Thermocouple: principle of operation, device

There are many different devices and mechanisms for measuring temperature. Some of them are used in everyday life, some - for various physical studies, in production processes and other industries.

One such device is a thermocouple. The principle of operation and the scheme of this device will be considered in the following sections.

The physical basis of the thermocouple

The principle of operation of a thermocouple is based on ordinary physical processes. For the first time the effect, on the basis of which this device works, was investigated by the German scientist Thomas Seebeck.

The essence of the phenomenon, on which the principle of the action of the thermocouple holds, is as follows. In a closed electrical circuit consisting of two conductors of various types, electricity is generated when a certain ambient temperature is applied.

The resulting electric current and the ambient temperature acting on the conductors are linearly dependent. That is, the higher the temperature, the greater the electric current produced by the thermocouple. This is how the thermocouple and resistance thermometer operate.

In this case, one thermocouple contact is at the point where it is necessary to measure the temperature, it is called "hot". The second contact, in other words - "cold", - in the opposite direction. The application for measuring thermocouples is allowed only if the room temperature is lower than at the measurement site.

This is a short scheme of the thermocouple operation, the principle of operation. We will look at the types of thermocouples in the next section.

Types of thermocouples

In each industry, where temperature measurements are required, the thermocouple is mainly used. The device and principle of operation of different types of this unit are given below.

Chromel-aluminum thermocouples

These thermocouple circuits are used in most cases for the production of various sensors and probes, which allow controlling the temperature in industrial production.

Their distinctive features can be called a fairly low price and a huge range of measured temperature. They allow you to fix the temperature from -200 to +13000 degrees Celsius.

It is not advisable to use thermocouples with similar alloys in workshops and facilities with a high sulfur content in the air, since this chemical element adversely affects both chromium and aluminum, causing disturbances in the functioning of the device.

Chromel-Copel Thermocouples

The principle of the thermocouple, the contact group of which consists of these alloys, is the same. But these devices operate mainly in a liquid or gaseous medium with neutral, non-aggressive properties. The upper temperature index does not exceed +8000 degrees Celsius.

A similar thermocouple is used, the principle of its operation allows it to be used to determine the degree of heating of any surfaces, for example, to determine the temperature of open-hearth furnaces or other similar structures.

Iron-constantan thermocouples

This combination of contacts in a thermocouple is not as common as the first of the considered varieties. The principle of operation of the thermocouple is the same, but this combination proved to be well in a rarefied atmosphere. The maximum level of the measured temperature should not exceed +12500 degrees Celsius.

However, if the temperature starts to rise above +7000 degrees, there is a danger of a violation of the accuracy of the measurements due to a change in the physical and chemical properties of iron. There are even cases of corrosion of the iron contact of the thermocouple when there are water vapors in the surrounding air.

Platinum / Platinum Thermocouples

The most expensive thermocouple in manufacturing. The principle of operation is the same, but it differs from its counterparts by very stable and reliable temperature readings. Has a decreased sensitivity.

The main area of application of these devices is the measurement of high temperatures.

Tungsten-rhenium thermocouples

Also used to measure ultrahigh temperatures. The maximum limit, which can be fixed with this scheme, is 25 thousand degrees Celsius.

Their application requires certain conditions. Thus, during the measurement of temperature, the surrounding atmosphere must be completely eliminated, which adversely affects contacts as a result of the oxidation process.

To this end, tungsten-rhenium thermocouples are usually placed in protective casings filled with an inert gas that protects their elements.

Above we considered each existing thermocouple, the device, the principle of its operation, depending on the alloys used. Now let us consider some design features.

Thermocouple designs

There are two main types of thermocouple designs.

• With the use of an insulating layer. This design of the thermocouple provides for isolating the working layer of the device from electric current. Such a scheme allows the use of a thermocouple in the process without isolating the input from the ground.

• Without the use of an insulating layer. Such thermocouples can only be connected to measuring circuits whose inputs do not have contact with the ground. If this condition is not met, the device will have two independent closed circuits, as a result of which the readings obtained with a thermocouple will not correspond to reality.

Running thermocouple and its application

There is a separate version of this device, called "running." The operating principle of the traveling thermocouple is now considered in more detail.

This design is used mainly to determine the temperature of steel billet during its processing on turning, milling and other similar machines.

It should be noted that in this case it is possible to use a conventional thermocouple, however, if the manufacturing process requires high accuracy of the temperature regime, the running thermocouple can not be overestimated.

When this method is used, its contact elements are pre-sealed into the preform. Then, during processing of the blank, these contacts are constantly exposed to the cutter or other working tool of the machine, as a result of which the junction (which is the main element in the removal of temperature indicators) "runs" through the contacts.

This effect is universally applied in the metalworking industry.

Technological features of thermocouple designs

When manufacturing the working circuit of a thermocouple, two metal contacts are welded together, which are known to be made of different materials. The junction is called the "junction".

It should be noted that it is not necessary to make this connection using spikes. It is enough just to twist together two contacts. But this way of production will not have a sufficient level of reliability, and can also give errors in the removal of temperature indicators.

If it is necessary to measure high temperatures, the metal spike is replaced by welding. This is due to the fact that in most cases the solder used in connection has a low melting point and is destroyed when its level is exceeded.

Schemes for which welding was used, can withstand a wider temperature range. But this method of connection has its drawbacks. The internal structure of the metal under the influence of high temperature during welding can change, which will affect the quality of the data obtained.

In addition, the status of the thermocouple contacts during operation should be monitored. Thus, it is possible to change the characteristics of metals in the circuit due to the influence of an aggressive environment. There can be oxidation or mutual diffusion of materials. In such a situation it is necessary to replace the working circuit of the thermocouple.

Types of thermocouple junctions

Modern industry produces several designs that are used in the manufacture of thermocouples:

• With open junction;

• With an isolated junction;

• With grounded junction.

A feature of thermocouples with an open junction is a poor resistance to external influences.

The following two types of construction can be used to measure temperatures in corrosive environments that have a destructive effect on the contact pair.

In addition, the industry is currently mastering the production of thermocouples for semiconductor technology.

Measurement error

Correctness of the temperature parameters obtained with a thermocouple depends on the material of the contact group, as well as external factors. The latter include pressure, radiation background, or other causes that can affect the physicochemical parameters of the metals from which the contacts are made.

The measurement error consists of the following components:

• Random error caused by thermocouple manufacturing features;

• The error caused by the violation of the temperature regime of the "cold" contact;

• The error caused by external interference;

• Inaccuracy of the control equipment.

The advantages of using thermocouples

The advantages of using such devices for temperature control, regardless of the field of application, include:

• A large range of indicators that can be fixed with a thermocouple;

• The spike of the thermocouple, which directly participates in taking readings, can be placed in direct contact with the measuring point;

• Simple process of manufacturing thermocouples, their strength and durability of operation.

The disadvantages of measuring temperature with a thermocouple

The disadvantages of using a thermocouple include:

• The need for constant monitoring of the temperature of the "cold" thermocouple contact. This is a distinctive feature of the design of measuring instruments, based on a thermocouple. The principle of operation of this scheme narrows the scope of its application. They can be used only if the ambient temperature is lower than the temperature at the measuring point.

• Violation of the internal structure of metals used in the manufacture of a thermocouple. The fact is that as a result of the influence of the external environment, the contacts lose their uniformity, which causes errors in the obtained temperature indexes.

• During the measurement, the contact group of the thermocouple is usually exposed to the negative influence of the environment, which causes disturbances during operation. This again requires the sealing of contacts, which causes additional costs for the maintenance of such sensors.

• There is a danger of electromagnetic waves acting on a thermocouple, the design of which provides for a long contact group. This can also affect the measurement results.

• In some cases, there is a violation of the linear relationship between the electric current generated in the thermocouple and the temperature at the measurement site. A similar situation requires the calibration of control equipment.

Conclusion

Despite the existing shortcomings, the method of measuring temperature with thermocouples, which was first invented and tested in the 19th century, found its wide application in all branches of modern industry.

In addition, there are applications where the use of thermocouples is the only way to obtain temperature data. And having become acquainted with this material, you have thoroughly understood the basic principles of their work.