Test Quantity of Diamagnetic, Paramagnetic Particles in a Viscous Moving Liquid

I want to be able to quantify the amount of very small Diamagnetic and Paramagnetic particles in a moving stream of viscous liquid. A hall effect device comes to mind, but I do not know how to make the circuit, or if there is already a device that can do that.

I am not as sharp as I once was, past 76years old and nothing works like it used to, especially after a fight with Covid so bear with me.

Would it require a PM on one side of the column and the Hall effect on the other? Would spacing make a difference in measuring the total particles? Would velocity of the fluid make a difference? I kind of picture like cars of two different colors passing by a sensor. It only counts the number of each color cars, not the speed.

Do they make high speed, high resolution Hall Effect sensors?

Any help is sincerely appreciated.