Calculating tau from measured time and temperature

[blairj]

so i have two data points. T1 = 64.68, t1 = 200.7 and T2 = 84.15, t2 = 243. I want to use this data to find the tau value of the exponential curve the data came from.

I was trying to use simultaneous equations but got stuck. Can anyone help me with this?

64.68 = Tf(1-exp(-200.7/Tau))
84.15 = Tf(1-exp(-243/Tau))

I got to 19.47 = 84.15*(exp-200.7/Tau) - 64.68*(exp-243/Tau)

I want to get a value for Tau but i can't figure out how to simplify/remove the exponential terms.
Any help is appreciated

 

[Feldoh]

What are you trying to fit? A function of the form

f(t) = T_f(1 - exp(\frac{-t}{\tau}))

Your best bet it to evaluate it numerically, don't think it can be analytically solved...

I get tau = -327.156 and Tf = -76.3792