Regarding the compensation wire for thermocouples

I’m not sure which length the original poster is referring to The K-type compensation wires here run from the field all the way to the DCS cabinet room, over a distance of about 400 meters; there is no such thing as a specific length requirement!

Care should be taken to distinguish between compensation wires: those of the compensation type and those of the extension type, as their applicable temperature ranges differ

You need to understand that the compensation wire of a thermocouple here is actually the signal wire of the thermocouple. Since thermocouples require cold-junction compensation, ordinary signal wires cannot be used for signal transmission; instead, a compensation wire that is suitable for that particular thermocouple must be used as the signal wire. So of course, the length of the compensation wire must be the same as that of the signal cable, because you are using it as the signal cable!

I’d like to ask a question as well: what is the difference between compensatory and extended types? How is the compensatory type used, and how is the extended type used?

Compensating wires are primarily used for thermocouples made of precious metals (such as platinum, rhodium, nickel, chromium, and other metals). Compensating wires of the compensatory type are wires made from two inexpensive metals, whose thermoelectric properties are identical to or similar to those of the thermocouple’s hot junction within a certain temperature range (0–100°C or 0–200°C). They serve as specialized cold-end extensions for such thermocouples, allowing their cold ends to be extended into the control room. For thermocouples that are already not expensive, there is no need to use another type of metal wire as a replacement for the extension wire; instead, a wire made of the same material as the thermocouple wire can be used as the cold-end extension wire, which is known as a compensating extension wire. These two types of compensation wires are collectively referred to as compensation wires. The function of a compensation wire is not to compensate for the temperature at the cold end; rather, it serves to extend the cold end of the thermocouple to an environment with a more constant temperature, thereby enabling manual and automatic compensation, reducing the costs associated with the extended wiring, and improving the physical and mechanical properties of the wiring. Platinum-rhodium 30-platinum 6 thermocouples do not require a compensation wire; a shielded cable can be used as an extension wire. Since the thermoelectric potential at the cold end of this thermocouple remains below 3 μV within the temperature range of 0–50°C, it has little impact on the measurement results.

Such issues related to the use of compensation wires involve basic knowledge and concepts of instruments. I hope colleagues in the instrumentation field will read more professional books on instrumentation. Such as chemical process automation instruments, instrument engineering construction manuals, etc

The problem has been solved. Thank you all. To summarize: 1. The compensation wire of a thermocouple is actually the signal wire for that thermocouple. Since thermocouples require cold-junction compensation, ordinary signal wires cannot be used for signal transmission; instead, a compensation wire specific to that thermocouple must be used as the signal wire. Therefore, the length of the compensation wire must naturally be the same as that of the signal cable, since it is used as the signal cable itself. 2. Compensation wires are divided into compensatory compensation wires and extension-type compensation wires (with models KC and KX respectively; for details, refer to the Manual of Commonly Used Devices in Automatic Control, page P204). Among them, KC is further divided into KPC copper and KNC constantan, while KX is divided into KPX nickel-chromium and KNX nickel-silicon. Compensatory compensation wires are made from two inexpensive metals, and their thermoelectric properties are identical or similar to those of the thermocouple’s hot junction within a certain temperature range (0-100°C or 0-200°C). They serve as specialized cold-junction extensions for such thermocouples, extending their cold junctions to the control room. For thermocouples that are already not expensive, there is no need to use another type of metal wire as a replacement for the extension wire; instead, a wire made of the same material as the thermocouple wire can be used as the cold-end extension wire, which is known as a compensating extension wire. These two types of compensation wires are collectively referred to as compensation wires. The function of a compensation wire is not to compensate for the temperature at the cold end; rather, it serves to extend the cold end of the thermocouple to an environment with a more constant temperature, thereby enabling manual and automatic compensation, reducing the costs associated with the extended wiring, and improving the physical and mechanical properties of the wiring. (In simple terms, KC needs to be more economical.)

What are the precautions when using compensation wires? http://bbs.hcbbs.com/viewthread.php?tid=594518&extra=page%3D2%26amp%3Bfilter%3Dtype%26amp%3Btypeid%3D3

The last edit to this post was made by broken01 on 2010-1-9 at 23:50. In the document titled “Professional Skill Assessment **Question Bank – Petrochemical Sub-library: Advanced Theoretical Knowledge Test for Instrument Repair Technicians”, there is a question like this: (The answer is provided in the materials.) There is a nichrome-aluminum constantan compensation wire with specifications of 2 1.5 (resistance value of 0.77 ohms/m), and its length connecting the thermocouple and the moving-coil instrument is 15 meters. Can this compensation wire be used? Solution: The total length of the compensation wire is 15*2=30 m. The resistance of this compensation wire is 30*0.77=23.1 ohms. Since moving-coil meters generally require an external wire resistance of 15 ohms, it is not appropriate to use a compensation wire of this diameter. It can be seen that there are still requirements regarding length.