Replacing thermcouples

[Chinook82]

I am looking at replacing thermocouples with 4-20mA temperature transmitters and, instead of incurring the expense of re-wiring with regular copper analog shielded wiring, re-used the existing thermocouple extension wires?

Presently the thermocouples (close to 150) are hard wired back to inputs on a number of Honeywell MUX boxes located some 600- 800 feet away.

I can see no reason not to use the existing wiring, and use proper wiring practice. Just thought I would check with you guys first.

 

[itsmoked]

Why change?
800ft is typical.

However. I don't see any problem except the original TCs pass virtually no current and now you are talking about 20mA. You need to check the resistance this is going to add to your current loop and make sure that your 4-20 supply has adequate compliance to bury that resistance.

Kapeesh?

Also 150 times 2 times the price of several suitcases of transmitters and receivers is gonna cost a bundle and require a bunch of space on both ends. Maybe it would be less expensive to modify the installation to solve whatever problem you are running from.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[danw2]

Are these 2 wire loop powered transmitters you plan on installing to get the 4-20mA?

Have you considered that the wiring currently runs to a MUX, which probably means that the newly installed transmitter(s) will only be powered from the MUX when the MUX selects that channel, because otherwise the loop is (most likely) broken/open when the channel is not selected?

Will the transmitter initialize in the time provided by scan time that the MUX connects to any particular channel?

Is your accuracy requirement outside of the conditions under which accuracy is normally specified, which is always a warm, powered-up-for-a-certain-period, stabilized transmitter?

Were you planning on using HART (not a bad idea for 150 odd transmitters)? Will the relatively slow frequency that HART runs work during the limited connection of the scan time of the MUX?

The use of T/C wire depends, as Keith mentions, on the type of wire and its resistance.

Page 2 of the pdf below shows resistances for various wire sizes:

http://www.thermometrics.com/assets/images/thermcpl.pdf

20 gauge type K is 0.4 ohm per foot.
600 ft x 2 x 0.4 = 480 ohms which approaches the 500 ohm load that many 2 wire loop powered transmitters can drive with a 24Vdc power supply. Other wire types and gauges vary.

Measuring the actual resistance is tricky because a T/C generates an EMF when it has a temperature gradient across it.

I, too, am curious as to what problem you're tying to solve with a boatload of new transmitters. What's wrong with MUX'd T/C readings?

Dan

 

[hacksaw]

if you can't make the original extension wires and t/c work why re-use the extension wires?

replace the wiring or make the orig. system work.

my two cents

 

[Loopee]

What is the problem with the thermocouples in the first place?

I agree with everyone that the loop resistance would keep this from being a practical solution.

Do the exisiting thermocouples all come to a common junction box or are they all routed individually back to the existing mux?

 

[Chinook82]

Thanks everyone for the good info. The "K" thermcouples are pretty old and starting to drift (not uncommon I know) So I'm concidering Class "A" RTD's to put an end to the problem. I think I'll try a few first and go from there, it's starting to sound quite expensive.

 

[hacksaw]

just remember rtd's require a lot more care and feeding, they are not always a one-for-one replacement, especially where drift is already a problem. Wish they were,

good luck

 

[glowing]

you can replace a lot of TCs for the price of all of those transmitters.