There are many different devices and mechanisms that allow you to measure temperature. Some of them are used in everyday life, some - for various physical research, in production processes and other industries.
One such device is a thermocouple. We will consider the principle of operation and the scheme of this device in the following sections.
Physical basis of thermocouple operation
The working principle of a thermocouple is based on ordinary physical processes. For the first time, the effect on the basis of which this device works was studied by the German scientist Thomas Seebeck.
The essence of the phenomenon on which the principle of operation of a thermocouple rests is as follows. In a closed electrical circuit, consisting of two conductors of different types, when exposed to a certain ambient temperature, electricity arises.
The resulting electrical flow and the ambient temperature acting on the conductors are in a linear relationship. That is, the higher the temperature, the greater the electric current produced by the thermocouple. On thethis is the principle of operation of the thermocouple and resistance thermometer.
In this case, one thermocouple contact is located 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. Use for measuring thermocouples is allowed only if the air temperature in the room is lower than at the place of measurement.
This is a brief diagram of the operation of a thermocouple, the principle of operation. The types of thermocouples will be discussed in the next section.
Types of thermocouples
In every industry where temperature measurements are needed, the thermocouple is the main application. The device and principle of operation of various 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 that allow you to control the temperature in industrial production.
Their distinguishing features include a fairly low price and a huge range of measured temperatures. They allow you to fix the temperature from -200 to +13000 degrees Celsius.
It is not advisable to use thermocouples with similar alloys in shops and facilities with a high sulfur content in the air, as this chemical element negatively affects both chromium and aluminum, causing malfunctions in the device.
Chromel-Kopel thermocouples
The principle of operation of a thermocouple, the contact group of which consists of these alloys, is the same. But these devices operate mainly in a liquid or gaseous medium, which has neutral, non-aggressive properties. The upper temperature index does not exceed +8000 degrees Celsius.
A similar thermocouple is used, the principle of which 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 a thermocouple is the same, but this combination has shown itself 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 measurement accuracy violations due to changes in the physical and chemical properties of iron. There are even cases of corrosion of the iron contact of the thermocouple in the presence of water vapor in the ambient air.
Platinorhodium-platinum thermocouples
The most expensive thermocouple to manufacture. The principle of operation is the same, but it differs from its counterparts in very stable and reliable temperature readings. Has reduced sensitivity.
The main application of these devices is the measurement of high temperatures.
Tungsten-rhenium thermocouples
Also used to measure ultra-high temperatures. The maximum limit that can be fixed using this scheme reaches 25 thousand degrees Celsius.
Their application requires compliance with certain conditions. Thus, in the process of measuring temperature, it is necessary to completely eliminate the surrounding atmosphere, which has a negative effect on the contacts as a result of the oxidation process.
For this, tungsten-rhenium thermocouples are usually placed in protective casings filled with an inert gas to protect their elements.
Above, each existing thermocouple, device, its principle of operation, depending on the alloys used, were considered. Now consider some design features.
Thermocouple designs
There are two main types of thermocouple designs.
- With an insulating layer. This design of the thermocouple provides for isolating the working layer of the device from electric current. This arrangement allows the thermocouple to be used in the process without isolating the input from 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 develop two independent closed circuits, resulting in invalid thermocouple readings.
Traveling thermocouple and its application
There is a separatea kind of this device, called "running". We will now consider the principle of operation of a running thermocouple in more detail.
This design is mainly used to detect the temperature of a steel billet during its processing on turning, milling and other similar machines.
It should be noted that in this case it is also possible to use a conventional thermocouple, however, if the manufacturing process requires high temperature accuracy, it is difficult to overestimate the running thermocouple.
When applying this method, its contact elements are soldered into the workpiece in advance. Then, in the process of processing the blank, these contacts are constantly exposed to the action of a cutter or other working tool of the machine, as a result of which the junction (which is the main element when taking temperature readings) seems to “run” along the contacts.
This effect is widely used in the metalworking industry.
Technological features of thermocouple designs
In the manufacture of the working circuit of a thermocouple, two metal contacts are soldered, which, as you know, are made of different materials. The junction is called a junction.
It should be noted that it is not necessary to make this connection using soldering. Simply twist two contacts together. But such a production method will not have a sufficient level of reliability, and may also give errors when taking temperature readings.
If you need to measure hightemperatures, the soldering of metals is replaced by their welding. This is due to the fact that in most cases the solder used in the connection has a low melting point and breaks down when it is exceeded.
Circuits that have been welded can withstand a wider temperature range. But this method of connection also has its drawbacks. The internal structure of the metal when exposed to high temperatures during the welding process may change, which will affect the quality of the data obtained.
In addition, the condition of the thermocouple contacts should be monitored during its operation. So, it is possible to change the characteristics of metals in the circuit due to the impact of an aggressive environment. Oxidation or mutual diffusion of materials may occur. In such a situation, the working circuit of the thermocouple should be replaced.
Types of thermocouple junctions
Modern industry produces several designs that are used in the manufacture of thermocouples:
- open junction;
- with insulated junction;
- with grounded junction.
A feature of open-junction thermocouples is poor resistance to external influences.
The following two types of design can be used when measuring temperatures in aggressive environments that have a devastating effect on the contact pair.
In addition, the industry is currently mastering schemes for the production of thermocouples using semiconductor technologies.
Measurement error
The correctness of the temperature readings obtained using a thermocouple depends on the material of the contact group, as well as external factors. The latter include pressure, radiation background or other reasons that can affect the physico-chemical parameters of the metals from which the contacts are made.
Measurement error consists of the following components:
- random error caused by the manufacturing process of the thermocouple;
- error caused by violation of the temperature regime of the "cold" contact;
- error caused by external interference;
- error of control equipment.
The benefits of using thermocouples
The benefits of using these temperature control devices, regardless of application, include:
- large range of indicators that can be recorded using a thermocouple;
- The junction of the thermocouple, which is directly involved in taking readings, can be placed in direct contact with the measuring point;
- Thermocouples are easy to manufacture, strong and durable.
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" contact of the thermocouple. This is a distinctivedesign feature of measuring instruments, which are based on a thermocouple. The principle of operation of this scheme narrows the scope of its application. They can only be used 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 thermocouples. The fact is that as a result of exposure to the external environment, the contacts lose their uniformity, which causes errors in the obtained temperature indicators.
- During the measurement process, the thermocouple contact group is usually exposed to the negative influence of the environment, which causes disturbances in the process. This again requires sealing the contacts, which causes additional maintenance costs for such sensors.
- There is a risk of exposure to electromagnetic waves on a thermocouple, the design of which provides for a long contact group. This may also affect the measurement results.
- In some cases, there is a violation of the linear relationship between the electric current that occurs in the thermocouple and the temperature at the measurement site. This situation requires calibration of control equipment.
Conclusion
Despite its shortcomings, the method of temperature measurement using thermocouples, which was first invented and tested in the 19th century, has found its wide application in all branches of modern industry.
In addition, there are applications where the use of thermocouplesis the only way to get temperature data. And having read this material, you have quite fully understood the basic principles of their work.
