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Okay to twist thermocouple wires?

  1. Jan 19, 2015 #1
    I am trying to make some K-type thermocouples. I have wire. Is it okay to twist the ends together at the sensing tip? There is no tension in the wires, so is it okay? I don't see why everyone says to weld them or solder them.
    I don't have a welder and the solder won't stick (yes I used flux).
     
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  3. Jan 19, 2015 #2

    Averagesupernova

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    Crimp them with some kind of crimp-style connector.
     
  4. Jan 19, 2015 #3

    jim hardy

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    You'll get by for some time with twisting them .

    Cut off the part that has solder on it.

    There exist nickel crimps for that purpose


    here's the best place i know of for thermocouple accessories.
    And a good introduction to them.
    http://www.omega.com/guides/thermocouples.html

    but i didn't find those crimps.
     
  5. Jan 19, 2015 #4

    OCR

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    Great link Jim... thanks... :oldcool:
     
  6. Jan 20, 2015 #5

    jim hardy

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  7. Jan 20, 2015 #6
    The thermocouple is one of the most misunderstood pieces of lab gear. There are plenty of folks that swear it can only be make by welding, others accept soldering, another group accepts twisting... Then you get into junctions and the arguments start up all over again.
    Fortunately, General Electric used to train their engineers and technicians in most everything, and they wrote a complete book on thermocouples. It wasn't a very thick book, but it put most all arguments to rest.

    Simply put, if you succeed in making a clean bond between the two wires and the bond experiences the measured temperature uniformly, then it doesn't matter what incidental metals are involved. Thus twisting, welding, soldering, crimping - they all work, until... Until corrosion sets in between the connections. Thus, twisting is the least preferable over time because the metal is neither fused nor crushed together sufficiently to form what's termed "a gas tight connection."

    That said, almost every experiment I ever run used twisted connections because it is easy.

    If you're attempting to measure the temperature of electronic components for reliability and safety assessments, then you really want a tiny junction, a poor thermal conductor for the TC, and to pass the across the body of the part to reduce the error due to heat being sucked away by the TC wires.
     
  8. Jan 21, 2015 #7
    I don't want to add some crimp because it will heavily increase the time constant. I want to keep response pretty fast (don't we all).
    How about I twist them and dip them in a little epoxy? I only need to go to 175C at the most. They will be at room temp 99.9% of their life. Good idea?
     
  9. Jan 21, 2015 #8

    dlgoff

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    I'm curious why you can't solder these. I've soldered hundreds of type K thermocouples with a 15 watt iron using a low silver content solder from Radio Shack. This would give you a smaller "time constant" than from twisting. And Epoxy = Good insulator => larger "time constant"

    http://demandware.edgesuite.net/sits_pod26/dw/image/v2/AASR_PRD/on/demandware.static/Sites-radioshack-Site/Sites-master-catalog/default/v1421881229251/images/06400013_00.jpg?sw=350&sh=350&sm=fit
     
  10. Jan 21, 2015 #9

    jim hardy

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    I was told by an old timer to never ever solder a thermocouple. So i never have.
    The idea got reinforced when i had to replace an extension wire that had been soldered. It was creating a 2 degF error in a ~430 degree measuement,

    I think the trouble with soldering them is some fluxes can contaminate the thermocouple wire where it's heated , by diffusion, changing its microvolts per degree. Who knows what flux the splices in my example above were soldered with.

    Here's an Australian government publication that gives instructions for soldering thermocouples.
    I note they specify what flux to use.
    http://casa.gov.au/wcmswr/_assets/main/rules/1998casr/021/021c99s2c16.pdf

    I expect also that with some practice you could weld them by touching the ends together while powering with a car battery.
    That would be a handy skill to have.

    dlgoff seems to have got along okay. Let us know how you come out...



    old jim
     
  11. Jan 21, 2015 #10

    jim hardy

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    ps Omega sells fast miniature thermocouples....

    i'd try twisting the end tightly, then apply a bead of epoxy above the twisted tip to stabilize it against untwisting.
     
  12. Jan 21, 2015 #11
    I'll point back to the sacred book of thermocouples by GE. What matters is that you have an electrically sound connection, even if you have transient metals in the connection region. As long as it's all the same temperature, the energy transitions will cancel out as you go from one material to the next. Solder's good if you have an alloy that will wet the two alloys.
     
  13. Jan 21, 2015 #12

    dlgoff

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    For extensions, you need to use the proper metals. For K-Type, you would want to use something like these; from the Omega link.

    OSTW-CC_l.jpg

    Exactly. The reason I brought up the soldering was to provide the OP with a smaller, less "time delaying" joint.
     
  14. Jan 21, 2015 #13
    As a kid, I worked in a TV shop, and my senior tech advised me that "If I were told a pink elephant comes out of the set at 5:00 everyday, I didn't have to believe it, but I did have to show up at 5:00 with a pink elephant gun."
    Since then, I've experienced all matter of "inconceivable" things. Which has me itching because my trusty training is at odds with Jim's observation and I tend to trust both.
    That said, I'm itching to perform a most deliberate experiment in which I fabricate a twisted thermocouple and another thermocouple that deliberately has a section of solder between the leads. Then wire them opposing in series and place them on the same isotherm in an oven.
    The difficulty being that I don't have a decent oven or a microvolt-meter, though I'm sure I can cobble the latter. I suspect a weekend project is brewing...
     
  15. Jan 21, 2015 #14

    jim hardy

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    A small clean joint ought to be the key.

    The microvolts are actually produced along the wires leading away from the joint, not at the joint itself.
    So it's important to not alter the metal in the area where it experiences a temperature gradient.




    http://www.capgo.com/Resources/Temperature/thermocouple/thermocouple.html


    What we found in that extension i mentioned was a soldered splice about every fifty feet.
    It was the correct thermocouple extension wire.
    We replaced it with a single piece, no splices, and our measurement improved to almost perfect.

    http://www.azom.com/article.aspx?ArticleID=1208

    I think that our extension wire had been soldered with a gas torch and got way too hot.
    But i really don't know. Soon as i saw those soldered joints the old-timer's words came back to me so we pulled new extension wire, about 400 feet of it..

    Mike: what does the GE book say about soldering ? Any precautions about overheating?

    Refind : how much precision do you need? Are you after precise numbers, or response time?


    ps thanks all for the kind words....

    old jim
     
  16. Jan 21, 2015 #15

    dlgoff

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    bold by me

    http://www.ge-mcs.com/download/validation/Validation_TC_Datasheet_final.pdf

    Note the bold. It does make a difference.
     
  17. Jan 22, 2015 #16

    Bystander

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    And with NO thermal "lagging," I'll bet. The headaches in keeping all joints, connectors at constant T sell lots of aspirin.
     
  18. Jan 22, 2015 #17

    jim hardy

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    Here's what Omega has to say

    http://www.omega.com/temperature/z/pdf/z021-032.pdf
    Sounds to me like solder should be okay provided one is attentive to cleanliness and uses modest heat. Anecdotal evidence for problems exists so technique must be a factor.


    OP's difficulty was in soldering to Chromel and Alumel
    Here's an article that describes how one person silver-soldered Chromel - Alumel extension wire for his airplane.

    http://www.aeroelectric.com/articles/excerpt.pdf
    page 7 of 12
     
    Last edited: Jan 22, 2015
  19. Jan 22, 2015 #18
    Unfortunately, my handy GE thermocouple book has been lost through the years. It didn't cover the underlying physics in any case. There is a book I found by Daniel D. Pollock that seems to explain more than anyone healthy would care to know - it goes for over $300.

    If there's a three paragraph explanation to the underlying physics behind these things, I'd love to hear it. I've constructed them in thin film and still don't have a good explanation for why they behave as they do.
     
  20. Jan 23, 2015 #19

    jim hardy

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    My understanding is not deep.
    The simple mental image i keep in my head, and which i use to explain to folks, goes like this:

    We know that any metal contains a sea of electrons that are loosely bound, in the very outer shells of its atoms.
    That's why metal conducts so well.
    We also know that temperature is molecular motion.
    Thirdly we know that different atoms have different affinities for electrons.

    Pardon the jargon here, trying to paint a simple mental picture:
    Heating one end of a wire rattles the atoms there
    loosely bound electrons in the hot region are shaken out of their proper place and migrate toward the cool end
    causing a small potential difference along the part of the wire that's experiencing the temperature gradient.
    Not surprisingly that potential difference depends on the makeup of the wire.

    So: if i could measure the voltage between the ends of a single wire experiencing a temperature gradient,
    i could plot its microvolts per degree.
    Of course that'd be impractical because one of my my voltmeter's wires, the one touching the hot end, would also experience a temperature gradient.

    But if i joined two dissimilar metal wires and heated the joint
    at their far end i'd measure the difference between their temperature induced voltages, and at room temperature.

    That's stated more eloquently here:
    http://www.mstarlabs.com/sensors/thermocouple-cold-junctions.html

    For me the light came on when i remembered as a kid shaking a tree to get the fruit.
    Different varieties of tree have different affinities for their fruit, as do different metals for their conduction electrons.
    I grew up in subtropics shaking mango and loquat trees.
    Surely you folks from temperate zones noticed the same thing?

    I hope this preposterous oversimplification is some help.
    We must oversimplify and exaggerate to get our thinking started along a right path
    and refine from there.
    When our microscopic and macroscopic views come into agreement, and our thought experiments based on the two predict the same results, is i believe when we are beginning to understand.

    Imagine those poor guys trying to figure it out before discovery of the electron...
    http://thermoelectrics.caltech.edu/thermoelectrics/history.html

    old jim
     
  21. Jan 23, 2015 #20

    jim hardy

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    i have to take a break
    my internet has become unusable with PF

    ignores post button for ten minutes then posts nine copies
    will try again in a week or so

    old jim
     
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