What is an assembled thermocouple? Comprehensive Analysis of Working Principle, Model Selection and Application

1.What is an assembled thermocouple?

Assembled thermocouples, as the name suggests, refer to Temperature Sensors that are pre-assembled into a solid whole by components such as thermocouple wires (temperature-sensing elements), insulating materials, protective tubes, and junction boxes. It is a key component for contact temperature measurement in industrial production and is widely used in various equipment and processes in industries such as metallurgy, chemical engineering, power, machinery, and food. It directly measures the temperature of liquid, steam, gas media, and solid surfaces within the range of 0°C to 1800°C.

 

Its core features are a sturdy structure, easy installation, long service life, and the ability to adapt to various harsh industrial environments.

 

2. Working Principle of Assembled Thermocouples

The working principle of the assembled thermocouple is based on the classic Seebeck effect, also known as the "thermoelectric effect".

 

In simple terms, when one end of two conductors of different materials (referred to as thermoelectric electrodes) is welded together to form the measuring end (hot end), while the other end (reference end or cold end) remains open or is connected to a measuring instrument, if there is a temperature difference between the measuring end and the reference end, a thermoelectric potential (voltage) proportional to the temperature difference will be generated in the circuit.

 

The instrument measures this tiny electromotive force and then accurately calculates the temperature at the measuring end based on the known thermocouple scale table. The protective tube and other structures of the assembled thermocouple mainly serve to fix, protect and insulate, ensuring that the internal temperature-sensing elements can work accurately and stably.

 

3. Main models and scale Numbers of Assembled Thermocouples

Depending on the different thermoelectric electrode materials, assembled thermocouples are classified into various standard models, each with its specific scale number, temperature measurement range and characteristics. The following are several of the most common types

 

Thermocouple types with scale numbers, temperature measurement range (≈), characteristics and applications

K-type nickel-chromium - nickel-silicon has the largest usage at 0 ~ 1300°C, with high cost performance and good linearity. It is suitable for oxidizing and inert atmospheres.

S-type platinum-rhodium 10-platinum 0 ~ 1600°C features high precision and good stability, making it suitable for high-temperature and oxidizing environments. However, it is expensive.

E-type nickel-chromium-constantan has the largest thermoelectric potential at 0 to 800°C, high sensitivity, and is suitable for low-temperature and inert atmospheres.

N-type nickel-chromium-silicon - nickel-silicon has strong high-temperature oxidation resistance from 0 to 1300°C and better stability than K-type. It is an improved version of K-type.

T-type copper - constantan -200 to 350°C, suitable for low-temperature measurement, with good stability and moisture resistance.

How to choose a model? It is usually determined comprehensively based on the temperature measurement range, accuracy requirements, atmosphere conditions and budget. Type K has become the most common choice due to its wide applicability.

4.Typical Structures of Assembled Thermocouples

A standard assembled thermocouple mainly consists of three parts:

 

Thermocouple wire (temperature-sensing element) : The core part is made by welding two metal wires of different materials together.

 

Insulating sleeve (magnesium powder/ceramic tube) : It is used to isolate the electrical connection between two Thermocouple Wires and between them and the protective tube, preventing short circuits. Commonly used materials include magnesium oxide, aluminum oxide, etc.

 

Protective tube: In direct contact with the measured medium, it protects the internal components from mechanical damage, chemical corrosion and high-temperature oxidation. Material selection is of vital importance. Common ones include:

 

Metal tubes: such as 304/316 stainless steel (general), GH3030 high-temperature resistant alloy, etc.

 

Non-metallic tubes: such as ceramic and alumina tubes (suitable for extremely high temperatures).

 

Junction box: It is used to connect thermocouples and compensating wires, serving to prevent dust, splash and explosion. There are common type, waterproof type, explosion-proof type, etc.

5.How to Select the Right Type? Selection Guide for Prefabricated Thermocouples

Correct selection is the key to ensuring the accuracy of temperature measurement and the lifespan of the sensor. Please be sure to consider the following factors:

 

Temperature measurement range: This is the primary condition. The selected model must have a temperature measurement range that covers your actual working temperature and leave a margin.

 

Nature of the medium being measured: Is it gas, liquid or solid? Is it corrosive, oxidizing or reducing? This directly determines the choice of pipe material for protection.

 

Pressure conditions: The level of working pressure determines the wall thickness and structural strength of the protective pipe. In high-pressure environments, protective pipes with strong pressure-bearing capacity should be selected.

 

Installation environment: Consider the installation method (threaded, flanged, welded, etc.), insertion depth, and whether there is vibration or shock on site.

 

Accuracy requirements: Thermocouples of different scale numbers and grades have different accuracies. Select Grade I or Grade II accuracy according to the process requirements.

 

Economy: Under the premise of meeting all technical requirements, select the solution with the highest cost performance.

6.Frequently Asked Questions (FAQ)

Q1: What are the differences between assembled thermocouples and Armored Thermocouples?

A: The main difference lies in structure and flexibility. Assembled thermocouples usually have relatively thick and rigid protective tubes, which are suitable for fixed installation. Armored thermocouples are formed by drawing thermocouple wires and insulating material metal powders as a whole into a slender and flexible armored body, which has the advantages of fast response, flexibility and high pressure resistance.

 

Q2: Why use compensating wires?

The thermoelectric potential generated by A thermocouple is based on the temperature difference between the cold end and the hot end. However, in practical applications, the temperature at the cold end (junction box) varies with the environment. The function of compensating wires is to extend the cold end to a control room far from the high-temperature area where the temperature is more stable, thereby eliminating measurement errors caused by temperature changes at the cold end.

 

Q3: What could be the reasons for inaccurate temperature measurement by a thermocouple?

Common causes include: incorrect selection (such as corrosion of the protective tube), improper installation position (insufficient insertion depth), insulation deterioration, deterioration of thermocouple wire, failure of cold end compensation or loose wiring, etc.

 

Conclusion

As the cornerstone of industrial temperature measurement, the correct understanding and selection of assembled thermocouples are directly related to production safety, product quality and energy efficiency. It is hoped that through the detailed explanation in this article, you can have a comprehensive and clear understanding of the assembled thermocouple and choose the most suitable temperature sensing solution for your project.