In the field of industrial temperature measurement, movable thread mounted probe-type platinum resistance thermometers, movable thread mounted junction box type platinum resistance thermometers, and probe-type thermocouples with connecting tubes are three types of sensors designed for different working conditions. They have significant differences in structural design, working principle, installation method, and applicable scenarios. The following systematically explains their distinguishing features from a technical perspective.
I. Movable Thread Mounted Probe-Type Platinum Resistance Thermometer: A Precision and Stable Expert for Medium and Low Temperature Measurement
The movable thread mounted probe-type platinum resistance thermometer works based on the linear change of platinum metal resistance with temperature, conforming to the IEC 60751 international standard. Its core consists of high-purity platinum wire wound on a ceramic or mica skeleton, covered with a stainless steel protective tube and filled with high-insulation alumina powder, forming a rigid probe structure. This type is directly screwed into pipes or equipment interfaces through standard threads (such as M8, G1/2), allowing for quick disassembly and calibration. It is particularly suitable for medium and low-temperature environments (-200℃ to 600℃) requiring frequent maintenance and extremely high accuracy, such as chemical reactors and pharmaceutical sterilization systems. Its installation requires pre-machining threaded holes on the equipment wall, tightening with a wrench, and using thermal conductive grease or sealing gaskets to ensure airtightness. This design gives it the advantages of robustness, good sealing, and removable maintenance, but the response speed is relatively slow because heat needs to be transferred from the medium to the pipe wall and then to the sensing element, resulting in a certain thermal inertia. Its accuracy can reach ±0.15℃ (Class A), and it has excellent long-term stability with an annual drift of less than 0.05℃, making it a benchmark choice for industrial measurement and process control. The junction box is usually integrated into the tail of the probe, resulting in a compact structure, but it should be noted that its rigid structure may lead to performance degradation due to mechanical stress in strong vibration environments. II. Movable Threaded Connection Junction Box Type Platinum Resistance Thermometer: A Flexible Medium-to-High Pressure Temperature Measurement Solution
The movable threaded connection junction box type platinum resistance thermometer features a separate design with a movable thread and an independent junction box. The probe is connected to the equipment via a thread, while the junction box is fixed to the outer wall of the equipment via a thread or ferrule. This separate design facilitates on-site adjustment of the junction box position, adapting to complex installation environments. Its core component is also a platinum resistance element, encapsulated in a protective tube, suitable for medium-to-high pressure environments (such as steam pipelines and chemical reactors), and capable of withstanding high pressure. The flexibility of the movable thread facilitates maintenance and calibration. Its response speed is between that of the movable threaded probe type and the fixed flange type, due to the longer heat transfer path, but it is superior to the movable threaded probe type. The accuracy is comparable to the movable threaded probe type, and it offers excellent long-term stability, making it suitable for applications requiring precise measurement in high-pressure environments. The junction box is typically explosion-proof or waterproof, facilitating signal output and protection, and its independent design reduces the impact of equipment vibration on the wiring.
III. Probe-Type Thermocouple with Connecting Tube: A Convenient Option for High-Temperature, Fast Response
The probe-type thermocouple with a connecting tube is entirely based on the Seebeck effect. It consists of two different metal conductors (such as nickel-chromium and nickel-silicon for K-type) welded together at the measuring end to form a hot junction. It does not require an external power supply, relying on the temperature difference to generate a microvolt-level thermoelectric potential for temperature measurement. Its structure consists of thermocouple wires, a metal protective tube (such as stainless steel or ceramic), a connecting tube, and a junction box. The protective tube is fixed inside the equipment through the connecting tube, allowing for quick installation and removal. The connecting tube design makes installation extremely simple, suitable for applications requiring frequent replacement or maintenance. This design results in a faster response speed, with a thermal response time as low as 0.5 seconds, suitable for high-temperature, rapidly changing temperature environments such as steelmaking furnaces, glass melting furnaces, and gas turbines (up to 1800°C). Its simple installation method, with the connecting tube ensuring airtightness, makes it suitable for medium-to-low pressure environments. Compared to platinum resistance thermometers, thermocouples are less expensive and have a wider measurement range, but their accuracy is relatively lower, with a typical tolerance of ±1.5°C (Class II), and they are susceptible to electromagnetic interference, requiring the use of a cold junction compensation circuit. The junction box is usually splash-proof or waterproof, facilitating signal output and protection.
IV. Core Differences and Application Selection
The essential differences among the three are: the movable threaded platinum resistance thermometer relies on resistance changes, prioritizing accuracy and stability; the movable threaded junction box type platinum resistance thermometer adds the flexibility of a junction box to this; and the probe-type thermocouple with a connecting tube relies on thermoelectric potential, prioritizing speed and convenient installation. When making a selection, a comprehensive assessment should be made based on temperature range, pressure requirements, ease of installation, and maintenance frequency: Movable threaded platinum resistance thermometers are suitable for precise measurements at medium and low temperatures; movable threaded junction box type platinum resistance thermometers are suitable for medium and high pressure applications requiring flexible adjustment; and probe-type thermocouples with connecting tubes are suitable for high-temperature applications requiring rapid response and frequent maintenance. Understanding these differences can prevent misuse caused by confusion regarding appearance or installation methods, ensuring the efficiency and safety of the temperature measurement system.
