Time derivative of a surface integration in 4.3

Hi

I'm using the magnetic field (mf) physic in time dependent study. I want to calculate the voltage V(t), as v(t) = - d/dt(flux phi) (time derivative of the magnetic flux). I serched the docs and found that's accomplished using the differentiation operator : d(variable,t) on the simulation result.

Since the magnetic flux is not supported as a variable by comsol. I have to evaluate it using surface integration (derived values node), as flux = surface integral (B ds) with B the magnetic flux density evaluated by comsol.

The problem is that the surface integration give me the magnetic flux phi as a defined value in a table and I just can't use it to for the time derivative I'm needing.

Is there any solution de get a surface integration of the flux density B as an actual variable or is there any other way to calcutate the magnetic flux Phi ?

Thanks a lot !