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T/C extension wires
Sensor technologies. topic
Posted by d- on 3 March, 2013 - 1:14 pm
Just interested in how many plants use neutral extension wires with thermocouple measurements? and do the t/c monitoring i/o allow comensation for that case?


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Posted by David on 3 March, 2013 - 4:06 pm
I hope not many, but I have seen it.

If by "neutral extension wires" you mean that connection from the thermocouple to the analog input is not thermocouple extension wire, but copper wire, then

1) if a thermocouple transmitter is used and connected with thermocouple wire to its input, then copper wire for the 4-20ma current output is normal and cold junction compensation is done by the transmitter.

2) if a thermocouple transmitter is not used, and the copper wire connection is to a thermocouple input, my experience shows that it is unlikely that any external cold junction (CJ) compensation is used, resulting in some level of CJ error.

There are provisions on some thermocouple inputs (I/O) for "external CJ reference"; - one being a fixed temperature reference junction where the temp is specified and used for CJ,
- one being a corollary of the first: an ice bath at 0C,

- another being an RTD or thermistor wired separately to an 2nd AI channel and its temperature used for CJ.

The only implementation I've ever seen using remote CJ is the ice bath used for calibration purposes.

For the 'thermocouple/copper wire' situations I've run across they're all commissioning mistakes, CJ being done at the analog input terminals but connection to the thermocouple with copper wire.


Posted by d- on 3 March, 2013 - 6:16 pm
I had not seen it used in plants either, but ran across the item in a measurement handbook that dealt with the case of neutral t/c extension wires (i.e. copper wires).

The handbook did not advocate its use, it just presented the analysis.

Just curious if this has been deployed in plant applications and if it were, how the reference junction compensation was dealt with in the I/O cabinet.

Still interested to see if it has been intentionally used.


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Posted by David on 4 March, 2013 - 2:18 pm
Neutral extension leads (as described in "Temperature Measurement", 1999, CRC Press) require that the transition point from T/C wire to 'neutral' copper wire be the CJ measurement point and that the CJ temperature must be known/measured and somehow supplied to the instrument for CJ compensation.

I know where 'neutral' extension wires were used half a century ago. Way 'back when', my colleague's father (now deceased) did calibrations in US steel mills post WWII and onwards.

In those days, the instruments didn't have local cold junction at the terminal strip, like they do now. Instead, an underground point that remained at a stable temperature throughout the seasons was used as a cold junction reference.

He had a notebook noting various underground reference locations, for instance:
Inside the NW corner of Bldg 42A, 3' east, 3' south of the inside corner beam, at 7' down, 54F.

The field thermocouple wire was run to the reference point and then copper wire connected the reference point to the recorder/controller/indicator input. The instrument had an adjustment for the stable reference 'cold junction' temperature.

Maintaining a stable CJ reference with an icebath that uses neutral copper wire extension is considered somewhat of an art by the purists in the cal lab. Why anyone would bother with 'neutral' extension leads for reasons other than lab accuracy nowadays isn't clear to me, since all commercial T/C inputs have some form of local CJ (some better than others, admittedly).


Posted by d- on 4 March, 2013 - 7:26 pm
Excellent bit of history, I'd seen it in labs, but in the early plants it was matching t/c wires not "extension leads" back to the recorder, no splices and in those days "extension wires" in todays usage did not exist. Control was done with filled systems...

The CRC handbook Meas. Instumentation & Sensors by Webster, discusses the errors for neutral(Cu/Cu) extension leads along with other sources of errors, as you indicate in your responses, pretty rare.


Posted by Curt Wuollet on 5 March, 2013 - 12:19 am
I remember the old Leeds and Northrup gear. It's such a simple concept and has so many ways to go wrong that I seldom believe a TC setup that I haven't examined from one end to the other. Even replacing a terminal screw you have to match material. It's a sinking feeling when you find a screw up in a place with dozens of TCs and you hope it's not replicated.

Regards
cww


Posted by David on 5 March, 2013 - 5:13 pm
Curt,

Those L&N's you remember are probably not that much different from the one on display at the Smithsonian, representative of the one used to record background microwave radiation now considered residual big-bang remnants.

http://airandspace.si.edu/collections/artifact.cfm?id=A19970342000


Posted by Curt Wuollet on 5 March, 2013 - 6:04 pm
Yes. I messed with those, but the L&N device we started with for TC trouble shooting was the last wooden cased instrument I think I used. Someone has one on ebay:

271163410934
Item number:

This was at Control Data a fairly "modern" company.

Regards
cww


Posted by Russ on 5 March, 2013 - 8:55 pm
We use copper for extension of the TC feedback to a PLC controlling resistance heaters. We are controlling ~3KW heaters and maintaining ~ 300-400F. We've found that there can be a few degrees of inaccuracy, but we are not trying to maintain a precise temperature, and all the 'neutral' junctions are between 70-100F. Rather, we are giving the PLC an indication of temperature for PID control, to control the heater power and maintain a constant (albeit a few degrees inaccurate) temperature. The operator has the ability to tweak the temperature (and does often) as he finds the process requires adjustments, so an exact/precise temperature is not our requirement.


Posted by Curt Wuollet on 6 March, 2013 - 12:36 am
Yes, TC amd thermopiles are sometimes used as indicators with no compensation. Engine temperature gauges and the like. Accuracy is poor but the cost is low and it's simple.

Regards
cww

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