I am modelling an axisymmetric piezoelectric transmitter in COMSOL 5.6 and use Exterior Field Calculation (full integral) to find the pressure outside my simulation domain. This works fine when I want to find the pressure in a single point (e.g. acpr.pext(1,0)), but I want to find the average pressure over a plane surface coaxial with the transmitter, i.e. a line average due to the axial symmetry.

I haven't been able to define a line average in "Result -> Derived Values" since there is no boundary outside my simulation domain, and if I add a line in the geometry outside my simulation domain, it does not get meshed. An alternative approach would be to export acpr.pext at the line of interest (with a suitable sampling) to a text file and compute the average outside COMSOL, so far I haven't been able to export the pressure for multiple points.

Does anybody have an idea as to how I could compute the average pressure?