α = R100 - R0 / R0 * 100
R100 - is the resistance of RTD at 100 degrees celsius. For pt100 the resistance is 138.5
After substituting the value in the formula the value of alpha (temperature coefficient) will be 0.00385/degree celsius
So let's say we have the resistance value measured as 80 ohms now let's convert it to the temperature.
80 = 100(1 + 0.00385t)
After doing the math our temperature will be,
t = -51.9 degrees celsius.
Here is an image showing an RTD connected inside to a vessel taken from my mobile in an industry