Structural Differences: The movable threaded connection type thermocouple consists of three parts: an independently removable metal protective tube, a rotating threaded interface with sealing function, and a sealed junction box. The thermoelements are insulated from the protective tube by insulating material, and the signal is led out through the junction box terminals, achieving both mechanical and electrical separation. The snap-ring type thermocouple, however, is an integrated structure without a junction box. The thermoelements are encapsulated within a metal sheath, and the tail end has a built-in elastic spring mechanism. The spring force forces the sensing end against the measured surface. The leads are either directly exposed or covered with an insulating layer, without any rotating, detachable, or sealing units, making it a single-use temperature sensing device where the probe itself is the contact element.
Installation Method and Operation Logic: The movable threaded type requires manual screwing into a pre-set threaded hole on the equipment. After installation, the probe can still be freely rotated 360° to precisely align with the temperature measurement point, adapting to curved pipe walls, spatial interference, or installation deviations. Torque control is used to ensure sealing. The snap-ring type uses non-invasive compression installation. The probe only needs to be pressed against the measured surface (such as injection molding machine screws, mold walls, or pipe outer walls). The spring is compressed by manually pressing or tightening the retaining ring to achieve thermal contact. Operation is extremely simple, but it is entirely dependent on the flatness and cleanliness of the measured surface. Any oil, rust, or unevenness will lead to temperature measurement lag or errors, and the angle or position cannot be adjusted after installation.
Maintenance and Replacement Capabilities: The movable threaded type allows for the replacement of only the protective tube and sensing probe. The junction box, cable, and seals can be reused for a long time. Single maintenance takes 5-10 minutes, suitable for periodic calibration and fault isolation. For the snap-ring type, once the spring fatigues, the sheath oxidizes, the leads break, or contact is poor, the entire probe must be scrapped and replaced. There are no detachable or repairable parts, making it a disposable design. Replacement frequency is high, and maintenance costs are concentrated on frequent purchases.
Environmental Adaptability and Reliability: The movable threaded type, with its metal protective tube and IP65 or higher sealed junction box, can withstand high temperatures (up to 1200°C), high pressure, strong vibrations, condensation, and cleaning environments, making it suitable for continuously operating industrial systems. Although the sheath material of the spring-loaded type is mostly stainless steel or nickel-based alloy, and the rated temperature limit is usually ≤400°C, it lacks electrical shielding and mechanical support structures. Long-term thermal cycling can easily lead to spring failure, increased contact resistance, and detachment of the sensing end. It has weak resistance to impact and fluid erosion, and is only suitable for medium-low temperature, static, and low-disturbance surface temperature measurement scenarios, such as injection molding machines, extruders, hot runner plates, and packaging sealing strips.
Typical Application Scenarios: The movable threaded type is widely used in demanding environments requiring long-term stability, maintainability, and industrial compliance, such as pharmaceutical GMP reactors, food CIP cleaning lines, nuclear power auxiliary pipelines, and chemical high-pressure reactors. The spring-loaded type is mainly used in surface temperature acquisition scenarios requiring rapid deployment, non-invasive monitoring, and temporary measurement, such as injection molding equipment, plastic extruders, food processing heating rollers, rubber vulcanizing machines, and laboratory furnaces, especially for equipment inspection where drilling or shutdown is not possible.
Standards and Specifications: The movable threaded type strictly adheres to GB/T 30429-2013 "Replaceable Sheath" and IEC 60584-3 "Removable Installation" specifications, possessing standardized interfaces and traceability. The spring-loaded type has no independent national standard; its design and performance are based on the "Surface Type Thermocouple" clause in GB/T 16839.1-2021, and it is a non-standard customized product. There are no unified specifications for spring pressure, sheath length, and lead wire material among different manufacturers, resulting in poor interchangeability.
Core Selection Decision: If long-term operation, repeatable maintenance, vibration resistance, and industrial compliance are required, a movable threaded type must be chosen. If only non-invasive rapid temperature measurement, temporary inspection, and situations where drilling or shutdown are not possible are needed, and the measured surface is flat and clean with a moderate temperature, the spring-clip type is an economical and efficient surface temperature measurement solution.
Operation Precautions: For the movable threaded type, welding or forcibly fixing the junction box is strictly prohibited, otherwise its core advantages will be completely lost. For the spring-clip type, long-term use in high-temperature, high-pressure, severe vibration, or fluid erosion environments is prohibited, and using excessive torque to tighten is strictly prohibited, otherwise it will lead to sheath deformation, thermocouple breakage, or insulation layer damage, affecting temperature measurement accuracy and safety.
