In the field of industrial temperature measurement, metal tube probe with junction box type platinum resistance thermometers and fixed flange type probe platinum resistance thermometers represent two fundamentally different technological approaches, with significant differences in their design philosophy, material selection, and application scenarios. Understanding these differences is crucial to avoiding selection errors and ensuring the reliability of the measurement system.
I. Structural Essence: Modular and Maintainable vs. Integrated and Fixed
The core of the metal tube probe with junction box type is its modular maintainability. Its structure consists of three parts: a metal protective tube (usually 316L stainless steel or Inconel 625), an internal high-purity magnesium oxide (MgO) insulation layer, and an independent junction box module. The junction box is connected to the probe via threads or clips, forming a separable electrical interface. This design gives it two major advantages:
Online replacement capability: When the sensor ages or the lead wires are damaged, only the probe needs to be unscrewed and replaced with a spare part; the junction box and cable can be reused. This design keeps the mean time to repair (MTTR) within 15 minutes, greatly reducing downtime losses in continuous production systems.
Electrical isolation: The junction box separates the high-temperature area from the control room, preventing thermal radiation from damaging the instrument, and simultaneously achieves a Faraday cage effect through grounding bolts, shielding against electromagnetic interference.
The fixed flange type probe adopts an integrated fixed design, where the metal protective tube and flange are directly welded or mechanically connected, forming a non-removable integral structure. The flange interface is usually ANSI 150# or DIN PN16 standard, and sealing is achieved through a metal gasket. This design gives it extremely strong mechanical stability, suitable for high-pressure, high-flow rate, or high-vibration environments, but maintenance requires overall replacement, making local repair impossible.
II. Performance Boundaries: Extreme Challenges of Temperature and Pressure
In terms of temperature resistance, the metal tube type has significant advantages:
Long-term operating temperature can reach 850℃, and can withstand over 1000℃ for short periods, suitable for extreme thermal environments such as power plant boilers and cracking furnaces. The operating temperature range of fixed flange type sensors is typically -50℃ to 850℃, but the flange sealing material (such as graphite gaskets) may age at high temperatures and require regular replacement.
In terms of pressure resistance, the fixed flange type is superior:
It can withstand high pressures of 10–20 MPa, suitable for high-pressure environments such as steam pipelines and reactors.
The threaded connection of metal tube type sensors has weaker pressure resistance, usually ≤6.0 MPa, and ≤4.0 MPa when using PTFE gaskets with flange connections.
In terms of mechanical impact resistance, the fixed flange type, through the rigid connection between the flange and the pipe, can withstand high-speed fluid scouring of 5–10 m/s and mechanical vibration; while the metal tube type relies on the strength of the protective tube itself, and its impact resistance is relatively weaker.
III. Application Scenarios: Heavy Industry vs. Chemical and Energy Industries
The metal tube probe with junction box type is the exclusive choice for heavy industry, suitable for:
Boiler heating surfaces and turbine cylinders in thermal power plants, requiring resistance to high temperatures of 850℃ and steam pressure of 10 MPa.
Cracking furnaces and hydrogenation reactors in the petrochemical industry, requiring resistance to highly corrosive media and mechanical vibration.
Cooling systems in nuclear power plants, requiring long-term reliability in extreme environments.
The fixed flange type probe is more suitable for high-pressure scenarios in the chemical and energy industries:
Distillation columns and high-pressure reactors in the chemical industry, requiring resistance to pressures of 10–20 MPa and highly corrosive media.
Steam pipelines and heat exchangers in the energy industry, requiring long-term stable operation under high temperature and high pressure.
IV. Selection Principles: Maintenance Convenience vs. Structural Strength
Choose the metal tube probe with junction box type: When you are facing environments with high temperature, high pressure, high flow rate, and strong vibration, and maintenance convenience is a priority, it is the only guarantee.
Choose the fixed flange type probe: When you need extremely high structural strength and sealing performance, and the maintenance frequency is low, it is the better choice. V. Maintenance Logic: Repairable Assets vs. Disposable Consumables
The core value of the metal tube type lies in its maintainability:
The separate design of the junction box and probe allows for partial replacement, reducing long-term maintenance costs.
The independent grounding and shielding design of the junction box ensures signal stability.
The fixed flange type, on the other hand, tends towards single-use:
Overall replacement leads to accumulating maintenance costs.
Flange seals require regular inspection to prevent leaks.
VI. Summary: The Ultimate Trade-off in Engineering Value
Both meet the IEC 60751 Pt100 accuracy standard, but the structure defines the functional boundaries:
The metal tube type is a "maintainable fortress" for heavy industry, resisting extreme environments with its modular design.
The fixed flange type is a "high-pressure guardian" for chemical and energy industries, achieving long-term sealing with its integrated design.
Choosing the wrong model can be costly: using a fixed flange type in high-temperature steam pipelines will lead to extended downtime due to inconvenient maintenance; using a metal tube type in a chemical high-pressure reactor will cause leakage accidents due to insufficient sealing. Understanding the underlying logic is crucial to achieving maximum reliability at minimum cost.
VII. Application Case Comparison
Metal Tube Type Case: Temperature measurement in a power plant boiler superheater, using a metal tube probe inserted into a junction box. It operates stably at 850°C and 10 MPa steam pressure. During maintenance, only the probe needs to be replaced, reducing system downtime to 15 minutes.
Fixed Flange Type Case: Temperature measurement in a chemical plant distillation column, using a fixed flange type probe. It provides long-term sealing under 20 MPa pressure and in highly corrosive media, but requires complete sensor replacement every two years.
