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Thermocouple probe related knowledge
Jun 21, 2018

Thermocouple / probe the basic principles of temperature measurement:

The two different materials of the conductor or semiconductor A and B welded together to form a closed loop, when the conductor A and B between the two points 1 and 2 there is a temperature difference between the two will produce electromotive force between the loop In the formation of a size of the current, this phenomenon known as the thermoelectric effect. Is the so called Sebeck effect. Thermocouple / probe is the use of this effect to work.

Conductors A and B are called thermal electrodes. (T> called the working end (usually welded together); the lower end of the temperature (To) called the free end (usually at a constant temperature>).

According to the relationship between thermoelectric potential and temperature function. Can be made thermocouple indexing table. The indexing table is obtained at the free end temperature To = 00C. Different thermocouples have different indexing tables.

When the third metal material is connected to the thermocouple circuit, the thermoelectric power generated by the thermocouple will remain unchanged as long as the temperature of the two contacts of the material is the same, that is, not affected by the third metal access circuit. Therefore, the thermocouple temperature measurement, access to measuring instruments, measured thermoelectric power, you can know the temperature of the measured medium. In theory, any two conductors can be formulated into thermocouples, but in fact not all materials can produce thermocouples, so the thermal electrode material must meet the following: (1) thermocouple material by temperature Can produce a higher thermoelectric potential, the relationship between thermoelectric potential and temperature is best linear or nearly linear single-valued function; (2) can measure the higher temperature, and in a wide temperature range of domestic applications, (3) the requirements of the material temperature coefficient of resistance is small, high resistivity, good electrical conductivity, heat capacity is small; (4) reproducibility is good, easy to mass production and the production of high-performance, (5) good mechanical properties, material uniform; (6) resource-rich, cheap.

Thermocouple / probe type and structure:

   

    First, the type of thermocouple probe Thermocouple probe can be divided into:

 

   (1) standard thermocouple / probe (2) non-standard thermocouple probe two categories.

  The so-called standard thermocouple probe is the national standard specifies the relationship between its thermoelectric power and temperature, allowing the error, and a unified standard thermocouple probe, it has a matching display instrument available. Non-standardized thermocouple / probe in the scope of use or magnitude are less than the standardization of thermocouple / probe, generally there is no uniform sub-table, mainly for some special occasions, the measurement.

Standard Thermocouple Probes China Since January 1, 1988, thermocouple / probe and thermal resistance are all produced according to IEC international standards and designated S, B, E, K, R, J, T seven standard thermocouple / probe Designed for our country unified thermocouple / probe.

Second, the thermocouple / probe structure In order to ensure the thermocouple / probe reliable and stable work, its structure requirements are as follows: ① thermocouple probe composed of two thermal electrode welding must be strong; ② two hot electrodes with each other Should be well insulated to prevent short circuit; ③ compensation wire and thermocouple / probe free end of the connection should be convenient and reliable; ④ protective sleeve should be able to ensure that the thermal electrode and harmful media fully isolated. (3). Cold junction temperature compensation.

As the thermocouple probe 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 the compensation wire thermocouple / probe cold (Free end) extends to a relatively stable temperature control room, connected to the instrument terminal. It must be noted that the thermocouple / probe compensating wire acts only to extend the thermal electrode so that the cold end of the thermocouple / probe moves to the instrument terminal of the control room itself and does not eliminate the effect of cold junction temperature variations on temperature Play a compensatory role. Therefore, other correction methods need to be used to compensate the cold junction temperature t0 ≠ 0 ℃ on the temperature of the impact.

When using the thermocouple / probe compensation wire must pay attention to the model match, polarity can not be wrong, compensation wire and thermocouple / probe connection side of the temperature can not exceed 100 ℃.


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