In the field of industrial temperature measurement, movable threaded probe-type platinum resistance thermometers, fixed flange-mounted junction box type platinum resistance thermometers, and non-metallic protective tube junction box type thermocouples are three types of sensors designed for different working conditions. They have significant differences in structural design, working principles, installation methods, and applicable scenarios. The following systematically explains their distinguishing features from a technical perspective.
I. Movable Threaded Probe-Type Platinum Resistance Thermometer: A Precise and Stable Expert for Medium and Low Temperature Measurement
The movable threaded 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 frame, 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 using 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 ease of 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 some thermal inertia. Its accuracy can reach ±0.15℃ (Class A), with excellent long-term stability and an annual drift of less than 0.05℃, making it a benchmark choice for industrial measurement and process control. The junction box is usually a simple type, facilitating signal output and protection, but it should be noted that its rigid structure may experience performance degradation due to mechanical stress in strong vibration environments.
II. Fixed Flange Mounted Junction Box Type Platinum Resistance Thermometer: A Stable Temperature Measurement Solution for Medium and High-Pressure Environments
The fixed flange mounted junction box type platinum resistance thermometer features a fixed flange design, directly connecting to the equipment via the flange, ensuring sealing and mechanical strength in high-pressure environments. Its core is a platinum resistance element, encapsulated in a protective tube. The flange is non-movable and fixed to the equipment with bolts. This design makes it suitable for medium and high-pressure environments (such as steam pipelines and chemical reactors) and capable of withstanding high pressures. The stability of the fixed flange facilitates long-term fixed temperature measurement. The response speed is between that of the movable threaded and movable flange types, due to the longer heat transfer path, but superior to the movable threaded type. The accuracy is comparable to the movable threaded type, and it offers excellent long-term stability, making it suitable for scenarios requiring precise measurement in high-pressure environments. The junction box is usually explosion-proof or waterproof, facilitating signal output and protection, and its double protection design effectively reduces measurement errors caused by pressure fluctuations.
III. Non-Metallic Protective Tube Junction Box Type Thermocouple: A Fast Response Option for Highly Corrosive Environments
The non-metallic protective tube junction box type thermocouple is entirely based on the Seebeck effect, consisting 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 excitation power supply and relies on the temperature difference to generate a microvolt-level thermoelectric potential for temperature measurement. Its structure consists of thermocouple wires, a non-metallic protective tube (such as ceramic or PTFE), and a junction box. The protective tube's non-metallic material provides corrosion resistance, making it particularly suitable for highly corrosive media environments such as strong acids and bases. This design results in a faster response speed, with a thermal response time as low as 0.5 seconds, suitable for chemical, pharmaceutical, and other highly corrosive environments. Installation is typically done via threads or flanges, but the flexibility of the non-metallic protective tube requires avoiding excessive bending during installation to prevent tube breakage. Compared to platinum resistance thermometers, thermocouples are inexpensive and have a wide measurement range, but their accuracy is relatively lower, with a typical tolerance of ±1.5℃ (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. However, it's important to note that its temperature resistance range is typically lower than that of metal-sheathed thermocouples, making it suitable for medium-to-low temperature corrosive environments.
IV. Core Differences and Application Selection
The fundamental differences among the three are: the movable threaded platinum resistance thermometer relies on resistance changes, prioritizing accuracy and stability; the fixed flange platinum resistance thermometer adds stability in high-pressure environments; and the non-metallic sheathed thermocouple relies on thermoelectric potential, prioritizing reliability and corrosion resistance in highly corrosive environments. When choosing, a comprehensive assessment should be made based on the temperature range, pressure requirements, environmental corrosiveness, and maintenance frequency: Movable threaded platinum resistance thermometers are suitable for precise measurements in medium-to-low temperature ranges; fixed flange platinum resistance thermometers are suitable for medium-to-high pressure environments; and non-metallic sheathed thermocouples with junction boxes are suitable for highly corrosive environments. Understanding these differences can prevent misuse caused by confusion regarding appearance or installation methods, ensuring the efficiency and safety of the temperature measurement system.
