Core Structure Differences
|
Feature Dimension |
Non-metallic Protective Tube Thermocouple |
Screw-in Thermocouple |
|
Overall Structure |
Separate design: The thermoelements are encapsulated in a rigid non-metallic protective tube (such as alumina, quartz), connected to a separate junction box via high-temperature resistant cable, with the signal output physically separated from the sensing end. |
Integrated design: The thermoelements are embedded in a metal or ceramic sheath, with an integrated threaded interface at the end. There is no external junction box, and the signal wires are directly led out from the tail. |
|
Protective Tube Material |
High-purity alumina ceramic (Al₂O₃), quartz (SiO₂), zirconia (ZrO₂), insulation resistance >10⁹ Ω·cm, chemical corrosion resistance, no metal ion precipitation. |
Typically 304/316L stainless steel, Inconel 600, or armored ceramic-coated structure, balancing mechanical strength and thermal conductivity, allowing for trace metal contact. |
|
Installation Method |
Inserted into the equipment opening through a flange, bracket, or threaded sleeve. The protective tube is fixed at the temperature measurement point, and the junction box is suspended externally. |
Directly screwed into the equipment's threaded interface (e.g., M27×2, 1/2"NPT). The probe is rigidly connected to the equipment body, requiring no drilling, and installation is complete upon screwing in. |
|
Signal Output |
Signal is led out from a separate junction box via shielded cable, requiring additional wiring. The junction box has an IP65 or higher protection rating. |
Signal wires are directly led out from the probe tail or integrated with a plug-in terminal. No separate junction box, compact structure, strong anti-interference ability. |
|
Heat Conduction Path |
Heat needs to penetrate the non-metallic insulation layer, resulting in slow heat conduction and a response time >3s, suitable for steady-state temperature measurement. |
Heat is directly conducted through the metal sheath, resulting in low thermal inertia and a response time <1s, suitable for dynamic temperature monitoring. |
Material Performance and Temperature Limit
Non-metallic Protective Tube Material Characteristics:
Alumina Ceramic: Maximum operating temperature 2050℃, suitable for glass melting furnaces, ceramic sintering kilns, and high-temperature reactors.
Quartz glass tube: Softening point 1730℃, resistant to all acidic media except hydrofluoric acid, used in high-purity semiconductors and pharmaceutical processes.
Zirconia ceramic: Strong thermal shock resistance, suitable for molten steel and molten metal temperature measurement (1600–1700℃), but not resistant to reducing atmospheres.
Screw-type thermocouple material characteristics:
304/316L stainless steel sheath: Temperature resistance 800℃, resistant to chloride ion corrosion, suitable for chemical pipelines, injection molding machines, and food machinery.
Inconel 600: Temperature resistance 1100℃, resistant to sulfide stress corrosion, used in gas turbines and boiler outer walls.
Armored structure (MgO insulation): Outer diameter can be as small as 0.5mm, fast response, suitable for confined spaces, bearings, and mold surface temperature measurement.
Installation Specifications and Engineering Error Control
Non-metallic protective tube type:
The insertion depth should be ≥ 8–10 times the outer diameter of the protective tube, otherwise serious thermal hysteresis will occur.
The junction box must be sealed and moisture-proof (IP65+), otherwise the insulation resistance will drop to <10MΩ, leading to signal drift.
Calibration is based on JJF 1637-2017, and comparative calibration must be performed in a tube furnace, with ≥4 forward and reverse cycles.
Screw type:
Clean the threaded interface before installation, and use high-temperature sealant or metal spiral wound gaskets to prevent media leakage.
The tightening torque must comply with equipment specifications (e.g., M27×2 recommended 40–50 N·m), overtightening can easily cause sheath deformation or thermocouple breakage.
Typical error sources: Thread gap thermal resistance, sheath wear caused by media erosion, and thermal expansion stress accumulation.
Typical Application Scenario Comparison
|
Application Scenario |
Recommended Type |
Reason |
|
Temperature measurement in acidic atmosphere inside glass melting furnace |
Non-metallic protective tube type |
Ceramic does not release metal ions, avoiding glass discoloration and contamination. |
|
Temperature measurement of highly corrosive media in chemical reactors |
Non-metallic protective tube type |
Avoids metal contamination, resistant to strong acids and bases. |
|
Boiler steam pipeline wall temperature monitoring |
Screw-in type |
Quick installation, vibration resistant, good sealing, no need to shut down. |
|
Gas turbine combustion chamber outlet temperature |
Screw-in type |
Metal sheath resistant to high temperature and pressure, fast response, resistant to thermal shock. |
|
Injection molding machine mold surface temperature control |
Screw-in type |
Threaded screw-in installation, fast response, suitable for frequent start-stop cycles. |
|
Laboratory standard temperature calibration |
Non-metallic protective tube type |
Removable, easy to replace, conforms to metrological specifications. |
Engineering Selection Decision Tree
Select Non-metallic Protective Tube with Terminal Box:
Medium is highly corrosive (acid, alkali, molten salt)
Requires no metal contamination (food, pharmaceuticals, semiconductors)
Temperature >1200℃ and no mechanical shock
Not suitable for high pressure, vibration, or applications requiring fast response
Select Screw-in Thermocouple:
Requires online installation and replacement without shutdown
Equipment has standard threaded interface (such as NPT, M27)
Presence of vibration, high pressure, and rapid temperature changes
Not suitable for highly corrosive non-metallic media (such as HF, molten sulfur)
