Parts Per Million. [one part silver to one million parts water] [micrograms of silver per cubic centimeter of water by molecular weight] PPM has nothing to do with particle size.
PPM is measured by comparative weight. There are instruments available that measure Total Dissolved Solids via a conductivity reference but the calibration is for a solution for various minerals and salts dissolved in water, NOT a suspension of pure metal such as colloidal silver. The readings will be a relative indicator of concentration but will not reflect the actual PPM or total silver content. Meters do not register the colloidal ‘suspended’ portion you can see with a laser…only the ionic ‘dissolved’ portion and then only as a unit of conductivity..not PPM
The more pronounced the ‘TE’ [Tyndal Effect] , the higher the ratio of particles to ions.
[TDS meters measure microsiemens like all other meters [an electrical conductance unit] and use an industry standard fudge factor for salt water. [Actually 3 standards depending on what salt]