Quote Originally Posted by nicholai View Post
I want to use an arduino with a SSR on an immersion heater to get something similar to this.
Go for it. But keep in mind that PID controllers are quite sophisticated in calculating how much heat to apply to a bath to regulate the temperature without fluctuation. It includes a self training function to tune the parameters to the bath.

Given how cheap PIDs are, your project will certainly be done for love of hacking an arduino than for expediency. But have fun! You'll learn a lot I'm sure.