Thermocouple Connection Question

ndzied1

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Join Date
Aug 2002
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Hi All,

We are building a box for a customer that has several Type J thermocouples entering the box that need to be able to be connected and disconnected depending on a test setup. These eventually end up at a National Instruments 9213 Card

They have asked us to use a spring pin connector on the box to get the wire through the wall because they have used them before. This is a nickle plated brass part and not made for T/C use. It would look something like this.
attachment.php



I suggested we use a T/C connector and T/C wire all the way to the input card like this:

attachment.php


They are saying the card should be able to compensate for the extra cold junctions in the spring clips. I have been trying to read tons of stuff on this and still can't convince myself one way or the other o_O. Maybe the "Thermocouple Law of Intermediate Metals" means that they are OK using these???

If I use the spring pins, what wire should go between the clips and the T/C card?

Any help or suggestions appreciated.

Thanks, Norm

SpringClips.png Connector.png
 
I am by no means an expert on this, so...
If the temperature of the the spring clamps are different from the terminal at the module, the reading will be off. The cjc on the module is measuring the temp of the modules terminals (or as close as it can).

Whenever I put connections for thermoucouples, I always use thermocouple specific connectors and wire from the connector to the A/D. Why not get thermocouple connectors and panel jacks from Omega? You can buy pre-made jack panels for not that much money.
http://www.omega.com/ppt/pptsc.asp?ref=SJP&Nav=temg15
 
Hi All,

They are saying the card should be able to compensate for the extra cold junctions in the spring clips.


The only way you can compensate for temp at the cold junction is by measuring also the temp at the cold junctions. If whomever made this statement was correct, then there would be no need to ever bother using real thermocouple junctions.
 
In agreement with Damian and g.mccormick...spring junctions would require constant temperature monitoring and ensuing calculations to maintain accuracy
 
I have been trying to read tons of stuff on this and still can't convince myself one way or the other

wire up a quick "show and tell" demonstration with some "non-thermocouple" type junction blocks to "splice" the connections between a thermocouple and a transmitter ...

heat the junction blocks (just a few degrees should be enough) ... a common hairdryer works well for this ...

it's been years since I've done this – but it makes a very convincing demonstration ...
 
True, Ron, we teach a similar demo using an ice bath...This method also alows the junction to achieve a point where the error will remain constant.
 
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This method also allows the junction to achieve a point where the error will remain constant.

yes, if I remember correctly, the non-thermocouple junctions will introduce an "offset" (three or four degrees) which (theoretically) COULD be compensated for ...

BUT ...

it's when the non-thermocouple junctions are at a DIFFERENT temperature than the other components that things get really nutty – really quick ...

AND ...

naturally the card couldn't possibly compensate for that type of problem – without knowing/sensing the amount of temperature difference at the make-shift junctions ...
 
Thanks all for the replies so far.

So to keep trying to wrap my head around this, if the temp of the Nickle plated connections changes, then my reading will change but if I stick with the T/C elements through the circuit and heat those up that will not affect my measurement?

I'm not sure I have the hardware to check this right now but will definitely try to round it up.
 
Some time ago I had several panels built many with Type J T/Cs. I specified T/C termination terminal blocks. We have been using WAGO terminal blocks our vendor claimed that T/C blocks were not necessary. Needless to say a lengthy discussion ensued. My vendor provided the attached documents from WAGO. So we went forward without T/C blocks. We did challenge the documents and tested by changing some of the Blocks to T/C after install. We could see no difference in reading and do not use T/C blocks and have continued with the WAGO blocks. We have had no issues. Further, we have used other blocks and done similar field tests and could see no difference.
BTW: my post is meant as an endorsement for WAGO. But, simply sharing my experience.

View attachment 8400_Thermocouple test report.pdf

View attachment Thermocouple Application.doc
 
here's a reference from the Omega Temperature Handbook that pretty well sums it up ... (the added emphasis is mine) ...

http://www.omega.com/temperature/Z/PracticalGuidelinesforTemperatureMeasurement.html

the tricky part is whether or not there is a TEMPERATURE DIFFERENCE across the non-thermocouple connections ... IF (that's a big IF) it can be assured that there will NOT be a temperature difference in that part of the wiring, then you could probably get away with using non-thermocouple connections ...

otherwise -

well - we'll just have to see whether everything works "close enough" for our purposes - or not ...

so ...

(as I understand it) the purpose of these connections is to take the thermocouple leads THROUGH the side of a box ... that begs the question: can we be ASSURED that the temperature INSIDE the box will always be the same as the temperature OUTSIDE the box? ...

.

omega.PNG
 
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I disagree. Nickel plated intermediate junctions can work, given isothermal conditions. The "Thermocouple Law of Intermediate Metals" does apply, but the corrolary (sp?) law is that BOTH sides of the intermediate junction MUST remain isothermal: at the same temperature. Any difference in temperature results in an additional EMF that affects the circuits EMF.

The problem with the hair dryer demonstration is that the application of heat does not guarantee that both sides of the junction increase in temperature identically. The error seen in the hair dryer demo is the difference in temperature between the two sides of the junction. The polarity of the error (whether the indicated temperature increases or decreases) depends on which side of the junction is hotter or cooler than the other side.

The purpose of the cold junction measurement is to measure the terminal block temperature at the termination point where the EMF voltage is actually measured and doesn't really affect the intermediate junction issue.

The statement that "the card compensates for nickel junctions" has no bearing; the AI card can't know what the field wiring conditions are, it just measures an EMF and a cold junction that is supposed to be its own terminal block temperature.

It's the Law of intermediate metals that makes nickel plated junctions possible, if not practical in many cases.

In an environment where an intermediate junction can be shielded from convective (drafts), conductive and radiant heat sources, it's theoretically possible to use non-thermocouple materials for junctions.

I once encountered an exposed terminal strip mounted directly above the slotted steel chassis housing of a PLC power supply with free hanging thermocouple wire (not cut to length) laid atop the chassis. It was subject to temperature changes from all three heat sources
- conductive heat source through the wiring
- convective heat source from the ventilation slots
- radiant heat from the power supply and surrounding electronics.
It was a horrible intermediate junction installation.

Whether both sides of an intermediate junction remain at the same temperature can be dicey.

edit: I see that Ron came to the same conclusion while I was typing
 
from danw ...

I disagree.

I'm not trying to start an argument – or even a debate ... but what part of the discussion do you not agree with? ...

Nickel plated intermediate junctions can work, given isothermal conditions.

yes (I fully agree with you) ... but can the installation under discussion GUARANTEE those isothermal conditions? ... specifically, will the temperature INSIDE the box ALWAYS be the same as the temperature OUTSIDE the box? ... if not, then the isothermal conditions won't be present – and the temperature reading will be "off" ... "off" enough to matter? ... maybe – maybe not ... but it will be "off" ...

Any difference in temperature results in an additional EMF that affects the circuits EMF.

and that's the point that some of us are concerned about ... maybe there WILL be a temperature difference – and enough of a difference to affect the system being built ...

The problem with the hair dryer demonstration is that the application of heat does not guarantee that both sides of the junction increase in temperature identically.

I'm sorry that I didn't make myself fully understood ... the SUCCESS of the hairdryer demonstration is that it shows (conclusively) that UNLESS the temperatures of the junctions are IDENTICAL – then the temperature reading WILL be affected ...

a little background information ...

years ago we were building lab equipment for tech school students to use in hands-on experiments for temperature measurement ...

for various reasons, we wanted to "splice" some thermocouple wire ...

the question arose, can we just use regular old terminal blocks for this – or will we need special thermocouple type connections? ...

we researched – and then did the hairdryer experiment ...

our decision was that YES, we CAN use regular old terminal blocks – AS LONG AS the terminal blocks remained at the same room temperature as all of the other connections ...

BUT ...

for every class of students, we would always perform the hairdryer experiment to demonstrate that "out there in the field" the connections might NOT always remain at the same temperature ...

therefore ...

the students had to understand the reason that "thermocouple type" junctions ARE commercially available – and FOR A GOOD REASON – and that in SOME cases those special type junctions are absolutely essential for reliable temperature readings ...

we were afraid that if all the students saw was regular old terminal blocks being used in the lab – then they would assume that regular old terminal blocks could ALWAYS be used in the field too ...

that would be an incorrect assumption ...

once again, I'm not disagreeing with you (at all) ... it's just that I'm not sure that you understood my point ... again, I apologize for not being more clear ...

================

EDIT ...

Whether both sides of an intermediate junction remain at the same temperature can be dicey.

edit: I see that Ron came to the same conclusion while I was typing

and I see that we ARE together in thought - just slightly offset in our typing/timing ...

.
 
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Ron, I was typing while you and Tom were posting replies. After I posted it, I saw what you'd posted, so I added that quick edit statement at the bottom.

My 'disagreement' statement was with what I interpreted as assertions that nickel plated (non-thermocouple materials) are not suitable for intermediate junctions.

I misinterpreted your "original" hair dryer demo.

We are in agreement.

Sorry for the confusion.

Dan
 
Thermocouples are such a pain - that's why I seldom use them. I really think most of the time an RTD would serve the application better.

Discussions of special thermocouple terminals and termocouple wire always make me think of a project in Arizona many years ago where I was using RTDs for measuring air temperature. The electrical contractor (EC) had apparently had some experience with T/Cs and called me on the telephone:

EC: "I see these are platinum RTDs."
Me: "Yes."
EC: "Do I have to use platinum wire to connect them to the control panel"
Me: Loooooooong pause "Um, no, regular shielded twisted pair will be OK"
My Sales Rep to Me after the EC hung up: "You idiot - you should have told him yes, and that he could only get it from you!"
 

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