What is a screw-type thermocouple

A "screw-type thermocouple" is a common term in industrial settings for a threaded, assembled thermocouple. Its core characteristic is not a distinct sensor type, but rather a temperature sensing device that is mechanically secured by being directly screwed into a pre-existing threaded hole in the equipment via an external threaded interface. It is one of the installation forms of assembled thermocouples, conforming to the international standards IEC 60584 and ASTM E230, and is widely used in low-to-medium pressure, small-diameter applications requiring frequent maintenance.

 

Structure and Material System

Screw-type thermocouples consist of four core components:

Thermocouple Element : Uses standardized alloy wires, such as K-type (nickel-chromium-nickel-silicon) , E-type, or J-type, generating thermoelectric potential based on the Seebeck effect;

Insulation Layer: High-purity magnesium oxide powder is filled inside the protective tube to ensure electrical insulation (insulation resistance at room temperature ≥100MΩ) and efficient heat conduction;

Protective Tube: Commonly uses 304 stainless steel or 316L stainless steel, which is corrosion-resistant and resistant to mechanical shock, suitable for sanitary environments such as chemical and food industries;

Threaded Interface: Standard external thread, common specifications are M27×2, M20×1.5, or 1″NPT, achieving rigid engagement with the equipment's threaded hole.

Some models use an armored structure, where the thermocouple element, insulation layer, and metal sheath are integrally formed by drawing, improving vibration resistance and response speed (thermal response time ≤10s).

 

Model Naming Rules and Industry Codes

Domestic industrial models commonly follow the JB/T 8622 standard naming system, with a typical format of:

WRNK-230

W: Temperature instrument

R: Thermocouple

N: Nickel-chromium-nickel-silicon (K-type)

K: Armored type (optional)

2: Fixed thread mounting

30: Thread specification M27×2

Other common models include:

WZP-230 (resistance thermometer, same structure)

WRNT-02 (imitation imported type, M6×1.5 thread)

WRNKD2-04B (explosion-proof type)

 

Structural Variations and Functional Extensions

Type	Features	Application Scenarios
Standard Type	Straight rod type, with ordinary junction box	General pipeline and equipment surface temperature measurement
Armored Type	Flexible metal sheath, resistant to bending	Environments with high vibration and limited space
Right-Angle Type	90° elbow design	Compact space, wall-mounted
Explosion-proof type	Explosion-proof junction box (Ex d IIC T6)	Petrochemical industry, flammable and explosive areas
High-temperature type	Protective tube made of GH3030 or corundum ceramic	Vacuum sintering furnace, semiconductor annealing (up to 2200℃)

 

Technical Parameters and Performance Limits

Parameter	Typical Value	Description
Temperature Measurement Range	-200℃ ~ 800℃ (K-type)	Platinum-rhodium type can reach 1600℃, but rarely used for threaded type
Accuracy Class	Class I: ±1.5℃ or ±0.4%	Complies with IEC 60584-2
Protection Class	IP65	Dustproof and waterproof, suitable for humid environments
Pressure Resistance	≤10MPa	Depends on thread strength, not suitable for high-pressure main circuits
Response Time	≤10s	Armored type can be shortened to ≤2s
Installation Torque	M27×2: 40–60 N·m	A torque wrench must be used; brute force is prohibited

 

Comparison with Similar Installation Methods

Installation Method	Connection Mechanism	Applicable Scenarios
Screw-in type (threaded)	Threaded connection	Small diameter, medium and low pressure, removable
Flange type	Flange bolt fastening	Large diameter, high pressure, continuous operation
Welded type	Permanent welding	No leakage requirements, non-removable
Compression fitting type	Compression fitting + nut locking	Requires adjustment of insertion depth

 

Special Working Condition Applications

Explosion-proof environment: Uses explosion-proof junction box (such as JWB/P series), meeting Ex d IIC T6 Explosion-proof rating, suitable for refineries and chemical reactors;

Vacuum/inert atmosphere: Uses tungsten-rhenium thermocouples (W-5Re/W-26Re), with the protective tube filled with inert gas to prevent high-temperature oxidation, suitable for semiconductor wafer annealing furnaces;

Hygienic applications: Protective tube with mirror polishing (Ra≤0.4μm), compliant with GMP standards, used in pharmaceutical sterilization autoclaves and food filling lines.

The essence of a screw-type thermocouple is its "installation method" rather than its "sensing principle." Its value lies in achieving accurate contact temperature measurement at a low cost and with high maintainability, making it one of the most widely deployed basic sensing units in industrial automation systems.