How much do you know about these continuous thermocouples/thermopile thermocouples?

Thread Content

The special temperature-measuring thermosensitive cable is a temperature sensor that represents a new temperature measurement technology developed based on thermocouples. It is also known as a Continuous Thermocouple or a Heating Seeking Thermocouple. Thermistive cables utilize the thermoelectric effect of thermocouples, but they measure not the temperature at the tip of the junction, rather the temperature at the highest temperature point along the length of the thermoelectrode. Due to its unique functionality, it was initially adopted by the United States as a high-tech device and installed in the compartments of aircraft carriers and destroyers, as well as in military aircraft, spacecraft, armory facilities, and other such equipment. The thermocable produced by Changhui Instruments can operate within a temperature range of -40 to 900°C; it is resistant to corrosion and vibration impacts, and can be used in situations where conventional temperature-sensitive cables or optical fibers are not suitable, thereby helping to prevent and reduce accidents and losses caused by \"overheating\". Continuous thermocouple yunrun.com.cn/product/2513.html Structure and characteristics of thermistive cables 1. Structure of thermistive cables Thermistive cables are mainly composed of thermal electrodes, insulating materials, and protective tubes. A thermoelectric electrode consists of a pair of parallel wires separated by a certain distance from each other. The grading, material, and standards of these electrodes are similar to those of standard thermocouples; in China, K-type thermocouples are commonly used in motors. The electromotive force of a thermocable is directly proportional to temperature. The insulating material that fills the space tightly between the thermoelectrodes is a thermistor material with a negative temperature coefficient (NTC), manufactured using specialized processes; it has a high resistance at low temperatures and acts as an insulator, whereas its resistance drops sharply as the temperature rises. Thermistor materials are key to manufacturing thermistive cables. Different negative temperature coefficient thermistor materials have been used as isolation materials; for example, the thermistive cables developed in the 1980s used manganese dioxide as such a material (see U.S. Patent 4647710). Currently, there are several types of thermistor cables available on the market, made from different negative temperature coefficient thermistor materials. These cables have an outer protective metal coating, such as heat- and corrosion-resistant alloys, stainless steel, or double-layer polytetrafluoroethylene, which provides them with good mechanical strength and flexibility, making them easy to install and use. There are connectors at the ends, allowing its length to be extended through those connectors. http://yunrun.com.cn/upload/201905/31/201905310216140378.png 2. Features of armored continuous thermocouples: ① Simple structure. Any point along a continuous thermocouple can detect temperature and generate a millivolt signal automatically, without the need for an external power source; therefore, it can operate safely in explosive environments. ②Durable and robust. The armored structure boasts both good toughness and sufficient strength; it can be bent during installation, is easy to clamp and secure, requires almost no routine maintenance, and is resistant to moisture. ③Divisibility. The relationship between the thermoelectromotive force of a thermosensitive cable and temperature is given in the calibration tables for continuous thermocouples provided by the manufacturers of such cables, eliminating the need for on-site calibration. ④Strong compatibility: a. The millivolt signal from the thermocouple cable is converted into a 4-20mA two-wire signal using a dedicated continuous thermocouple temperature transmitter YR9031X-X ； b. The signal from the thermocouple cable is converted into a digital signal using the dedicated continuous thermocouple signal acquisition module YR-9740A, and this digital signal is transmitted to the PLC and DCS via RS485 communication. ⑤ The alarm points can be set arbitrarily on the control system. ⑥ The continuous thermocouple operates continuously within a temperature range of 80–880°C until it is damaged. ⑦ It is possible to measure the rate at which temperature rises. ⑧ The highest temperature reached can be measured (not the average temperature). ⑨ No on-site calibration or activation is required. The output electromotive force of the thermocouple cable is slightly lower compared to that of a standard K-type thermocouple; this results in a temperature difference of a few degrees. (At the same temperature, the temperature consistency of the continuous thermocouple is inferior to that of ordinary thermocouples, with deviations of up to ±10°C.) http://yunrun.com.cn/upload/202003/15/202003150042441376.png Working principle of thermal sensitive cables: A thermal sensitive cable is a new type of sensor used to measure the temperature at the point of highest temperature along the length of a thermocouple. Its temperature measurement principle is the same as that of a thermocouple. Its pair of thermoelectrodes consists of a pair of thermocouple wires. When the temperature at any point along the length of the continuous thermocouple (T1) is higher than that of other sections, the insulation resistance between the thermoelectrodes at that point decreases, resulting in the formation of a “temporary” thermocouple measuring junction. At this point, it functions as a regular thermocouple; by measuring the thermoelectric potential at the reference end of the thermoelectrodes, the temperature at the junction (“temporary” hot junction), i.e., T1, can be determined. If another point on the thermocouple cable experiences a situation where T2 is higher than T1, the insulation resistance between the thermoelectrodes at that point will become lower than that at point T1, resulting in the creation of a new \"temporary\" thermocouple measurement point. The thermoelectric potential measured at the reference terminal of the thermoelectrodes will then correspond to the temperature at the newly formed T2 point on the thermocouple. This is the principle behind why the “temporary” thermocouple junction can track the highest temperature point on the thermocouple cable, as shown in the figure. http://yunrun.com.cn/upload/201905/31/201905311550139913.png As can be seen from the aforementioned operating principle, since the two electrodes are not in perfect contact with each other and are separated by a thermistor with low resistance, the thermoelectric voltage measured at the measurement end is slightly reduced, resulting in a certain temperature measurement error. Generally speaking, the higher the temperature and the wider the high-temperature range, the smaller the error. To reduce this error, some manufacturers add another pair of thermocouple wires, also using negative temperature coefficient thermistors as isolation materials; by utilizing this pair of wires for auxiliary measurement, the error can be corrected to a certain extent. If it is necessary to determine the location of the high-temperature point, an additional distance measurement cable must be added. This cable has three conductors: one with low resistance and two with high resistance, whose resistivities are ρ1 and ρ2 respectively. They are also separated from each other by thermistors with a negative temperature coefficient; in the event of a high-temperature spot, the three conductors are effectively connected to each other through two low-value resistors r, as shown in the diagram. http://yunrun.com.cn/upload/201905/31/201905311605393786.png By measuring the resistance RAC between the low-resistance wire and the high-resistance wire 1, as well as the resistance RBC between the low-resistance wire and the high-resistance wire 2, and taking the difference between these values, it is possible to calculate: RAC-RBC = R1+R+r-(R2+R+r) = R1-R2. In the formula, R1 represents the resistance value of the high-resistance wire 1 from the measurement and control terminal to the thermal contact point ； R2 is the resistance value of the high-resistance wire 2 from the measurement and control terminal to the thermal contact point ； R is the resistance of the low-resistance wire from the measurement and control terminal to the thermal contact. As can be seen from this formula, the contact resistance r has been eliminated from the aforementioned calculation results; furthermore, the distance L between the measurement and control terminal and the thermal contact can be determined using the formula L=K×(R1-R2)/(ρ1-ρ2). Product technical parameters: ① Outer protective tube: Inconel 600 heat-resistant alloy. ② Temperature sensing element: Standard K-type thermocouple (it exhibits a certain temperature offset compared to traditional K-type armored thermocouples; there is a significant difference in the relationship between the thermoelectromotive force and temperature for continuous thermocouples, so a specific calibration table provided by the manufacturer is required for use). ③ Output signal: DC millivolt signal corresponding to the temperature at the highest point of the cable. ④ Normal operating temperature: 80–880°C. ⑤ Extreme temperatures: -40–900°C. ⑥ Outer diameter: 2 mm or 3 mm; other diameters require customization in consultation with the factory. ⑦ Bending radius: 15–20 times the diameter of the armored cable. ⑧ Companion compensation wire: 2×0.5 mm2 – specialized high-temperature compensation wire or high-temperature shielded compensation wire (the calibration values for general-purpose K-type thermocouple compensation wires differ from those of specialized ones). ⑨ Output impedance of the thermocouple: The output impedance of a continuous thermocouple in operation is very high, so the signal cannot be directly connected to standard instruments/PLCs/DCS systems. It is required that the input impedance of the accompanying instruments be greater than 40 MΩ, and that they include a built-in calibration table for continuous thermocouples; standard K-type display instruments/temperature transmitters/system I/O cards are unable to detect this signal. ⑩Thermocouple calibration tables for heat search/Calibration tables for continuous thermocouples/Calibration tables for special temperature-sensing cables http://yunrun.com.cn/upload/202002/10/202002101327324106.png Installation of continuous thermocouples: 1. Method of installing continuous thermocouples on the surface of high-temperature equipment: ① Several nuts are welded to the surface of the equipment, which are used to install the all-stainless-steel fixing components for the continuous thermocouples ； ②Fully stainless steel fixing components are used to secure the pressure plate and packing ； ③The continuous thermocouple is pressed against the surface of high-temperature equipment by a pressure plate and adjusting screws ； ④The dedicated compensation wire is connected to the junction box through a conduit (the junction box contains terminal blocks, a continuous thermocouple temperature transmitter/continuous thermocouple data acquisition module, and grounding terminals). Surface temperature measurement in gasifiers is one of the typical applications of continuous thermocouples. 2. Two installation methods for continuous thermocouples: In ordinary thermocouples, the wire at the measurement end needs to be welded together. Since the point where the positive and negative terminals of the thermocouple are connected (i.e., the weld point) is fixed, temperature can only be measured through this connection point; therefore, ordinary thermocouples have a compensation wire connected to their cold end. If a break occurs in the thermoelectric element between the measuring end and the cold end of a conventional thermocouple, the thermocouple is damaged and can no longer be used. The wire strands of a continuous thermocouple are parallel to each other and not welded together. When the highest temperature occurs along the length of the cable, the two wire strands at that location are temporarily connected to form a joint similar to those found at the measurement ends of ordinary thermocouples. It is this feature that allows a continuous thermocouple to measure the temperature at the point where the highest temperature occurs, and it is precisely because of this characteristic that a continuous thermocouple can detect the highest temperature at any point along its cable length. Continuous thermocouples can have compensation wires connected at one or both ends of the temperature measurement cable (see Figures 1 and 2). Changhui Instruments will explain the differences between the two wiring methods in use: ① One end of the continuous thermocouple is connected to a compensation wire, as shown in Figure 1. This wiring method is exactly the same as that of ordinary thermocouples. When there is a break in one of the wire strands in the middle of the temperature sensing cable, the continuous thermocouple can measure the highest temperature along the length of the cable between the break point and the cold end; as a result, the length of the cable that can sense temperature is reduced. ②Compensation wires are connected to both ends of a continuous thermocouple, as shown in Figure 2. When there is one break in the filler wire at the middle of the temperature measurement cable, the continuous thermocouple can measure the highest temperature along the length of the cable, excluding the location of the break ； If there are 2 or more break points in the filler wire at the middle of the temperature measurement cable, the continuous thermocouple can measure the highest temperature along the length of the cable outside the positions of the two farthest break points, resulting in a reduction in the temperature-sensitive length of the cable. Connecting compensation wires at both ends of the temperature measurement cable to a continuous thermocouple is known as the continuous thermocouple redundancy structure. This wiring method is recommended for the continuous thermocouples produced by Changhui Instruments. Figure 1: One end of the continuous thermocouple is connected to a compensation wire. http://yunrun.com.cn/upload/202009/21/202009211622366838.png Figure 2: Both ends of the continuous thermocouple are connected to compensation wires (with redundancy). When ordering a continuous thermocouple, it is also necessary to order components for fixing it, such as clamps/fillers, cable trays, junction boxes, specialized compensation wires, as well as dedicated temperature transmitters/signal collectors. ◆The material of the continuous thermocouple mounting assembly is 304 steel; each set includes 1 threaded screw, 2 flat washers, 2 spring washers, and 2 nuts. The fixed component quantity equals the number of nuts welded on the field equipment, with a surplus of about 6% at the time of factory delivery. http://yunrun.com.cn/upload/202005/16/202005161314458498.png ◆ Press plates and fillers: A total of several meters of press plates and fillers are required (each turn is approximately equal to the circumference of the equipment’s surface). Industrial applications of continuous thermocouples: Distributed temperature monitoring and over-temperature alarm systems that use K-type armored continuous thermocouples as temperature sensing elements have been widely used in China’s coal chemical industry. They are primarily employed for monitoring the surface temperature of large-scale equipment such as gasifiers, as well as for providing over-temperature alerts. In addition to being used for temperature measurement on the surface of gasification furnaces, it can also be applied for measuring temperatures in coal storage bins, in polycrystalline silicon ingot casting furnaces, in air preheaters in thermal power plants, on boiler shells, and for monitoring the temperature in locomotive engine compartments. Changhui Instruments manufactures, according to customer requirements, complete systems that complement K-type armored continuous thermocouples (such as continuous thermocouple temperature transmitters, continuous thermocouple acquisition systems, K-type thermocouple compensation wires, high-temperature resistant connectors, mounting clamps, etc.), and provides on-site installation guidance services. Source: Changhui Instrument Network