Non-Metallic Protective Tube Junction Box Type Thermocouple
Structural Essence: Composed of a high-purity alumina (Al₂O₃≥99%) or fused silica (SiO₂) ceramic protective tube, a mandatory sealed junction box, and a rigid flange connector. These three components form an inseparable integral structure with no detachable or moving parts.
Core Features:
Must be equipped with a junction box for insulation, moisture sealing, and wire terminal fixing; omitting the junction box violates GB/T 30429-2013;
The protective tube is a single-piece sintered ceramic, without metal cladding, completely eliminating the risk of metal ion leaching, meeting semiconductor-grade (≤0.1 ppb) purity requirements;
Unique installation method: rigidly fixed by flange bolts; any form of plugging, screwing, or bare insertion is strictly prohibited.
Thermodynamic Behavior: Extremely low thermal expansion coefficient (≈5×10⁻⁶/K), relying on a pre-reserved ≥5mm axial gap to absorb thermal stress, without a compensation mechanism, resulting in a completely rigid structure.
Insulation Performance: Intrinsic insulation resistance ≥10¹² Ω·cm, completely blocking electromagnetic interference (EMI), suitable for strong interference environments such as frequency converters and induction furnaces.
Application Scenarios:
Semiconductor CZ method single crystal pulling
Glass kiln melting zone temperature measurement
Molten salt thermal storage systems (NaNO₃/KNO₃)
High-purity catalyst reactors
Exposed Thermocouple
Structural Essence: Refers to a metal thermocouple without a protective tube or junction box, where the thermoelement measuring junction is directly exposed. It consists only of two different metal wires (e.g., K-type: NiCr/NiAl) welded together, without any encapsulation, insulation sleeve, or external casing.
Core Features:
Completely without junction box, wire ends are directly exposed, without sealing or protection;
No protective tube, the thermocouple electrodes are in direct contact with the measured medium, making them susceptible to corrosion, wear, and oxidation;
Insulation relies on air gaps, preventing short circuits solely through the spacing between wires, without ceramic or magnesium oxide insulation layers.
Installation Method: Directly inserted into the measured medium (such as liquid, gas, or solid surface), without flanges, threads, or fixing devices, suitable for temporary, manual, and low-frequency measurements.
Thermal Management Mechanism: Fast response speed, but only suitable for temperatures ≤300℃ (such as E-type and T-type thermocouples), strictly prohibited for use in high-temperature, high-pressure, or corrosive environments.
Application Scenarios:
Rapid temperature measurement in laboratories (e.g., beakers, condensers)
Temporary calibration of household equipment such as ovens and constant temperature water baths
Surface temperature inspection of electronic components
Teaching demonstrations and prototype verification
Key Differentiation Matrix
|
Dimension |
Non-metallic Protective Tube with Junction Box |
Exposed Thermocouple |
|
Does it include a protective tube? |
Must have, alumina/quartz ceramic |
Completely absent, bare thermoelements |
|
Does it include a junction box? |
Mandatory, cannot be omitted |
Completely absent, direct wire output |
|
Protective tube material |
High-purity alumina / fused silica |
None |
|
Maximum operating temperature |
1600–1700℃ |
≤300℃ |
|
Installation method |
Rigid flange bolt only |
Exposed insertion only, no fixing device |
|
Is contact with the medium allowed? |
Direct contact is prohibited, protective tube provides isolation |
Must be in direct contact, no barrier |
|
Insulation mechanism |
Intrinsic ceramic insulation (≥10¹² Ω·cm) |
Air insulation, susceptible to humidity |
|
Resistance to metal contamination |
No ion precipitation, semiconductor-grade purity |
No risk of metal contamination (because there is no metal tube), but the wires may oxidize |
|
Resistance to mechanical damage |
High (ceramic is impact resistant) |
Extremely low (wires are easily broken and oxidized) |
|
Safety risks |
Thermal shock cracking (if misused during insertion) |
Poor contact, short circuit, electric shock, contamination |
|
Industry Standards |
GB/T 30429-2013 (mandatory junction box + flange) |
IEC 60584-1 (allows bare wire structure) |
Core Conclusion:
"Exposed thermocouples" are bare wire structures without any encapsulation, while non-metallic thermocouples must have a ceramic protective tube and a mandatory junction box. The two are completely mutually exclusive in structure and cannot be confused.
"Alumina bare head" or "ceramic exposed type" are false concepts; there are no compliant non-metallic exposed thermocouples. Any such product is an illegal modification and a serious violation, and its use is strictly prohibited.
Selection Rule of Thumb: Safety and Compliance > Temperature Requirements > Measurement Convenience.
In processes above 1400℃ with zero tolerance for metal contamination, a non-metallic sensor + junction box + flange is the only legitimate solution;
In temporary, non-critical laboratory or commissioning scenarios at ≤300℃, exposed thermocouples are an economical choice, but they are not suitable for continuous industrial operation.
Terminology Clarification: "Exposed type" ≠ "without protection tube" ≠ "without junction box" – these three are necessary attributes of the same structure, and none can be omitted. Misuse will lead to three risks: short circuit, contamination, and measurement failure.
