Max Recommended Distance That Thermocouple Wire Can Be Run

[JORMAC]

Hello,

I trying to determine the maximum length that thermocouple (extension) wire can be run from the connection head directly to a PLC T/C board. Is there a standard (API, ISA, IEEE, etc.) which addresses this installation question?


Best,

KC

 

[hacksaw]

depends on the input impedence of you a/d and the size of you extension wires. At one time runs of several hundreds of meters were common in large plants. These days it is a toss-up as to the cost of a transmitter at some satellite point versus the use of extension wires. with the plc you have to confront CMRR and CMR voltagesif you are using grounded sensors and using exceptional run distances.

it is a engineering not a standards item although some customers may have recommended practices that you have to comply with.

 

[khan101]

I used to work at a very ancient plant as an Instrument Tech back in 80's and T/C wires upto 500 yards were very common there (both "K" and "J" type).

 

[ilan7]

Both are right. I used compensation cable of 500 meters at the past. Nowadays we prefer to install a 4-20 ma transmitter at the Therocouple and run the 4-20ma signal to the PLC. I had problems with direct connecting TC to the PLC and using RTD (PT100) for direct connection or using transmitters for long wires. Transmitters are at the 80 to 120$ range, and you can get good transmitters from Mescon (

http://www.Mescon.com),

RP Electronics (

http://www.RPElectronics.dk),

Minco (Minco.com) and others.