Non-Metallic Protection Tube Junction Box Type Thermocouple
Structural Essence: Uses high-purity alumina (Al₂O₃≥99%) or fused silica (SiO₂) ceramic protection tubes, integrally sintered and non-removable. The junction box is fixed by rigid flange bolts, forming a stress-free rigid system.
Core Features: Must be equipped with a junction box for sealing, insulation, and wire connection; omission or simplification is prohibited. Any "without junction box" design violates GB/T 30429-2013 and constitutes a serious safety hazard.
Installation Method: Limited to rigid flange connection; direct insertion, screw-in, or exposed probe installation is absolutely prohibited. Ceramic materials cannot withstand mechanical stress; forced direct insertion will cause thermal shock cracking.
Thermodynamic Behavior: Extremely low thermal expansion coefficient (≈5×10⁻⁶/K), relying on a pre-reserved ≥5mm axial gap to absorb thermal deformation, without any compensation structure.
Application Scenarios: Extreme processes with zero tolerance for metal contamination and temperatures ≥1400℃, such as semiconductor CZ crystal growth, glass furnaces, molten salt thermal storage, and high-purity chemical reactors.
Simplified Probe Type Thermocouple
Structural Essence: Refers to metal thermocouples without a junction box, where the probe is directly exposed or simply encapsulated. The protection tube is made of metal materials such as 316L or Inconel, with an exposed measuring end or a short tubular probe at the end, and no independent junction box component.
Core Features: Omits the junction box; the wires are directly led out from the end of the protection tube or connected through a simple insulating sleeve. The structure is compact and low-cost, suitable for temporary, low-frequency, and non-harsh environment temperature measurement.
Installation Method: Uses direct insertion or screw-in method, which can be directly inserted into the pre-reserved hole of the equipment, without the need for flanges or bolts, supporting quick replacement.
Thermal Management Mechanism: Thermal stress is borne by the metal tube body, suitable for conventional operating conditions with temperatures ≤1300℃ and without severe thermal cycling.
Application Scenarios:
Laboratory rapid temperature measurement
Household appliances such as ovens, fireplaces, and gas stoves
Temporary calibration of injection molding machine barrels
Temporary spot measurement during on-site inspection
Key Differentiation Matrix
Dimension | Non-metallic Protective Tube with Junction Box | Simple Probe Type |
Does it include a junction box? | Must be equipped, cannot be omitted | Completely without a junction box, direct wire output |
Protective tube material | Corundum / Quartz (ceramic) | 316L stainless steel / Inconel 600 (metal) |
Maximum operating temperature | 1600–1700℃ | ≤1300℃ |
Installation method | Rigid flange only, direct insertion prohibited | Direct insertion/threaded insertion only, flange prohibited |
Structural detachability | Single-use, not repairable | Replaceable probe, supports repeated use |
Insulation performance | ≥10¹² Ω·cm, intrinsic insulation | Dependent on magnesium oxide/coating, ≤10⁹ Ω·cm |
Resistance to metal contamination | No ion precipitation, semiconductor-grade purity | Metal dissolution exists, prohibited for high-purity processes |
Safety risks | Thermal shock cracking (if direct insertion is misused) | Poor contact, insulation failure, short circuit |
Industry Standards | GB/T 30429-2013 (mandatory junction box) | IEC 60584-2 (allows structure without junction box) |
Core Conclusion:
The "simple probe type" is a metal thermocouple without a junction box, while non-metallic thermocouples must be equipped with a junction box; the two are completely mutually exclusive in structure.
Non-metallic thermocouples must never omit the junction box. Any "corundum bare head" or "ceramic direct insertion" product is an illegal and counterfeit product, posing an explosive safety risk. "Simplified" ≠ "Low-cost" ≠ "Universal" – only suitable for non-critical, low-temperature, and non-corrosive environments. Strictly prohibited for high-purity, high-temperature, and continuous operation processes.
Selection Ironclad Rule: Safety and Compliance > Temperature Requirements > Cost and Convenience. In scenarios above 1400℃ and with metal ion concentration ≤0.1ppm, non-metal + junction box + flange is the only legitimate solution; in scenarios ≤1300℃ and for temporary temperature measurement, metal + no junction box + direct insertion is the economical choice.
Terminology Clarification: A "junction box" is a mandatory component of non-metal thermocouples, while "without a junction box" is a simplified form of metal thermocouples – the two should not be confused, as misuse will lead to three types of accidents: contamination, short circuit, and explosion.
