In the field of industrial temperature measurement, thermocouples with connecting tubes and platinum resistance thermometers with fixed flanges and junction boxes are two common types of temperature sensors. They exhibit 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. 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. 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 Fixed Flange and Junction Box
The core feature of a platinum resistance thermometer with a fixed flange and junction box lies in its fixed flange connection and separate junction box structure. It typically uses a fixed flange (such as DN50) to secure it to the surface of the object being measured with bolts, while the junction box is independently installed outside the equipment and connected to the probe via wires. The fixed flange design ensures a rigid connection between the probe and the equipment surface, making it suitable for high-pressure or applications requiring long-term stable measurement. For example, in the chemical or pharmaceutical industries, the fixed flange design ensures that the probe maintains a stable position on high-pressure pipelines, reducing the impact of vibration on measurements. Its structural design emphasizes the rigid connection of the flange fixing and the independence of the junction box. The fixed flange design reduces the influence of environmental factors on measurement accuracy and enhances resistance to mechanical shock and chemical corrosion. However, its installation process requires ensuring that the flange is completely in contact with the surface of the object being measured, which increases the complexity of installation. Furthermore, the rigid connection of the fixed flange may limit flexibility in scenarios requiring frequent adjustments.
II. Differences in Working Principles
1. Working Principle of Probe-Type Thermocouple with Connecting Tube
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, the magnitude of which 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 Fixed Flange Mounted Junction Box Type Platinum Resistance Thermometer
Platinum resistance thermometers are based on the characteristic that the resistance of a metal changes with temperature. The resistance value has a non-linear relationship with temperature and needs to be determined by consulting a table or using a formula (e.g., Pt100 has a resistance of 100Ω at 0°C, and the resistance value increases linearly with increasing temperature). 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 at medium and low temperatures (-200°C to 600°C), but they should be kept away from strong magnetic fields or mechanical vibrations to avoid affecting measurement accuracy.
III. Identification Methods
1. Appearance Inspection
Probe-type thermocouple with connecting tube: 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.
Fixed flange mounted junction box type platinum resistance thermometer: The head is usually covered with a metal protective tube, and the inside is a temperature-sensing element wound with platinum wire. The fixed flange part is fixed to the surface of the object being measured with bolts, and the junction box is installed independently. 2. Wiring Method
Thermocouple with connecting tube probe: Uses a two-wire system (positive and negative), with the terminal box marked "TC+" and "TC−". The leads are usually red (positive) and black/blue (negative).
Platinum resistance thermometer with fixed flange mounting and terminal box: Uses a three-wire system (R1, R2, R3), with the terminal box marked "R1", "R2", and "R3". The leads are usually red, white, and yellow.
3. Multimeter Measurement
Thermocouple with connecting tube probe: The resistance value is very small, usually only a few ohms.
Platinum resistance thermometer with fixed flange mounting and terminal box: The resistance value is approximately 100 ohms at room temperature (Pt100).
IV. Differences in Application Scenarios
1. Thermocouple with connecting tube 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 environments with high temperature, high pressure, and strong corrosive media.
2. Platinum resistance thermometer with fixed flange mounting and terminal box
Scenarios requiring fast response and close contact: For example, in the chemical or pharmaceutical industry, the fixed flange 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 fixed flange design facilitates installation and maintenance while providing additional protection.
V. Selection Suggestions
1. Thermocouple with connecting tube 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 fixed flange mounting and terminal box selection
Installation requirements: Select a probe with a fixed flange 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 fixed flange mounted 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 fixed flange mounted 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 fixed flange mounted 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.
