Your help is still great and so appreciated even under those laws, I can assure you.
As I've found, the dpm concentration is compured as:
DPM concentration = (Avg. particle mass in cell * Part resi. Time * Strength of particle) / Cell volume
Strength of particle =total particle flow rate / mass of single particle in the stream
DPM conc. = (Avg. particle mass in cell * Part resi. Time * Total particle flow rate) / (mass of single particle in stream * cell volume)
Among the mesh variable of the Custom Field Function there is "cell-volume-2D" . Maybe, if I would be able to substitute the cell volume in the dpm concentration formula with this one, I could solve the problem. As I've understood, this variable means to compute the volume with a uniform depth and not by considering the 3d geometry behind the axial symmetry.
To calculate the DPM_concentration with a new formula (generally speaking), I just need to write the new formula in the Custom Field Functions space using the variables listed and then by activating the custom laws in the injection panel, am I correct?