In the field of industrial temperature measurement, movable flange mounted junction box type thermocouples and fixed threaded probe type platinum resistance thermometers are two common types of temperature sensors. They have significant differences in structure, working principle, application scenarios, and appearance/wiring. This article will discuss in detail how to distinguish between these two sensors, helping readers to better understand and apply them.
I. Structural Differences
1. Structural Characteristics of Movable Flange Mounted Junction Box Type Thermocouples
Movable flange mounted junction box type thermocouples mainly consist of a thermoelement, insulation sleeve, protective tube, movable flange connection device, and junction box. The thermoelement is the core component of the thermocouple, composed of two metal wires of different materials (such as nickel-chromium/nickel-silicon, platinum-rhodium/platinum, etc.), with their ends welded together to form the measuring end. The insulation sleeve is usually made of ceramic material (such as corundum tube, high-alumina tube), possessing high temperature resistance and insulation properties, used to isolate the two thermoelements and prevent short circuits. The protective tube's material is selected according to working conditions (such as carbon steel, stainless steel, corundum, Hastelloy), and its main function is to protect the thermoelement from corrosion or impact by the medium. High thermal conductivity is not required, as the hot end of the thermocouple directly contacts the medium, resulting in a fast response speed. The movable flange connection device connects to the equipment or pipeline via a flange, allowing for fine-tuning of the sensor during installation to adapt to different installation positions and depths. The junction box is used to connect the thermoelement and compensation wires, and contains a terminal block. Cold junction temperature stability (avoiding proximity to heat sources) is a key consideration, and the sealing rating is usually no less than IP65.
2. Structural Characteristics of Fixed Threaded Probe Type Platinum Resistance Thermometers
Fixed threaded probe type platinum resistance thermometers mainly consist of a sensing element, protective tube, fixed threaded connection device, and junction box. The sensing element is the core temperature measuring component of the platinum resistance thermometer, usually made of wound platinum wire, requiring sufficient heat exchange with the medium. The protective tube is made of stainless steel or nickel-based alloy, possessing high mechanical strength and corrosion resistance, protecting the sensing element from mechanical damage and chemical corrosion. The fixed threaded connection device connects to the equipment or pipeline via threads, ensuring the sensor's position is fixed after installation and cannot be finely adjusted. The junction box is used to connect the temperature sensing element and signal wires, and contains a built-in terminal block, supporting three-wire or four-wire connections (to compensate for wire resistance errors). Cold junction compensation is not required; only moisture-proof sealing is needed.
II. Differences in Working Principles
1. Working Principle of Thermocouples
Thermocouples work based on the Seebeck effect, which states that when two different conductive materials form a closed circuit, a thermoelectric potential is generated in the circuit if the two junctions have different temperatures. The thermoelectric potential of a thermocouple is directly proportional to the temperature difference between the measuring end and the reference end. By measuring the magnitude of the thermoelectric potential, the temperature of the measuring end can be indirectly determined. Thermocouples have a fast response speed and are suitable for measuring high temperatures and rapidly changing temperatures.
2. Working Principle of Platinum Resistance Thermometers
Platinum resistance thermometers work based on the characteristic that the resistance of a metal changes with temperature; that is, the resistance value of a metal increases with increasing temperature. There is a certain functional relationship between the resistance value of a platinum resistance thermometer and the temperature. By measuring the resistance value of the platinum resistance thermometer, the temperature of the measured medium can be calculated. The fixed threaded probe-type platinum resistance thermometer ensures close contact between the sensing element and the medium through the fixed threaded connection device, improving measurement accuracy and stability.
III. Differences in Application Scenarios
1. Application Scenarios of Thermocouples
The movable flange-mounted junction box type thermocouple is suitable for high-temperature measurement applications requiring fixed installation and fast response, such as furnace temperature and molten metal temperature. Its fast response speed allows it to quickly reflect temperature changes, making it suitable for applications requiring rapid response. In addition, thermocouples have a wide measurement range, from -200℃ to +1800℃.
2. Application Scenarios of Platinum Resistance Thermometers
The fixed threaded probe-type platinum resistance thermometer is suitable for medium and low-temperature measurements, such as the temperature measurement of liquids, gases, and solid surfaces. Its fixed threaded connection design allows it to closely fit the measured surface, improving measurement accuracy. Platinum resistance thermometers have high measurement accuracy, making them suitable for applications requiring high precision, especially in the medium and low-temperature range (-200℃ to +500℃). In addition, the fixed threaded mounting junction box type platinum resistance thermometer is easy to install, making it suitable for applications with limited space or requiring quick installation. IV. Appearance and Wiring Differences
1. Thermocouple Appearance and Wiring
The head of a thermocouple usually does not have a significant change in diameter because its hot junction directly contacts the medium, eliminating the need for additional temperature sensing elements. The junction box generally contains two or four wires (for dual thermocouples). The compensating wires have positive and negative polarities and must be connected correctly to avoid measurement errors.
2. Platinum Resistance Thermometer Appearance and Wiring
The fixed-thread probe-type platinum resistance thermometer has a distinct threaded connection device at its head, a unique design used for connecting to equipment or pipes. The terminals are usually integrated directly into the junction box and generally have three or four wires (three-wire or four-wire system) to compensate for lead resistance errors. Platinum resistance thermometers do not require distinguishing between positive and negative terminals because their working principle is based on resistance value changes rather than electromotive force.
V. Summary
The movable flange-mounted junction box type thermocouple and the fixed-thread probe-type platinum resistance thermometer have significant differences in structure, working principle, application scenarios, and appearance and wiring. Thermocouples are suitable for high-temperature measurement applications requiring fixed installation and fast response, while fixed-thread probe-type platinum resistance thermometers are suitable for medium and low temperatures and surface temperature measurements requiring high accuracy, and are easy to install. In practical applications, the appropriate sensor should be selected according to specific needs. By understanding the structural characteristics and working principles of these two sensors, their application scenarios can be better understood, thus providing more accurate and reliable solutions for industrial temperature measurement.
