Analytic function accepts only [m] and not [mm]

Dear all,

I have a trouble using an analytic function: I get values only using the SI unit of measure, and not with the MPa one.
The model is a first attempt of a welding simulation, and the analytic function should simulate the heat source distribution in the plates (Goldak's equation).
If I set SI unit measure (metres) both in the root and in model , I get an heat source distribution, of course in W/m^3, and let's say with a peak of 7.22e10 W/m^3.
On the other hand, if I choose MPa in unit measure (thus mm), I should get the same heat source distribution but "scaled", hence with a peak of 72.2 W/mm^3 (from 1/m^3 to 1/mm^3), but this does not happen...I get an empty plot, it's =0 all over the domain...why?

I attach my model, it's in 4.2a.
Thank you for any suggestions