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. The principle of operation and the circuit of this device we will consider 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 action of a thermocouple rests is as follows. In a closed electrical circuit, consisting of two conductors of various kinds, when exposed to a certain ambient temperature, electricity occurs.
The resulting electrical flow and the ambient temperature acting on the conductors are linearly dependent. That is, the higher the temperature, the greater the electric current generated by the thermocouple. The principle of operation of a thermocouple and a resistance thermometer is based on this.
In this case, one contact of the thermocouple is at the point where it is necessary to measure the temperature, it is called "hot". The second contact, in other words, “cold,” is in the opposite direction. The use of thermocouples for measurement is allowed only if the air temperature in the room is less than at the measurement location.
This is a brief diagram of the thermocouple, the principle of operation. Types of thermocouples we will consider in the next section.
Types of Thermocouples
In every industry where temperature measurements are needed, a thermocouple is mainly used. The device and the 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 temperature control 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 impractical to use thermocouples with similar alloys in workshops and on objects with a high sulfur content in the air, since this chemical element negatively affects both chromium and aluminum, causing malfunctions of the device.
Chromel-Copel Thermocouples
The principle of operation of a thermocouple, the contact group of which consists of these alloys, is the same. But these devices work mainly in a liquid or gaseous medium with neutral, non-aggressive properties. The upper temperature indicator does not exceed +8000 degrees Celsius.
A similar thermocouple is used, the principle of which allows it to be used to establish 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, however, such a combination showed itself well in a rarefied atmosphere. The maximum level of measured temperature should not exceed +12500 degrees Celsius.
However, if the temperature begins to rise above +7000 degrees, there is a danger of a violation of the accuracy of measurements in connection with a change in the physicochemical 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.
Platinum Rhodium-Platinum Thermocouples
The most expensive thermocouple in manufacturing. The principle of operation is the same, but it differs from its counterparts in very stable and reliable temperature readings. Has a reduced sensitivity.
The main field of application of these devices is the measurement of high temperatures.
Tungsten-Rhenium Thermocouples
Also used for measuring ultra-high temperatures. The maximum limit that can be fixed using this scheme reaches 25 thousand degrees Celsius.
Their use requires compliance with certain conditions. So, in the process of measuring temperature, it is necessary to completely eliminate the surrounding atmosphere, which has a negative effect on contacts as a result of the oxidation process.
For this, tungsten-rhenium thermocouples are usually placed in protective casings filled with inert gas protecting their elements.
Above, each existing thermocouple, device, its operation principle, depending on the alloys used, were considered. Now consider some design features.
Thermocouple Designs
There are two main varieties of thermocouple designs.
Using an insulating layer. This design of the thermocouple provides for the isolation of 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, the inputs of which do not have contact with the ground. If this condition is not met, two independent closed circuits will appear in the device, as a result of which the readings obtained with the help of a thermocouple will not correspond to reality.
Running thermocouple and its application
There is a separate version of this device, called "running". The principle of operation of a running thermocouple, we now consider in more detail.
This design is mainly used to determine 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 possible to use a conventional thermocouple, however, if the manufacturing process requires high accuracy of the temperature regime, a traveling thermocouple can not be overestimated.
When using this method, its contact elements are sealed in advance into the workpiece. Then, during the 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 when taking temperature readings) seems to “run” along the contacts.
This effect is commonly used in the metal 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 the two pins together. But this method of production will not have a sufficient level of reliability, and may also give errors when taking temperature readings.
If it is necessary to measure high temperatures, 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 is destroyed when its level is exceeded.
Schemes in the manufacture of which welding was used withstand a wider temperature range. But this connection method has its drawbacks. The internal structure of the metal when exposed to high temperature during welding can change, which will affect the quality of the data.
In addition, the condition of the thermocouple contacts during its operation should be monitored. So, it is possible to change the characteristics of metals in the circuit due to the influence of an aggressive environment. Oxidation or cross-diffusion of materials may occur. In this situation, replace the operating circuit of the thermocouple.
Varieties of thermocouple junctions
Modern industry produces several designs that are used in the manufacture of thermocouples:
A feature of open-junction thermocouples is their poor resistance to external influences.
The following two types of design can be used to measure temperatures in aggressive environments that have a devastating effect on the contact pair.
In addition, the industry is currently mastering semiconductor technology thermocouple production schemes.
Measurement error
The accuracy of the temperature indicators 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 physicochemical parameters of the metals from which the contacts are made.
The measurement error consists of the following components:
random error caused by the manufacturing features of the thermocouple;
the error caused by the violation of the temperature regime of the “cold” contact;
the error caused by external interference;
error of control equipment.
Benefits of Using Thermocouples
The advantages of using such devices for temperature control, regardless of application, include:
a large range of indicators that can be recorded using a thermocouple;
the thermocouple spike, which is directly involved in taking readings, can be placed in direct contact with the measurement point;
simple process of manufacturing thermocouples, their strength and durability.
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 distinctive feature of the design 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 the influence of the external environment, the contacts lose their uniformity, which causes errors in the obtained temperature indices.
During the measurement process, the contact group of the thermocouple is usually exposed to the negative effects of the environment, which causes disturbances during operation. This again requires sealing of the contacts, which causes additional costs for the maintenance of such sensors.
There is a danger 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 arising in the thermocouple and the temperature at the measurement site. This situation requires calibration of the control equipment.
Conclusion
Despite the shortcomings, the method of measuring temperature using thermocouples, which was first invented and tested in the 19th century, has found wide application in all sectors of modern industry.
In addition, there are applications where the use of thermocouples is the only way to obtain temperature data. And having familiarized yourself with this material, you have fully understood the basic principles of their work.