Characteristics of thermocouple and measurement principle
Thermocouple characteristics and measurement principle?
1.Thermocouple features are high precision measurement. Due to the thermocouple directly with the measured object contact, from the middle of the medium.
Wide range of measurements. Commonly used thermocouple from -50 ~ +600 ℃ can be continuously measured, some special thermocouple can be measured at -269 ℃ (such as gold and iron nickel chrome), up to +2800 ℃ (such as tungsten - rhenium). Simple structure, easy to use. Thermocouples are usually composed of two different wires, and are not limited by the size and the beginning, the outer protective sleeve, with very convenient to use.
2.Thermocouple measurement principle：
Thermocouple is a temperature sensor, it can convert the temperature signal into a thermoelectric power signal, through the electrical measurement instrument with the measurement can be measured temperature. The basic principle of thermocouple temperature measurement is thermoelectric effect.
In the closed loop consisting of conductors A and B of two different materials, the thermoelectric potential is generated in the loop when the two contacts of A and B are at different temperatures T and To. This is called the Seebeck effect. Conductors A and B are called thermal electrodes.
The lower end of the temperature (T> is called the working end (usually welded together); the lower end of the temperature (To) is the free end (usually at a constant temperature). According to the relationship between the thermoelectric potential and the temperature function, Into the thermocouple indexing table.
The indexing table is obtained under the condition of the free end temperature To = 00 C. Different thermocouples have different indexing tables.When the third metal material is connected in the thermocouple circuit, The temperature of the two contacts of the material is the same, the thermoelectric power generated by the thermocouple will remain unchanged, that is not affected by the third metal access circuit.
Therefore, the thermocouple temperature measurement, access to measuring instruments, After measuring the thermoelectric power, you can know the temperature of the measured medium.
3, thermocouple degree of standardization of thermocouple, according to IEC international standards. Thermocouple indexing number is mainly S, R, B, N, K, E, J, T and so on several. Which S, R, B are precious metal thermocouple, N, K, E, J, T are cheap metal thermocouple.
S indexing is characterized by strong antioxidant properties, should be in the oxidizing, inert atmosphere for continuous use, long-term use temperature 1400 ℃, short-term 1600 ℃. In all thermocouples, the S scale has the highest accuracy level, usually used as a standard thermocouple; R indexing number and S indexing number compared to the thermal electromotive force about 15%, other performance is almost the same; B points Degree at room temperature thermoelectromotive force is very small.
So the measurement is generally not compensation wire. Its long-term use temperature is 1600 ℃, short-term 1800 ℃. Can be used in oxidizing or neutral atmosphere, but also in short-term use under vacuum conditions.
N is the characteristics of high temperature oxidation resistance at 1300 ℃, long-term stability of the thermoelectromotive force and short-term thermal cycle of good reproducibility, resistance to nuclear radiation and low temperature performance Ye Hao, you can partially replace the S sub-degree Thermocouple; K sub-degree is characterized by strong antioxidant properties, should be in the oxidizing, inert atmosphere for continuous use, long-term use temperature of 1000 ℃, short-term 1200 ℃. In all thermocouples in the most widely used.
E sub-degree is characterized by commonly used thermocouples, the maximum thermoelectromotive force, that is, the highest sensitivity. Should be used in oxidizing, inert atmosphere, the use of temperature 0-800 ℃; $ vmeJ sub-degree is characterized by both for the oxidizing atmosphere (the use of temperature ceiling of 750 ℃), can also be used for reducing atmosphere (the use of temperature limit 950 ℃), and resistance to H2 and CO gas corrosion, and more for refining and chemical; T sub-degree is characterized by all the cheap metal thermocouple in the highest degree of accuracy, usually used to measure the temperature below 300 ℃.
4.The thermocouple cold junction temperature compensation As the thermocouple materials are generally more expensive (especially when the use of precious metals), and the temperature measurement point to the instrument are far away, in order to save thermocouple material, reduce costs, usually using compensation wire The cold end of the thermocouple (free end) extends to a relatively stable temperature control room, connected to the instrument terminal.
It must be pointed out that the role of the thermocouple compensation wire only extends the thermal electrode, so that the cold end of the thermocouple to move to the control room of the instrument terminal, which itself can not eliminate the cold junction temperature changes on the temperature, can not afford compensation.
Therefore, other correction methods need to be used to compensate the cold junction temperature t0 ≠ 0 ℃ on the temperature of the impact. In the use of thermocouple compensation wire must pay attention to the model match, the polarity can not be wrong, compensation wire and thermocouple connection side of the temperature can not exceed 100 ℃. Cold end temperature compensator model should be consistent with the model of the thermocouple, and within the specified temperature range; cold junction temperature compensator and thermocouple connection polarity can not be wrong; according to the balance of the compensator temperature adjustment instrument starting point.
So that the pointer at the equilibrium point temperature; with automatic compensation mechanism of the display instrument does not install the compensator; compensator must be regularly checked and verified.
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