×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Long Distance Thermoucouple signal cable

Long Distance Thermoucouple signal cable

Long Distance Thermoucouple signal cable

(OP)
I have one thermocouple K-type signal need to travel along 100 m to the transmitter. Are this will effect my accuracy of reading?

thanks.

RE: Long Distance Thermoucouple signal cable

If accuracy, meaning comparison to a standard, is really you're concerned with, then you should be using a special limit-of-error thermcouple.  The commercial catalog on my desk shows that extension wire for type J and type T comes in special limit of error, but types K, N & E do not.  For a long, 100m run of extension wire, you might consider using a type for which special limit of error extension wire is available.  I have no idea of whether the availability of special limit-of-error extension wire is industry wide or just this vendor.

Routing the extension wire to minimize thermal gradients across the extension wire (as opposed to the desired hot-to-cold-junction gradient) will minimize the potential for leg-to-leg errors in those gradient.

And I must ask, why not move the transmitter and achieve a short run to the T/C and do the long run on copper with 4-20 or whatever bus the transmitter talks?

 

RE: Long Distance Thermoucouple signal cable

Omega carries SLE extension wire for types J, N, K, T, and E.

But I agree with danw2, if possible, do the long run with a conditioned signal.

-AK2DM

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"It's the questions that drive us"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 

RE: Long Distance Thermoucouple signal cable

1)  Wire resistance at that distance

According to the chart on page 2
http://www.thermometrics.com/assets/images/thermcpl.pdf
20g (0.032") type K wire has a combined resistance of 0.5884 ohms per foot (chromel and alumel legs combined).

100m = 330± feet * 0.5884 ohms/ft = 194 ohms total circuit resistance

That might exceed what the input on the transmitter can handle.   For example, a Honeywell UDC single loop controller can handle 50 ohms per leg for a thermocouple input.   Good luck trying to establish a maximum input resistance for many instruments, it tends an elusive specification.

2) ungrounded thermocouple or isolated transmitter

Normal mode noise is more easily developed with a grounded T/C than an ungrounded T/C.  A grounded thermocouple is more likely to develope a ground loop between the T/C and transmitter.  Using an ungrounded thermocouple minimizes the likelihood of both.  An isolated transmitter accomplishes much the same task at about 50-100x the cost of an ungrounded T/C.

 

RE: Long Distance Thermoucouple signal cable

(OP)
[qoute]If accuracy, meaning comparison to a standard, is really you're concerned with, then you should be using a special limit-of-error thermcouple.  The commercial catalog on my desk shows that extension wire for type J and type T comes in special limit of error, but types K, N & E do not.  For a long, 100m run of extension wire, you might consider using a type for which special limit of error extension wire is available.  I have no idea of whether the availability of special limit-of-error extension wire is industry wide or just this vendor.

Routing the extension wire to minimize thermal gradients across the extension wire (as opposed to the desired hot-to-cold-junction gradient) will minimize the potential for leg-to-leg errors in those gradient.

And I must ask, why not move the transmitter and achieve a short run to the T/C and do the long run on copper with 4-20 or whatever bus the transmitter talks?  [/qoute]

Actually it was install in flare tip and the Transmitter will install on groun leve. for maintanence purpose.

So the thermocouple cable run along the way about 100m to transmitter. I think i hv to ask supplier to supply a special limit-of-error extension wire

RE: Long Distance Thermoucouple signal cable

That's understandable in that kind of service.  

Do you typically use dual elements and the high end transmitters that allow for input fail-over to the 2nd element should the first element fail?

RE: Long Distance Thermoucouple signal cable

(OP)
Hi. Danw2

We using dual element tc but not tc terminate to transmitter just for spare incare of one fail will change over. Because we have 3 tc one top all is dual element.

Danws regarding you mention the grounded tc. Will get more noise. what you mean ? why grounded tc will still go interference?
maybe i misundertand you answer.. can you help to explain more?

thanks

RE: Long Distance Thermoucouple signal cable

long thermocouple leads and a high-impedance device reading millivolt signals will pick up any stray EMF around.  
Advice you received earlier to have a short lead to a transmitter, and then to use a 4-20 ma signal to transit the long distance was very valid.  
Using a digital (HART or FFB) signal would be even better as the digitised zignal is error-checked, but more expensive.   

RE: Long Distance Thermoucouple signal cable

We use t/c's inside generators to measure stator temperatures without any problems. That's a pretty rich source of 'stray EMF' to say the least but there is very little differential-mode noise introduced into the signal because the t/c's and the extender cable is twisted pair; there's a fair bit of common-mode pickup but a good t/c amplifier will have sufficient CMRR to make common mode noise a non-problem.

I doubt you will need enough precision measurement at a flare tip to justify a high-end extender cable.

In my opinion some of the justification for using field converters and standard 4-20mA loops is financial, especially where the routes are long and the cost of extender cable is high. Reliability is reduced by introducing the converter with all its failure modes.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Long Distance Thermoucouple signal cable

New to this forum and chiming in a bit late.

On extension wire:  To optimize accuracy, request actual thermocouple wire rather than "extension grade" thermocouple wire which has worse limits of error than standard grade or special grade thermocouple wire.  Your distance should be no problem so long as your environment is not too noisy.  We have customers who need to get in a golf cart to get to the other end of their T/C wire so 100 m is no problem.  

If noise is a concern, select twisted, shielded wire.

On grounded versus ungrounded:  If you are not aware of your electrical environment you can get ground to ground loops that will play the devil with your instrumentation.  Grounded thermocouples are not all bad, though.  There are some advantages to grounded over ungrounded, here's a useful reference...

http://www.jms-se.com/catalog/Measuring_Junction_Types_and_Pros_Cons.pdf     

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources