Definition of diffusion coefficient function

Hi

I am trying to model the drying of a cylindrical part by using the "transport of diluted species" module.
In this model, I specified the temperature dependency of the effective diffusivity coefficient by an Arrhenius-type equation. The temperature in the simulation is a function of time. Therefore, the diffusion coeficent becomes a function of time. Running the simulation gives in total quite reasonable results for the concentration profiles. I attached this file.

However, my Arrhenius expression is only valid for a temperature range >50degC and applying it to lower temperatures returns an overprediction of the concentration. But a considerable part of the temperature profile (in terms of time) is at temperatures below 50 degC.

So here's my question: How can I change my simulation, so that I can add a linear or a constant expression for my diffusion coefficient to account for the initial part of my time-temperature profile.

I tried to add in a piecewise function, but I can't figure out how to chnage the syntax of Arrhenius expression of Deff then...

Anybody could help, please?

Cheers, Harry