In the field of industrial temperature measurement, probe-type thermocouples with connecting tubes and platinum resistance thermometers with fixed threaded mounting and junction boxes are two common types of temperature sensors. They have significant differences in structural design, working principles, performance characteristics, and application scenarios. The following provides a systematic comparison from multiple dimensions to clarify their core differences.
I. Differences in Structural Design and Installation Methods
1. Probe-Type Thermocouple with Connecting Tube
The core feature of a probe-type 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) to closely contact 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-nickel-silicon) welded together to form the measuring end. The connecting tube design allows the probe to form 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, and the bimetallic wires may oxidize in high-temperature environments, affecting long-term stability.
2. Platinum Resistance Thermometer with Fixed Threaded Mounting and Junction Box
The core feature of a platinum resistance thermometer with fixed threaded mounting and a junction box lies in its fixed threaded connection and separate junction box structure. It typically uses a fixed thread (such as M20×1.5) to achieve secure installation by screwing it onto the surface of the object being measured, while the junction box is independently installed outside the equipment and connected to the probe via wires. The fixed thread design ensures a rigid connection between the probe and the equipment surface, suitable for high-pressure or applications requiring long-term stable measurement. For example, in the chemical or pharmaceutical industries, the fixed thread 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 threaded fixing and the independence of the junction box. The threaded fixing 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 threads are completely in contact with the surface of the object being measured, which increases the complexity of installation. Furthermore, the threaded connection may loosen due to vibration during long-term use, affecting measurement accuracy.
II. Differences in Working Principles
1. Working Principle of Probe-Type Thermocouple with Connecting Tube
The thermocouple is 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 Fixed Threaded Junction Box Type Platinum Resistance Thermometer
The platinum resistance thermometer is based on the characteristic that the resistance of a metal changes with temperature. Its 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℃, 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 in medium and low temperatures (-200℃ to 600℃), but strong magnetic fields or mechanical vibrations should be avoided to prevent 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 threaded junction box type platinum resistance thermometer: The head is usually covered with a metal protective tube, and the inside is a temperature-sensing element made of platinum wire. The threaded part is screwed onto 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 fixed threaded 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 fixed threaded 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 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 fixed threaded mounting and terminal box
Scenarios requiring fast response and close contact: For example, in the chemical or pharmaceutical industry, the fixed thread 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 thread design facilitates installation and maintenance.
V. Selection Suggestions
1. Selection of Thermocouple with connecting tube type probe
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. Selection of Platinum resistance thermometer with fixed threaded mounting and terminal box
Installation requirements: Select a probe with a fixed thread specification that matches the equipment to ensure a secure connection.
Environmental conditions: Use in scenarios requiring precise measurement and fast response in medium and low-temperature environments, 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 threaded 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 fixed threaded mounting junction box type platinum resistance thermometer uses 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 threaded 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.
