Heres a link to a very good article on TDS Meters. The article doesn't
specifically address colloidal silver but it explains technical stuff in
a pleasant, easy to read fashion. (Quite an art). I knew TDS meters
really just measured conductivity but after reading this I understood
for the first time why ppm varies from fluid to fluid even though the
TDS readout may be the same in those fluids. Its because some ions are
much more conductive than others. (Maybe you all already knew this?)
Briefly , if I do understand it correctly, TDS meters are factory
calibrated to give PPM readouts for salt water. 1 PPM of salt creates a
specific level of conductivity that puts a 1 on the dial . 10 ppm of
salt puts a 10 on the dial, etc, etc. But salt is a lot more
conductive than CS. It takes twice as many CS ions (i.e. twice the
PPM) to make a number 1 appear on a TDS dial. Hence the usual
recommendation to double a TDS reading when measuring CS. (Yes, I
know, recommmendation 2, is to allow a 100% error factor!)
Of course, its more complicated than this and my terminology is
simplistic but I hope its not too far off.
PWT / uS meters aren't calibrated to give any PPM readout of course, but
it appears from this article that 1 ppm of salt produces about 2 uS.
The fact that CS is half as conductive as salt and that consequently 1
uS on a PWT (might) equal 1 ppm of colloidal silver is a nice coincidence.
http://www.reefkeeping.com/issues/2004-04/rhf/feature/
Another trick I use to try and understand this is to sustitute the word
'milligrams per litre' for PPM.
David
