In the field of industrial temperature measurement, thermocouples with connecting tubes and platinum resistance thermometers with double sheaths and junction boxes are two common types of temperature sensors. They differ significantly 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. Thermocouple with Connecting Tube
The core feature of a thermocouple with a connecting tube lies in its connecting tube fixation and bimetallic wire structure. It typically uses a metal connecting tube (such as stainless steel) that is tightly attached to the surface of the object being measured. The secure installation is achieved through the mechanical pressure of the connecting tube, while the interior consists of two different metal wires (such as nickel-chromium and nickel-silicon) welded together to form the measuring end. The connecting tube design allows the probe to make close contact with the equipment surface, improving measurement accuracy and response speed. For example, in mechanical manufacturing or electronic equipment, the connecting tube design ensures sufficient contact between the probe and the equipment surface, reducing heat loss during heat transfer. Its structural design emphasizes the tightness of the connecting tube fixation and the independence of the bimetallic wires. The connecting tube design reduces the influence of environmental factors on measurement accuracy and enhances resistance to mechanical shock. However, its installation process requires ensuring that the connecting tube is completely in contact with the surface of the object being measured, which increases the complexity of installation. Furthermore, the bimetallic wires may oxidize in high-temperature environments, affecting long-term stability.
2. Platinum Resistance Thermometer with Double Sheath and Junction Box
The core feature of a platinum resistance thermometer with a double sheath and junction box lies in its double sheath protection and separate junction box structure. It typically uses inner and outer metal sheaths (such as stainless steel) to enclose the platinum wire, achieving secure installation through the mechanical protection of the double sheath. The junction box is independently installed outside the equipment and connected to the probe via wires. The inner sheath directly contacts the object being measured, while the outer sheath provides additional protection. This design allows the probe to maintain positional stability in high-temperature, high-pressure, or corrosive environments, while also facilitating signal transmission and maintenance. For example, in the chemical or pharmaceutical industries, the double sheath design ensures that the probe is protected from mechanical damage and chemical corrosion in harsh environments. Its structural design emphasizes the protective nature of the double sheath and the independence of the junction box. The double sheath design reduces the influence of environmental factors on measurement accuracy and enhances resistance to mechanical shock and chemical corrosion. However, the installation process requires ensuring that the double sheath is in complete contact with the surface of the object being measured, which increases the complexity of installation. Furthermore, the double sheath structure may result in a slightly slower response time compared to a single sheath design.
II. Differences in Working Principles
1. Working Principle of Thermocouple with Connecting Tube Probe
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 Double Sheath Mounted Junction Box Platinum Resistance Thermometer
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 requires lookup tables or formulas for calculation (e.g., Pt100 has a resistance of 100Ω at 0°C, and the resistance value increases linearly with increasing temperature) to determine the temperature value. Platinum resistance thermometers have high sensitivity; a 1°C temperature change results in a significant change in resistance value. Their structure is simple, with no moving parts, making them suitable for precise measurements in medium to 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. Visual Inspection
Thermocouple with connecting tube probe: The head is usually covered with a metal protective tube, and the inside consists of two different metal wires welded together. The connecting tube part is in close contact with the surface of the object being measured.
Double sheath mounted junction box platinum resistance thermometer: The head is usually covered with inner and outer layers of metal protective tubes. The inside contains a temperature-sensing element wound with platinum wire. The double sheath part is in close contact with the surface of the object being measured, and the junction box is installed independently. 2. Wiring Method
Thermocouple with connecting tube type probe: Uses a two-wire system (positive and negative), the terminal box is marked "TC+" and "TC−", and the leads are usually red (positive) and black/blue (negative).
Platinum resistance thermometer with double-sheath mounting and terminal box: Uses a three-wire system (R1, R2, R3), the terminal box is marked "R1", "R2", "R3", and the leads are usually red, white, and yellow.
3. Multimeter Measurement
Thermocouple with connecting tube type probe: The resistance value is very small, usually only a few ohms.
Platinum resistance thermometer with double-sheath mounting and terminal box: The resistance value is approximately 100 ohms at room temperature (Pt100).
IV. Application Scenario Differences
1. Thermocouple with connecting tube type probe
Scenarios requiring fast response and close contact: For example, in mechanical manufacturing or electronic equipment, the connecting tube design ensures sufficient contact between the probe and the equipment surface, improving measurement accuracy and response speed.
High-temperature or corrosive environments: Suitable for high-temperature, high-pressure, and highly corrosive media environments.
2. Platinum resistance thermometer with double-sheath mounting and terminal box
Scenarios requiring fast response and close contact: For example, in the chemical or pharmaceutical industry, the double-sheath design ensures sufficient contact between the probe and the equipment surface, improving measurement accuracy and response speed.
Medium and low-temperature environments: Indoor or low-pressure scenarios. For example, in HVAC systems, its double-sheath design facilitates installation and maintenance while providing additional protection.
V. Selection Suggestions
1. Thermocouple with connecting tube type probe selection
Installation requirements: Select a probe with a connecting tube specification that matches the equipment to ensure a secure connection.
Environmental conditions: Use in scenarios requiring high-temperature or corrosive environment measurement, avoiding strong vibration or impact environments.
2. Platinum resistance thermometer with double-sheath mounting and terminal box selection
Installation requirements: Select a probe with a double-sheath specification that matches the equipment to ensure a secure connection.
Environmental conditions: Use in scenarios requiring precise measurement and fast response at medium and low temperatures, avoiding strong magnetic fields or mechanical vibration environments. VI. Summary and Complementary Relationship
The core difference between the connecting-tube type probe thermocouple and the double-sheath mounting junction box type platinum resistance thermometer lies in their working principles and applicable environments: the connecting-tube type probe thermocouple utilizes the Seebeck effect to provide flexible temperature measurement, suitable for scenarios requiring fast response and close contact; the double-sheath mounting junction box type platinum resistance thermometer utilizes resistance change to provide precise measurement in medium and low temperature ranges, also suitable for scenarios requiring fast response and close contact. When selecting a device, the core requirements must be clearly defined: the connecting-tube type probe thermocouple focuses on response speed and measurement accuracy in high-temperature environments, while the double-sheath mounting junction box type platinum resistance thermometer focuses on response speed and measurement accuracy in medium and low-temperature environments. Working together, these two types of sensors can meet the temperature measurement needs of different scenarios.
