Noah wrote

I understand that the Toolbox generally does not assume units, but it surely has to assume a unit of time?

Hi, thanks for trying the toolbox.

How and in what unit you interpret

*d/dt* is entirely up to you (and what other units you have chosen). But the general recommendation is to stick to a unit system (for example SI) to make it simple. For the convection-diffusion physics mode you would for example typically use the following SI units:

Concentration, c [mol/m3]
Diffusion coefficient, D [m2/s]
Reaction rate, R [mol/(m3/s)]
Velocities u, v, w [m/s]
Boundary flux, N [mol/(m2/s)]

But you could just as well substitute the time unit

*s* to minutes if you like, as long as it is consistent.