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Application of the principle of thermocouples
Thermocouples are among the most commonly used temperature sensing element. Thermocouple many advantages, namely:
1, high-precision measurement of temperature
Because the thermocouple in direct contact with the measured object, is not affected by the middle of the media. Temperature quickly and accurately.
2, wide measurement temperature range
Common side continued thermocouple can be measured from the -50 ~ + 1600 ℃, some special thermocouple lowest measurable to -269 ℃ (such as Gold and Iron nickel-chromium), up to + 2800 ℃ (such as tungsten - rhenium).
3, simple structure, easy to use
Thermocouples are usually composed of two different metal filaments, and without size restrictions beginning with the outer protective casing, it is very easy to use.
Thermocouple Fundamentals
The two different materials A and B conductors or semiconductors welded together to form a closed loop, as shown in Fig. 2-1-1. When the two attachment points when the temperature difference between 1 and 2, they produce conductors A and B between the two electromotive force, thus forming a size in the loop current, a phenomenon known as the thermoelectric effect. Thermocouple is to use this effect to work.
Thermocouple types and structure formation
Category (1) thermocouple
Commonly used standard thermocouple and thermocouple can be divided into two major categories of non-standard thermocouple. The call refers to the state standard thermocouple standard specifies the relationship between the thermoelectric power and temperature, tolerance, and a unified standard thermocouple indexing table, it has its support of the display instrument available. Non-standard thermocouple in the range of magnitude or not and standard thermocouple, generally there is no uniform indexing table, mainly used to measure certain special occasions.
Standard thermocouple China from January 1st 1988, thermocouples and RTD all according to IEC international standards, and designated S, B, E, K, R, J, T thermocouple seven kinds of standardization for our unified design Type thermocouple.
Form structure (2) thermocouple
In order to ensure thermocouple reliable, stable work, its structure requirements are as follows:
1, welding electrodes consisting of two hot thermocouple must be firm.
2, two hot electrodes to each other should be well insulated to prevent short circuits.
3, compensation wire thermocouple is connected to the free end of the convenient and reliable.
4, should be able to guarantee protection casing hot electrode and harmful media full isolation.
Thermocouple cold junction temperature compensation
Because thermocouple material are generally more expensive (especially the use of precious metals), and the distance measuring point to the instrument very far, in order to save the thermocouple material, reduce costs, usually compensation wire thermocouple cold end (free end) extends to the temperature is relatively stable in the control room, connected to the meter terminals. It must be noted, the role of thermocouple wires extending from the hot electrode only, so the cold end of the thermocouple is moved to the control room instrumentation terminals, which in itself does not eliminate the effect of temperature changes on the cold end of the temperature measurement, can not afford compensation. Therefore, the need to use other methods to compensate for correcting the cold junction temperature t0 ≠ effects on temperature of 0 ℃ time.
When using the thermocouple wire must be noted that the model match, can not pick the wrong polarity, compensation wire thermocouple connecting terminal temperature should not exceed 100 ℃.
Other armored thermocouple, sheathed platinum resistance.