How to calculate the flow rate

Hey,
i post it into that topic because its the same topic, if its not ok remove my post and i will start a new topic.

i have to calculate the mass flow over a threshold in a multiphase simulation. I create a slice at the threshold, and then the GenerateSurfaceNormls, but the Glyph are pointing to the side. Why the dont point in the normal direction ?
There is a screenshot below, showing the glyph.

I uploaded two videos, the show the result from the simulation.


Is it possible to measure the flow over the threshold ?

Best regards
Kai