Will a larger Thermocouple wire gauge change temperature readings? 1

[rfer]

I have a setup that came to me that I needed to make some changes on that is pretty much a black box. I can't open it to take measurements (more than once or twice) because it is constantly running tests at temp and RH%. No CAD for it. No one knows who made it, just that it came from another office and it works, but not well enough, so I have to improve it. Anyway, I finally track down who I think helped build it. I need to replace some Type T thermocouples inside of it so I ask the guy what the part number for the TCs are. He sends me a link, I buy them. I get around to opening this thing up to swap out parts and the TCs are the same type, but clearly different gauges. My TCs are like 20 AWG, the ones in there are like 40 AWG. When I swap the TCs in, the new TC measurements are all wonky.

Does the TC wire gauge affect the voltage output, thus affecting the temperature reading? So replacing a 40 AWG TC with a 20 AWG TC will not be a "plug and play" solution and require the TC reader re-calibrating?

I have no clue what the instrumentation is that is being used to read these, or what the closed loop PID the TC is communicating with looks like. I would assume since the wire gauge changed, the resistivity changed, thus the voltage output changed, so it will need re-calibrating, but would just like come confirmation (or correction).

Thanks!

 

[waross]

In some thermo-couple measuring schemes the resistance of the thermo-couple loop or circuit affected the calibration.
The amount that the measuring circuit "loads" the therm-couple voltage is critical and may be changed by a change in loop resistance.
If the readings are too far off to be accounted for by loop resistance it may be that the original TCs may have actually been a different type.
I would acquire a good quality TC transmitter and try both old and new TCs to see what I had.
If possible, two x-mitters and test both TCs at the same time in the same bath.
Then look at your measuring circuit.
Compare the measured results from you existing device with the results obtained from a good TC transmitter.

[link ]Bill[/url]
--------------------
"Why not the best?"
Jimmy Carter

 

[IRstuff]

It's possible that the reader accounts for the wire resistance, although, unlikely. The simplest thing is to make and ice bath and test it against a known good sensor. We just did that on a set of T thermocouples at our facility, using a thermos filled with ice and some water.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[EdStainless]

Yes gauge matters, both resistance and thermal conductivity are issues. You also need to look at the terminals very carefully, are they compensated alloy or just Cu? How is junction temperature compensation handled?
Check all of the old and new ones in ice water and boiling water. No so much do they read correctly but do they match.
Remember they are only providing mV level signals and any change in resistance can be a big deal.


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P.E. Metallurgy, consulting work welcomed

 

[itsmoked]

In what you describe, NO, the gauge does not matter except with regards to thermal constants. The temperature cannot move as fast as it can with smaller T/C wire.

The quickest way to get 'wonky readings' is to reverse the wires accidentally.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[moltenmetal]

Grounded junctions also tend to give wonky readings. Ground loops in thermocouple circuits are very common.

Most thermocouple reader circuits these days are very high impedance and don't give a rat's hind quarters about the gauge. They can totally be thrown for a loop, pun intended, by ground loops however.

The gauge of the wire will, however, affect the thermal conductivity and the heat capacity of the element. It is quite possible in some circumstances to have the thermal conductivity along the length of the element, affect the reading at the junction. It is also quite possible to make measurements with thermocouples that are actually accurate despite what you'd think would be terrible coupling between the junction and the thing being measured.

 

[che12345]

Dear Mr. rfer

Q1. " ...Does the TC wire gauge affect the voltage output, thus affecting the temperature reading? So replacing a 40 AWG TC with a 20 AWG TC will not be a "plug and play" solution and require the TC reader re-calibrating? ..."
A1. From the description I take it that the concern is on the (TC lead) []conductor size, while the conductor [material] and length are the same. I am of the opinion that the manufacturer knows better that the difference in [conductor size] is unlikely to have any [significant impact on the accuracy]. Therefore usually , NO re-calibration is needed.
A2. BTW the voltage generated across the TC is >0.1 mV and the current is in uA range. The difference in resistance between the conductor sizes would be in uOhm.
Che Kuan Yau (Singapore)

 

[waross]

We generally assume that existing installations worked properly.
That is not always the case.
The replacement TCs may be having thermal conductivity issues between the terminals and the junction.
If these are in thermo-wells, the replacements may not be making good contact with the thermo-wells.
Some old measuring circuits had to be calibrated for loop resistance.
You don't give us enough information to do other than make WAGs.
What are you measuring?
There may be an off the shelf solution that will give you better results than the old set ever did.


[link ]Bill[/url]
--------------------
"Why not the best?"
Jimmy Carter

 

[danw2]

>They can totally be thrown for a loop, pun intended, by ground loops however.

You betcha. (moltenmetal, your expression is so good I'm going to have to steal it).

There appear to be multiple thermocouples in this black box. The classic complaint when there's a ground loop is, "when I add the 3rd thermocouple to the AI card it throws all the readings off, even the first two that were OK before I added the 3rd." The ordinal numbers are examples only, it's the ability of one grounded thermocouple to affect an entire AI module, in particular, single ended AI card's with a common signal negative.

An ungrounded thermocouple element in an MgO assembly with a sheath costs maybe $2 more than the ungrounded version and ungrounded elements do have an inherent thermal lag due to the isolation of the junction from the sheath, but ungrounding the junction takes away the bad boy ground loop.

Keith mentioned polarity - if the extension wire gets connected reverse polarity, then the result is weird values too, but matching the color coded T/C wires should keep polarity straight.

 

[EdStainless]

There are a couple of things that are also possible.
If the sensor circuit does not have high enough impedance then changes in TC resistance matters.
The other thing that comes to mind is that the mounting hardware is temperature compensated (one being copper and the other constitan) and the polarity is correct but the alloy links are swapped. Trust me they look the same and are rarely marked.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[John2025]

Make sure you got the right type and don't forget the red wire is typically negative on thermocouples (type K anyway)

 

[waross]

You don't give us enough information to do other than make WAGs.
What are you measuring?

[link ]Bill[/url]
--------------------
"Why not the best?"
Jimmy Carter

 

[FacEngrPE]

 

[Shanaya]

Thicker wires mean that you heat the 'cold' end more (and, as in the commentary) by extracting more heat from the hot end. The no-load voltage is proportional to the temperature difference between the ends for a rough approximation.

Long thick wire (long enough the cold end does not heat appreciably) does not vary from a long thin wire pair if the alloys are identical and the current is negligible.

Think of the heat generator, if that makes sense, when you start drawing the resistance figures in the current as well as the heat flow down the wires.

You need to keep both ends at controlled temperatures to get a good reading. For rough measurements (maybe within 2-3 degrees C) you should place one end (the junction) in boiling water and the other end (the transfer junction(s) to copper on both sides) in an ice-water slurry.