Seebeck Voltages

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.