Thermocouples

GENERAL CHARACTERISTICS

Thermocouples are temperature sensors that operate by means of two different conductors, joined at their ends. Inside them there is an electrical circuit formed by two metallic conductors of different materials welded together at their ends.

In the presence of a temperature difference between the two junctions, one generates a circulating current, so-called electromotive force, in the event that is open one of the two junctions, which is proportional to the temperature difference, and these joints are called the hot junction (also commonly called “joint degree”), directly exposed to the temperature you want to measure and cold junction (or “reference junction”), corresponding to the junction between the wires of the thermocouple and measuring circuit, while the terminals are at room temperature, the potential difference between them is proportional to the difference between the temperature and the ambient temperature to detect .

The polarization and intensity of the electromotive force generated depends solely on the type of the two metals forming the thermocouple and the temperature at which they subjected the two joints.

For a given temperature, the thermocouple measuring process is necessary that the junction is at a given temperature (usually 0 ° C), so that the current loop generated in (electromotive force) depends solely on the temperature of the hot junction.

Main technical parameters:

• Temperature range in which the thermocouple may operate or maximum operating temperature
• Voltage supplied
• These parameters depend on the materials from which the thermocouple is made.

Advantages in the use of thermocouples:

• Ability to measure a wide temperature range (around 2000 ° C);
• Possibility of use of materials resistant to different environmental conditions
• Thermocouples are temperature sensors with low cost.
• There are, on the contrary, particular problems or disadvantages in their use, apart from the lack of linearity and low output voltage.
• The type of thermocouple depends on the materials comprising the conductors, and can be summarized as follows: drive or bulb capillary bulb thermostats), and standard voltage adjustment scale.

There are two structural types of thermocouples:

• Thermocouples with traditional insulation
• Mineral insulated thermocouples.

The thermocouples are made with traditional insulation:

Thermocouples with traditional insulation (composition):

Measuring junction (or hot junction) is the area where the two thermocouple wires are joined together, since its dimensions are small, we can consider the measure with point-type thermocouples. The execution of this joint must be done in a way that no mechanical stress on the two conductors (especially for thermocouples made of noble metals) as these, once in temperature would affect the proper functioning of the thermocouple.
2)Thermocouple wires: they must be properly sized according to the conditions of employment; same probe can be inserted into two or more thermocouples.
3)Ceramic insulators: they serve to keep the thermocouple wires insulated along the length of the probe with each other and the external sheath.
4) Protective sheath: the aim is to protect the thermocouple wires, in fact, being in direct contact with the process, it is essential that the sheath is the most suitable material and has specific dimensions. Normally made of metal, if the process temperature is very high, it can also be made ​​of ceramic material. In particular conditions, it is appropriate that the sheath is coated with an additional protection that takes the name of the thermowell.
5) Connection head: contains the terminal of insulating material, usually ceramic, which allows the electrical connection of the thermocouple, for use in explosive atmospheres, it is also possible to equip these thermocouples with heads of ATEX Explosion-proof connections. In place of the terminal you can install a converter with 4-20mA.

Mineral insulated thermocouples MgO (composition):

These thermocouples that can achieve high performance and extended operational life, due to excellent technology with which they are constructed and their mechanical properties, also encased in very small (0.5 mm in diameter), but also very long. Their protective sheath can have very tight bend radii.

Measuring junction, the two conductors forming the thermocouple are joined in the mineral oxide insulated cable. The hot junction may be insulated, grounded or exposed.
Thermocouple wires: in the MgO insulated cable can be two, four or six wires, so the thermocouple can be simple, double or triple.
Mineral insulated sheath: it is composed of a metal sheath containing the conductors insulated from each other and the sheath itself, and this is by means of pure metal oxides and highly compressed, standard insulation magnesium oxide MgO.
Connection head: contains the terminal of insulating material, usually ceramic, which allows the electrical connection of the thermocouple, use explosion-proof enclosures. for use in explosive atmospheres, it is also possible to equip these thermocouples with heads of ATEX Explosion-proof connections. In place of the terminal you can install a converter with 4-20mA.

Ithe following graph shows the time it takes for a thermocouple isolation

Mineral reach 63.2% of the temperature difference measured in water with a speed of 0.4 m / s

For a correct temperature measurement, it is essential that the transmission of the voltage signal produced by the thermocouple to the measuring instrument is not altered in any way and for this, this link allows the cable have to have very similar characteristics to those of the thermocouple.

There are three types of cables for thermocouples:

• Thermocouple cable normally used for the construction of thermocouple sensors, provides all the features of the thermocouple thermoelectric entire temperature range defined by the reference standard adopted (DIN, IEC, ANSI).
• Extension cable normally used to connect the thermocouple to the measuring instrument, its conductors have the same characteristics and thermoelectric properties of their respective thermocouples within a limited temperature range, in the range 0-200 ° C.
• Compensation cable used to connect the thermocouple to the respective measuring instrument, and his conductors have different characteristics than those of the thermocouples which are connected, although it maintains the same thermoelectric properties, with a limited temperature range, typically in the range 0 -100 ° C or 0-150 ° C). Just as an example, we can think of wire compensation for thermocouples with positive lead of copper (Cu) and the negative of copper-nickel alloy (Cu-Ni).

Below is shown a table that shows, respectively, by type of extension cords and compensation, the nature of the conductors and the tolerances in accordance with international standards.

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