# FOPDT model identification

So I'm trying to identify the model of a simple system (I'm pouring hot water (m [kg/s] into a water tank and displace the colder water inside) in order to tune a PID controller. I performed a doublet test and have the resulting data in this file (first column is time, second one the inout, third one the output). This is the plotted data:

Now I want to fit a function to this in order to derive the parameters $$K_p$$, $$\tau_p$$, and $$\theta_p$$ (which can be used to calculate the tuning parameters for a PID controller). However, I don't really know how to approach this. I've found a script but for some reason, the result is unusable ("optimized function" is pretty much just a straight line).