Solve Seebeck Voltage Conversion to Temperature Issues in Colorado

[physicsCU]

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!

 

[FredGarvin]

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...

 

[physicsCU]

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.

 

[physicsCU]

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

 

[FredGarvin]

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.

 

[physicsCU]

OK, thanks.

I pulled mine from National Instruments documentation on thermocouples, but I will be interested to see yours.

 

[FredGarvin]

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.

 

[physicsCU]

Thanks for that, but the problem with that is that converts from temp to volts, i need to go the other way.

 

[FredGarvin]

All right. Let me see what else I can dig up.

 

[FredGarvin]

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.