 400°C. How many types are there for the lead wiring of thermal resistors? What are the impacts? There are 3 types of lead wiring for thermal resistors: 2-wire, 3-wire, and 4-wire. 2-wire thermal resistors have simple wiring, but they introduce the additional error of lead resistance. Therefore, it is not suitable for manufacturing thermoresistors with Class A accuracy, and the leads and wires should not be too long when in use. The 3-wire system can eliminate the effect of lead resistance, resulting in higher measurement accuracy compared to the 2-wire system. As a process detection element, it is the most widely used. The 4-wire system can not only eliminate the effect of lead resistance but also eliminate this resistance when the resistance values of the connection wires are the same. For high-precision measurements, a 4-wire system should be used. How to choose thermocouples and thermal resistors? Selection is based on the temperature measurement range: thermocouples are generally used for temperatures above 500°C, while thermal resistors are typically used for temperatures below 500°C ； Choose based on measurement accuracy: select a thermal resistor for higher accuracy requirements, and a thermocouple for lower accuracy requirements ； Selection based on the measurement range: Thermocouples generally measure the temperature at a \"point\", while thermal resistors typically measure the average temperature in a space.