Fluid Flow-Particle Tracing-Electrostatics Coupling - Detect particles

Dear COMSOL community,

I am new to COMSOL and have been tasked with modelling the trajectory of particles being under the influence of a laminar fluid flow and the electric potential of a charged steel plate. Essentially the model is representative of depositing particles on to a steel surface.

It is my aim to calculate how many particles are attracted to the surface of the steel but I can find no effective way of calculating this number. I have attempted to set up a boundary probe (the boundaries being the steel surface) which detects the total number of particles in the selection but I get this error message;

Evaluation of result expression failed.

Failed to evaluate variable.
- Variable: part_count_sel
- Geometry: 1
- Boundary: 6 7 8 9 11 12 13 14 15 16 17 18 19 20 21
Failed to evaluate variable.
- Variable: comp1.fpt.Nsel
- Defined as: part_count_sel
Failed to evaluate expression.
- Expression: real(comp1.fpt.Nsel)
Failed to evaluate expression.
- Expression: real(comp1.fpt.Nsel)

Any advice would be much appreciated.