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Seebeck Voltages

  1. Sep 5, 2005 #1
    I have an issue with converting these to temperatures. I have the equation, voltage is in microvolts, but when I convert, say boiling water at 3600 microvolts to celsius, it gives me something like 90 C. It ought to be close to 95 C, I am in Colorado.

    I did it today in excel, and the matlab equation I wrote for it last week gave me the same answer. According to the lab handouts, the error should only be +/- 0.05 C.

    Thanks for any help!
     
  2. jcsd
  3. Sep 5, 2005 #2

    FredGarvin

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    Let's see what you are doing. Are you actually using a thermocouple? What metals are you using? You could possibly be using the wrong coefficients...
     
  4. Sep 5, 2005 #3
    Yes, we are using thermocouples made of Ni-Ch and Ni-Al

    Type K thermocouples (given by profs)

    Here is the equation

    T = 0.0 + 2.508355e-2*v + 7.860106e–8*v^2 + –2.503131e–10*v^3 + 8.315270e–14*v^4 + –1.228034e–17*v^5 + 9.804036e–22*v^6 + –4.413030e–26*v^7 + 1.057734e–30*v^8 + –1.052755e–35*v^9

    Again v is in microvolts, and T is in celsius.

    Because I am just a few degrees off, I am wondering if the polynomial is right, ie do we have the right thermocouple type.
     
  5. Sep 5, 2005 #4
    anyone else?

    I have checked this in matlab, and with other people, getting the same answer.

    but the LabVIEW VI software in the lab gives us the right answer
     
  6. Sep 6, 2005 #5

    FredGarvin

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    Looking at my ASTM tc reference, the K type polynomial is different than what you have. Give me a bit and I'll post it.
     
  7. Sep 6, 2005 #6
    OK, thanks.

    I pulled mine from National Instruments documentation on thermocouples, but I will be interested to see yours.
     
  8. Sep 6, 2005 #7

    FredGarvin

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    Without running some numbers I can't say if this will fix your offset, but I did forget that there are two exponential corrections for the first two terms. Take a look at the attached page.

    Also note the reference temperature of 0°

    Ref. ASTM Manual on the use of thermocouples in temperature measurement, 4th edition.
     
    Last edited: Nov 7, 2005
  9. Sep 6, 2005 #8
    Thanks for that, but the problem with that is that converts from temp to volts, i need to go the other way.
     
  10. Sep 7, 2005 #9

    FredGarvin

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    All right. Let me see what else I can dig up.
     
  11. Sep 8, 2005 #10

    FredGarvin

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    OK. Try this one. Hopefully this one will help. This is from NIST. It is a typical K type TC table, however at the very end there is a section for the inverse function of temp and emf. Give that a look see and maybe that will hlp you out.
     
    Last edited: Nov 7, 2005
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