I. First Choice: Determine the Calibration Type Based on the Temperature Range (Core Basis)
The "calibration type" of a thermocouple determines its temperature measurement capability, which is the first step in selection.
0-800℃ Common industrial applications → Prioritize K-type (nickel-chromium-nickel-silicon)
High cost-effectiveness and good stability, suitable for temperature measurement in most heating equipment, pipelines, motors, etc.
800-1300℃ High-temperature environments → Choose N-type or S-type
N-type has strong oxidation resistance and is suitable for long-term high-temperature operation; S-type has high accuracy and is often used in laboratories or calibration applications.
1300-1800℃ Ultra-high temperature → Choose B-type (platinum-rhodium 30-platinum-rhodium 6)
No compensation wires are needed, the thermoelectric potential is stable, suitable for extreme high-temperature environments such as kilns and metallurgy.
Below 400℃ Medium and low temperatures → E-type or T-type
E-type has high sensitivity and is suitable for fast response; T-type has excellent stability at low temperatures and is often used in food and pharmaceutical refrigeration.
Tip: If the temperature fluctuates significantly or requires long-term stable operation, it is recommended to choose an armored structure to improve durability.
II. Second Choice: Assess Material Suitability Based on the Operating Atmosphere
Different media environments have corrosive or reducing effects on thermocouple materials, so careful selection is necessary.
Oxidizing atmosphere (such as air, oxygen): K-type, S-type, and B-type perform well.
Reducing atmosphere (such as hydrogen, carbon monoxide): Avoid using S-type and B-type; N-type or a dense protective tube can be used.
Corrosive media (such as acid and alkali vapors): The protective tube should be made of 316 stainless steel or Hastelloy, and the sealing structure must be intact.
Vacuum or inert gas environment: K-type and S-type can be used, but attention should be paid to the possibility of metal evaporation affecting the lifespan.
III. Structure and Installation Form: Ensure Physical Compatibility
Even if the calibration type is correct, it's useless if it can't be installed. The installation interface and space limitations must be confirmed. Fixed Thread Type is the most common:Standard thread is M27×2, but M20×1.5, G1/2, etc., are also common.
Special Note: M27×2 is similar in outer diameter to the imperial G3/4′, which can easily cause confusion. Always verify before welding, otherwise installation will be impossible.
Protection Tube Diameter and Length:** Usually φ4~φ16mm, the insertion depth is recommended to be more than 10 times the diameter of the protection tube to reduce thermal conduction errors.
Whether Armored Protection is Needed:
Armored type offers faster response and vibration resistance, suitable for moving parts or confined spaces;
Ordinary assembled type is lower in cost and suitable for static equipment.
IV. Accuracy and Output Requirements: Interfacing with the Control System
Accuracy Level: Class I (±1.5℃) for high-precision control, Class II (±2.5℃) for general industrial needs.
Output Signal:
Direct mV signal output requires a temperature controller;
For long-distance transmission, it is recommended to use a transmitter with 4-20mA output for stronger anti-interference.
Explosion-proof Requirements: In flammable and explosive environments, an explosion-proof junction box should be selected, such as dIIBT4 certified products.
V. Comprehensive Selection Process (Recommended in this order)
Determine the temperature measurement range → Select the graduation number
Analyze the operating environment → Verify material compatibility
Clarify the installation method → Match the thread/flange/length
Evaluate the accuracy and signal requirements → Select the level and output form
Consider the protection level → Whether waterproof, explosion-proof, and corrosion-resistant
For example:
You need to measure the wall temperature of a heating furnace, the temperature is approximately 600℃, the environment is normal pressure air, and there is a reserved M27×2 threaded hole
