I am using a thermocouple, of Type N. But i am confused about choosing between the type of cable for the extension wire. Under what circumstances should Multicored cables (not twisted) be used and when should the Twisted Pair configuration be used?

I am new in this field. Any form of help is appreciated.

Thanks in advance.

 

You need to use thermocouple extension wire and thermocouple terminal blocks, having the same alloys as the junction.

Twisted pair cabling will reduce common mode noise induced into the cable. The requirement to use twisted, or shielded twisted pair cabling depends on how sensitive the equipment is to noise, how noisy the environment is, and how long the cable run is.

Search on "+Thermocouple", there are many posts on extension wire and compensation.

 

Hi.

Thanks. tried looking, did not get much info. Will certainly try again.

Again, thanks for replying.

 

Shrk,

www.omega.com is a very good resource for all kinds of sensors, particularly RTDs and T/Cs.

You must almost always use T/C extension wire for connecting T/Cs to monitors or PLCs or DCSs or other control systems--unless the device you are connecting the T/C to has the capability to determine the cold junction temperature in a remote location (usually an expensive option, and also requires more wire).

As for the comment about colored wires, usually T/C wires and T/C extension wires are specially colored to indicate the different types of metal used in the construction of the T/C and T/C extension wire so that the wires can be properly terminated. (Wire color is also indicative of polarity.)

Most T/C extension wire I've seen is usually twisted, and quite often twisted and shielded. As previously mentioned, twisting helps to avoid unwanted electrical noise from affecting the signal. Because the signal generated by T/Cs is DC millivolts it's always a good idea to use twisted pairs, and if you're not sure about proper signal level separation from high voltage/high current wires and cables in the run from the T/C to the control system then twisted, shielded pairs are the best insurance against unwanted electrical noise (of course, the shield drain wires must be properly terminated!).

As for using T/C terminal boards, that may or may not be necessary depending on the type of terminal board. It's usually a good practice to use terminal boards made with the same metals as the type of T/C, but if the terminations are not done correctly unwanted cold junctions can still be introduced.

So, what you want to be most concerned about are preventing cold junctions (you can look this up on omega.com or using your preferred Internet search engine) and using the proper type of T/C extension wire. (T/C extension wire is wire made using the same materials as the T/C is made of. This is necessary in order not to introduce any error in the signal, usually caused by cold junctions.)

Finally, you need to comply with the standards in use at your site (if any) for wiring, and any local or regional technical codes, regulations and/or standards for exterior color coding or fire retardancy, smoke reduction, etc.

Hope this helps!

 

It is seldom ever a good idea to run un-twisted pair thermocouple extension wire. Thermocouple signals are in the low mV range and care must be taken to minimize noise pick up from the ambient electromagnetic environment, and if the noise does get into the system, provide means to reduce the noise and maintain the integrity of the signal. Twisted pair works to cancel out common mode noise (e.g. identical noise that exist on both signal wires) that has gotten into the system, which most commonly comes from magnetically (inductive) coupled or ground loop noise. In refinery, chemical, and industrial facilities, good engineering practice would be to use shielded, twisted pair for TC extension cable and in addition, for the multi-conductor trunk an overall shield.

Some addition information can be found at :
http://www.testunlimited.com/pdf/an/5989-7722EN.pdf
https://www.tewire.com/noisred.php
http://isadenver.org/Docs/ReducingNoise.pdf

You can also look at API 552, "Transmission Systems," which covers all types of signal wiring for instrumentation, including thermocouples and IEEE 518-1982,"IEEE Guide for the Installation of Electrical Equipment to Minimize Electrical Noise Inputs to Controllers from External Sources."

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.

 

please note that T/C TYPE N lower cost compensated can be used depends up on your critical temperature measurement.
Regarding twisted paired cable shall be used because low mv will be affected buy electrical unwated signal which is noise.