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?
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.
[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
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?
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.
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.
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If we learn from our mistakes I'm getting a great education!
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...
