# Temperature due to resistance calculator?

1. Aug 21, 2010

### ISX

Think this question is more of a physics than math question. Anyways I am trying to figure out how to make a calculator to figure out what temperature something is at based on the ohm reading I get. I have several readings to go by but I think doing it exponentially is the wrong way to go about it since the readings I calculate are not very accurate based on the quadratic regression method I have been doing.

Wikipedia shows the formula on this page (http://en.wikipedia.org/wiki/Callendar-Van_Dusen_equation) but I don't know how to figure out any of the coefficients or anything.

Thanks for any help I can get!

2. Aug 21, 2010

### zhermes

By:
Do you mean using a thermistor, or just by plugging into any random piece of electronics?

3. Aug 21, 2010

### ISX

Yeah just a little thermistor.

Guess it would be this equation then. http://en.wikipedia.org/wiki/Steinhart–Hart_equation

Temperature range of interest would be -40 to 212F. I just need help on the coefficient stuff.

4. Aug 21, 2010

### lzkelley

Unfortunately I think your best (if not only) bet is to measure some values at known temperatures and fit those to the equation. You have 3 constants, so you need at least 3 measurements, more would be better. The boiling and freezing points of water would be the easiest to do. If you don't have/want-to-use a thermometer, you could use the temperature of boiling and freezing of both pure and salt water for 4 total points.

5. Aug 22, 2010

### ISX

I do have many measurements. Here are a few. kOhms is first number Temp (F) is the second.

73.00 5.0
65.00 6.6
56.00 13.0
39.00 28.0
25.80 43.0
24.57 46.2
18.96 53.7

6. Aug 22, 2010

### Sakha

I think you should use a thermistor because the temperature varies greatly with the heat-dissipation of the resistors.

7. Aug 22, 2010

### ISX

It is a thermistor..

8. Aug 22, 2010

### Q_Goest

Hi ISX
Do you have Excel or some other spreadsheet program? Just put the numbers you have for temperature and resistance and have the program do the curve fit to a polynomial for you.

For the values you provided, the chart is rather limited and I wouldn't suggest using it for values outside the tested limits, but attached is what you'd get. I'd suggest getting some more measurements, especially at the limits of your intended range (ie: test in boiling water, 212 F, and at -40 F).

Another alternative is to get an equation from the thermistor manufacturer.

Last edited: Aug 23, 2010