In the field of industrial temperature measurement, screw-type thermocouples and armored platinum resistance thermometers are two common types of temperature sensors, exhibiting significant differences in structural design, working principles, performance characteristics, and application scenarios. The following provides a systematic comparison from multiple perspectives to clarify their core differences.
I. Differences in Structural Design and Installation Methods
1. Screw-Type Thermocouple
The core feature of a screw-type thermocouple is its fixed threaded connection structure, typically using M27×2 or other standard thread specifications, achieving secure installation through mechanical engagement of the threads. This design allows the probe to form a tight physical connection with the equipment, suitable for scenarios requiring long-term stable monitoring and fixed installation positions. For example, in mechanical processing or electronic equipment, the threaded connection ensures the probe remains stable in vibrating or shock environments, while facilitating signal transmission and maintenance.
The probe of the thermocouple is encased in a metal protective tube (such as stainless steel), containing the thermoelectric elements (such as nickel-chromium-nickel-silicon alloy). Its structural design emphasizes the stability and sealing of the threaded connection, which may be equipped with sealing gaskets or welding processes to prevent media leakage. This design makes the thermocouple perform excellently in high-temperature, high-pressure, or corrosive environments, but the installation process requires the use of special tools (such as wrenches) to ensure tightening, increasing installation complexity.
2. Armored Platinum Resistance Thermometer
The core feature of an armored platinum resistance thermometer is its armored protective structure, typically using a metal sheath (such as stainless steel) to encase the platinum resistance element, and filled with insulating materials such as magnesium oxide, forming a robust armored layer. This design allows the probe to remain stable in harsh environments, suitable for scenarios requiring high protection levels or avoiding external interference. For example, in the chemical or pharmaceutical industries, the armored design can resist chemical corrosion and mechanical shock, ensuring long-term operation.
The probe of the platinum resistance thermometer is encased in an armored layer, containing the platinum resistance element (such as Pt100). Its structural design emphasizes the protective performance and flexibility of the armored layer, reducing the impact of environmental factors on measurement accuracy and improving resistance to mechanical shock and chemical corrosion. However, its installation process requires ensuring a secure connection between the armored layer and the equipment, and its sealing is relatively weak, which may not withstand extremely high pressure or highly corrosive media.
II. Differences in Working Principles
1. Working Principle of Thermocouples
Thermocouples are based on the Seebeck effect, where two different metal conductors generate a thermoelectric potential difference under a temperature gradient. When two metal conductors are connected to form a closed circuit, and the two junctions have different temperatures, an electromotive force is generated in the circuit. Its magnitude is related to the material properties and the temperature difference between the junctions. By measuring the electromotive force, the temperature value can be indirectly calculated. Thermocouples have high sensitivity; a 1°C temperature change results in an output potential change of approximately 5-40 microvolts. Their structure is simple, with no moving parts, making them suitable for high-temperature, high-pressure, and highly corrosive environments.
2. Working Principle of Platinum Resistance Thermometers
Platinum resistance thermometers are based on the characteristic that metal resistance changes with temperature. Their resistance value has a non-linear relationship with temperature and needs to be determined by consulting a table or using a formula (such as R=R₀[1+At+Bt²+C(t-100)³]). Platinum resistance thermometers have high sensitivity; a 1°C temperature change results in a significant change in resistance value (for example, Pt100 has a resistance of 100Ω at 0°C, and the resistance value increases linearly with increasing temperature). Their structure is simple, with no moving parts, making them suitable for precise measurements at medium and low temperatures (-200°C to 600°C), but strong magnetic fields or mechanical vibrations should be avoided to prevent affecting measurement accuracy.
III. Identification Methods
1. Appearance Inspection
Thermocouple: The head has no significant expansion structure, and the inside consists of two different metal wires welded together.
Platinum resistance thermometer: The head is usually covered with an armored layer, and the inside is a temperature-sensing element made of platinum wire. The armored layer is in the shape of a metal tube.
2. Wiring Method
Thermocouple: Uses a two-wire system (positive and negative), the junction box is marked "TC+" and "TC−", and the leads are usually red (positive) and black/blue (negative).
Platinum resistance thermometer: Uses a three-wire system (R1, R2, R3), the junction box is marked "R1", "R2", and "R3", and the leads are usually red, white, and yellow. 3. Multimeter Measurement
Thermocouple: The resistance value is extremely small, usually only a few ohms.
Platinum resistance thermometer: The resistance value is approximately 100 ohms at room temperature (Pt100).
IV. Differences in Application Scenarios
1. Screw-type Thermocouple
Industrial field: Chemical, petroleum, power generation, and other scenarios requiring long-term stable monitoring. For example, in boiler pipelines, the threaded connection ensures the probe is stable in high-temperature steam, providing continuous temperature data.
Special environments: High-pressure or highly corrosive media environments. For example, in a reactor, its sealed design prevents media leakage and ensures safety.
2. Armored Platinum Resistance Thermometer
High protection requirement scenarios: Scenarios requiring a high degree of protection or avoiding external interference. For example, in the chemical or pharmaceutical industry, the armored design resists chemical corrosion and mechanical shock, ensuring long-term operation.
Medium and low temperature environments: Indoor or low-pressure scenarios. For example, in a laboratory, its flexible design facilitates installation and maintenance.
V. Selection Suggestions
1. Screw-type Thermocouple Selection
Installation requirements: Prioritize selecting a probe with a thread specification that matches the equipment to ensure a secure connection.
Environmental conditions: In high-temperature, high-pressure, or corrosive environments, choose a metal protective tube and sealed design.
2. Armored Platinum Resistance Thermometer Selection
Installation requirements: Select a probe with an armored specification that matches the equipment to ensure a secure connection.
Environmental conditions: Use in scenarios requiring a high degree of protection or avoiding external interference, avoiding extremely high pressure or highly corrosive media.
VI. Summary and Complementary Relationship
The core difference between screw-type thermocouples and armored platinum resistance thermometers lies in their working principles and applicable environments: thermocouples provide high-temperature measurement based on the Seebeck effect and are suitable for harsh environments; platinum resistance thermometers provide precise measurement of medium and low temperatures based on resistance changes and are suitable for scenarios requiring high protection. When selecting, it is necessary to clarify the core needs: thermocouples focus on stability and environmental resistance in high-temperature environments, while armored platinum resistance thermometers focus on protection performance and measurement accuracy in medium and low-temperature environments. The two work together to meet the temperature measurement needs of different scenarios.
