【Daily Question 20090319】Are the thermoelectric properties of a compensation wire exactly the same as those of the thermocouple it is matched with? Please state the reason.
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【Daily Question 20090319】Are the thermoelectric properties of a thermocouple compensation wire exactly the same as those of the thermocouple it is matched with? Please state the reason. Posts that are duplicated or copied, as well as those that constitute plagiarism, will have their wealth points deducted upon confirmation. We hope that everyone will express themselves in their own words and participate actively; at the same time, valuable insights from those who take part in the discussions will be rewarded generously.
Since the materials used for thermocouples are generally expensive (especially when precious metals are employed), and the distance from the temperature measurement point to the instrument is quite large, in order to save on thermocouple material and reduce costs, compensation wires are typically used to extend the cold end (the free end) of the thermocouple into a control room where the temperature is more stable, thereby connecting it to the instrument terminals. It must be noted that the function of a thermocouple compensation wire is merely to extend the thermal electrode and move the cold end of the thermocouple to the instrument terminals in the control room; it cannot eliminate the impact of changes in the cold end temperature on temperature measurement, and thus it does not serve a compensating role. Therefore, other correction methods must also be employed to compensate for the effect of the cold-end temperature t0≠0℃ on temperature measurement. When using thermocouple compensation wires, it is essential to ensure that the models match, that the polarity is correct, and that the temperature at the connection point between the compensation wire and the thermocouple does not exceed 100°C. 1. The compensation wire must be used in conjunction with the corresponding thermocouple. 2. The positive terminal of the compensation wire is connected to the positive terminal of the thermocouple, and its negative terminal is connected to the corresponding negative terminal. The compensation wire must be selected properly based on the type of thermocouple being used and the application scenario. For example, a K-type thermocouple should be paired with a compensation wire of the same K-type, and the operating temperature range should be selected based on the application. Typically, the operating temperature for KX is -20 to 100°C, with a wider range of -25 to 200°C. The error for the standard grade is ±2.5°C, while for the precision grade it is ±1.5°C. I’m aware of the phenomenon of wires being connected in reverse; it was discussed before, and everyone can take a look at it. It turns out that one can also refer to the temperature difference calculation formula when choosing the wrong type of wire. This post was last edited by wopale3 on 2009-3-19 at 10:34.]
The thermoelectric properties of the compensation wire should be the same as or similar to those of the thermocouple it is used with; compensation wires are employed due to advantages such as lower cost. But strictly speaking, they’re not exactly the same. For example, the potential of an S-type thermocouple at 100 degrees is 0.645 mV, while the potential of its matching compensation wire, copper-copper-nickel, at 100 degrees is 0.643 mV.
The thermoelectric properties of the thermocouple compensation wire and the thermocouple it is matched with are not exactly the same, but only similar; compensation wires are used to extend the cold end (free end) of the thermocouple into a control room where the temperature is relatively stable.
Of course, they are not exactly the same; compensation wires are cheaper than thermocouples. Within the range of 0–100 degrees, their thermoelectric properties are only roughly similar to those of the thermocouple used. Therefore, using a compensation wire is like extending the thermocouple’s cold end to a control room that is far away from the heat source, where temperature changes are minimal； It’s like a very long thermocouple extending into the control room. :Lol. Additionally, in earlier years, temperature compensation involved adding a fixed temperature value, namely the room temperature in the control room; this also led to some errors. If the temperature in the control room changed, the value displayed after compensation would differ from the actual temperature. These days, cold-end compensation for thermocouples is done using the temperature of the control room (PT100) within the configuration; therefore, even if there are slight changes in the temperature of the control room, it does not affect the temperature reading displayed by the thermocouple. This post was last edited by chmyh on 2009-3-19 16:04]
Compensation wires are cheaper than thermocouples. Within the range of 0 to 100 degrees, their thermoelectric properties are identical to those of the thermocouple they are used with. Therefore, using compensation wires is like extending the thermocouple’s cold end to a control room or another location where it is convenient to take readings, and which is far away from the heat source； However, strictly speaking, the compensation wire and the thermocouple potential are not exactly the same. For example: the compensation wire made of copper-constantan has a potential of 4.27 mV at 100 degrees; the corresponding K-type thermocouple has a potential of 4.095 mV ； The compensation wire is copper-nickel-copper; its potential at 100 degrees is 0.643 mV, whereas that of the corresponding S-type thermocouple is 0.645 mV.
Similar to my post 【Daily Question 20090308】 at http://bbs.hcbbs.com/thread-409487-1-1.html, compensation wires are cheaper than thermocouples. Within the range of 0–100 degrees, their thermoelectric properties are identical to those of the associated thermocouple; therefore, compensation wires can be used to extend the cold end of the thermocouple to a control room or another location where it is convenient to take readings, and which is far away from the heat source； Strictly speaking, however, the compensation wire has the same thermocouple potential. This post was last edited by Mobei Yihai on 2009-3-19 16:27.]
Compensation wires are cheaper than thermocouples. Within the range of 0–100°C, their thermoelectric properties are identical to those of the associated thermocouple; therefore, using a compensation wire is like extending the thermocouple’s cold end to a control room or another location where it is convenient to take readings, and which is far away from the heat source； However, strictly speaking, the compensation wire and the thermocouple potential are not exactly the same. For example: the compensation wire made of copper-constantan has a potential of 4.27 mV at 100°C, while the corresponding K-type thermocouple has a potential of 4.095 mV ； The compensation wire is copper-nickel-copper; its potential at 100°C is 0.643 mV, whereas that of the matching S-type thermocouple is 0.645 mV. It’s the same as my post 【Daily Question 20090308】: http://bbs.hcbbs.com/thread-409487-1-1.html. The moderator is also troubled by this error; I suggest that this post be deleted!
Being exactly the same is not feasible; compensation wires are used because they are cheaper than thermocouples, thus helping to reduce costs. Their thermal characteristics only need to be similar to those of the thermocouples, and different thermocouples require different compensation wires.