What is a Thermocouple?

Thermocouple is a commonly used temperature measuring element in temperature measuring instruments. It directly measures the temperature and converts the temperature signal into a thermoelectromotive force signal. It is converted into the temperature of the measured medium through an electrical instrument (secondary instrument). The appearance of various thermocouples is often very different due to their needs, but their basic structure is roughly the same. They are usually composed of main parts such as thermal electrodes, insulation protection tubes and junction boxes. They are usually related to display instruments, recording instruments and electronic adjustments. Supporting device.

In the industrial production process, temperature is one of the important parameters that needs to be measured and controlled. In the temperature measurement, the application of the thermocouple is extremely extensive. It has many advantages such as simple structure, convenient manufacturing, wide measuring range, high precision, small inertia, and easy output of the remote signal. In addition, because a thermocouple is a passive sensor, it does not require an external power supply when measuring, and it is very convenient to use. Therefore, it is often used to measure the temperature of the gas or liquid in the furnace and pipes and the surface temperature of solids.

When two different conductors or semiconductors A and B form a loop, and the two ends are connected to each other, as long as the temperature at the two junctions is different, the temperature at one end is T, which is called the working or hot end, and the temperature at the other end is T0. , Called the free end (also referred to as the reference end) or the cold end, an electromotive force will be generated in the circuit. The direction and size of the electromotive force are related to the material of the conductor and the temperature of the two junctions. This phenomenon is called "thermoelectric effect", and the loop composed of two kinds of conductors is called "thermocouple", these two kinds of conductors are called "thermoelectrodes", and the generated electromotive force is called "thermoelectromotive force".

Thermoelectromotive force consists of two parts of electromotive force, one is the contact electromotive force of two conductors, and the other is the temperature difference electromotive force of a single conductor.

The size of the thermoelectromotive force in the thermocouple circuit is only related to the conductive material and the temperature of the two contacts, and has nothing to do with the shape and size of the thermocouple. When the two electrode materials of the thermocouple are fixed, the thermoelectromotive force is the two contact temperatures t and t0. The function is poor. which is

1. Simple assembly,

In order to ensure that the thermocouple is reliable,

Two different types of conductors (called thermocouple wires or thermal electrodes) are connected at both ends to form a loop.

Commonly used

In production, due to different measured objects, different environmental conditions, different measurement requirements, and thermal resistance installation methods and measures taken, there are many issues to be considered, but in principle, the accuracy and safety of temperature measurement 3, convenient maintenance to consider.

In order to avoid damage to the temperature measuring element, it should be ensured that it has sufficient mechanical strength. To protect the temperature sensing element from abrasion, a protective screen or protective tube should be added. To ensure safety and reliability, the installation method of the temperature measuring element should be determined according to the specific situation Such as the temperature, pressure of the medium to be measured, the length of the temperature measuring element and its installation position, form, etc.). Here are just a few examples to draw your attention:

All temperature-measuring components installed under pressure must be sealed. Thermocouples that work at high temperatures should be installed vertically to prevent the protection tube from deforming at high temperatures. If they must be installed horizontally, they should not be too long. Protect the thermocouples with a bracket. If the temperature measuring element is installed in a pipe with a large medium flow rate, it should be installed at an angle. In order to prevent the temperature measuring element from being eroded too much, it is best to install it at the bend of the pipeline. When the medium pressure exceeds 10 MPa, a protective jacket must be added to the measuring element. The installation location of the thermocouple / RTD should also consider sufficient space and space for disassembly, repair, and calibration. Thermocouples and RTDs with longer protection tubes should be easily removable.

The thermal response time is more complicated. Different test conditions will have different measurement results. This is because it is affected by the heat exchange rate between the thermocouple and the surrounding medium. If the heat exchange rate is high, the thermal response time is short. In order to make the thermal response time of thermocouple products comparable, national standards stipulate that the thermal response time should be performed on a dedicated water flow test device. The water flow speed of the device should be maintained at 0.4 ± 0.05m / s, the initial temperature is in the range of 5-45 ° C, and the temperature step value is 40-50 ° C. During the test, the temperature change of water should not be greater than ± 1% of the temperature step value. The insertion depth of the tested thermocouple is 150mm or the designed insertion depth (select the smaller value and indicate it in the test report).
Due to the complexity of the device, only a few units currently have this equipment, so the national standard stipulates that manufacturers are allowed to negotiate with users, and other test methods can be used, but the data given must indicate the test conditions.
Because the thermoelectric potential of the B-type thermocouple is small near room temperature, the thermal response time is not easy to measure. Therefore, the national standard stipulates that the S-type thermocouple of the same specification can be used to replace its own thermal electrode assembly, and then test.
During the test, the time T0.5 of the change in the output of the thermocouple to 50% of the temperature step change should be recorded. If necessary, the thermal response time T0.1 with a change of 10% and the thermal response time T0.9 with a change of 90% can be recorded. The recorded thermal response time shall be the average of at least three test results of the same test, and the deviation of each measurement result from the average shall be within ± 10%. In addition, the time required to form a temperature step change should not exceed one tenth of the T0.5 of the thermocouple being tested. The response time of the recording instrument or meter should not exceed one tenth of the T0.5 of the tested thermocouple.

1.Press

For the installation of thermocouples and thermal resistors, attention should be paid to the accurate temperature measurement.

The correct use of thermocouples can not only accurately obtain the temperature value, and ensure that the product is qualified,

The method of fault diagnosis of thermocouple input:

After correct wiring according to the meter wiring diagram, the meter first displays the thermocouple graduation number of the meter.

Since the materials of thermocouples are generally more expensive (especially when noble metals are used),

1. High measurement accuracy. Because it is in direct contact with the measured object, it is not affected by the intermediate medium.

2,

A thermocouple is formed by connecting two different conductors together.

Thermocouple (Figure 13)

When the measurement and reference connection points are at different temperatures, a so-called thermal electromagnetic force (EMF) is generated. Use of connection point The measurement connection point is the part of the thermocouple connection point at the measured temperature. The reference connection point is the portion of the thermocouple connection point that is maintained at a known temperature or that the temperature change can be automatically compensated.

In conventional industrial applications, thermocouple elements are usually terminated on the connector; however, the reference connection point is rarely located on the connector, and an appropriate thermocouple extension cable is used to transfer to a controlled environment with a relatively stable temperature. Connection point type The shell-type thermocouple connection point is physically connected (welded) to the probe wall, which enables good heat transfer-that is, heat is transferred from the outside to the thermocouple connection point through the probe wall. Shell-type thermocouples are recommended to measure the temperature of static or flowing corrosive gases and liquids, as well as some high pressure applications. In insulated thermocouples, the thermocouple connection point is separated from the probe wall and surrounded by a soft powder. Although the insulation type thermocouple has a slower response speed than the shell type thermocouple, it can provide electrical insulation. It is recommended to use an insulated thermocouple to measure the corrosive environment. Ideally, the thermocouple can be completely electrically insulated from the surrounding environment through a sheath shield. Open end thermocouples allow the tip of the connection point to penetrate into the surrounding environment. This type provides the best response time, but is limited to use in non-corrosive, non-hazardous and non-pressurized applications. The response time is expressed by a time constant, which is defined as the time required for the sensor to change 63.2% between the initial value and the final value in the controlled environment. The open-end thermocouple has the fastest response speed, and the smaller the probe sheath diameter, the faster the response speed, but the lower the maximum allowable measurement temperature. Extension line Thermocouple extension line is a pair of wires with the same temperature electromagnetic frequency characteristics as the thermocouple to which they are connected. When connected properly, the extension cable transfers the reference connection point from the thermocouple to the other end of the line, which is usually located in the controlled environment.

The following factors need to be considered when selecting a thermocouple:

1. Measured temperature range;

2. Required response time;

3. Type of connection point;

4. Chemical resistance of thermocouple or sheath material;

5, anti-wear or anti-vibration ability;

6. Installation and restriction requirements.