# Using resistance and temperature coefficient formula

What is the correct way to use the resistance and temperature correlation formula from http://hyperphysics.phy-astr.gsu.edu/hbase/electric/restmp.html?

In particular, does R have to be the higher resistance and R0 the lower or vice versa? The resulting calculated dT differs depending on this choice of R and R0.

Quick example of what I mean:

Choose R = 7.97 and R0 = 7.28, copper a ~= .00393

dT = 24.1171

Choose R = 7.28 and R0 = 7.97, copper a ~= .00393

dT = -22.0292

I believe the answer is to use the former (always choose R and R0 so R > R0) because this more closely correlates with my lab data.

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$R_0$ is the known reference value for resistance in some known reference temperature $T_0$. You find these for the material used from some table. If you then have measured some other value $R$ and want to know the temperature difference, you solve the function for $\Delta T$ and enter the known values to the equation. Remember that $\Delta R=R-R_0$ and $\Delta T=T-T_0$.

If $\Delta T$ is positive, measurement has been done in higher temperature than the reference measurement and if $\Delta T$ is negative, measurement has been done in lower temperature than the reference measurement.

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Correct, but my question remains. See my original example. 22 != 24. The core of my question stems from that it's a %dR due to the dR/R0. +x% is not the inverse operation of -x%. i.e. Start with a number A, go up 10%, go down 10%. This resulting number is not equal to A; it is closer to 0 than A. – MaxRunFast Jun 29 '12 at 18:41
In your example the $\Delta T$ is of course different if you have different denominator $R_0$ in the equation. R_0 is always the value you read for the reference temperature and R is what you have measured. – Edu Jun 29 '12 at 19:03
@RunHard It does answer your question. One value is the reference value and it always gets the $_0$. The other value is not the reference value and takes no subscript. That unambiguously sets the sign of both $\Delta R$ and $\Delta T$. – dmckee Jun 29 '12 at 19:04