curve fitting of a fluctuating curve

I observe a mass flow balance which should converge to a value of zero for a steady state (picture, real values are green, already fitted curve is read).

Currently it fluctuates around zero so I want to assume when a steady state ist reached. Which function can I use for this? I have already used the extrapol-function but its not considering the fluctuations which will become lower.

Thank you in advance!

Annemarie