Most people are under the mistaken impression that they can read the concentration of silver in ppm in their colloidal silver with a Total Dissolved Solids (TDS) meter. To understand why it will not read correctly, it is necessary to understand how a TDS meter works.
Inside of a TDS meter is an electrical circuit which measures the resistance between its two electrodes when immersed in a liquid. This circuit applies an AC voltage to the electrodes, and then measures the AC current which flows between the two electrodes. This reading is then corrected for temperature (if its a high quality instrument), and electrode geometry. The result is a measure of Total Ionic Content. The name Total Dissolved Solids is actually a misnomer, as a lot of dissolved solids will not read at all on a TDS meter since they do not ionize when they dissolve. Sugar is such a substance. If you were to dissolve a tablespoon of table sugar in a cup of distilled water and read it with a TDS meter, the resulting reading would be zero.
The reason AC is used instead of DC voltage on the electrodes is to keep the metals in the solution from plating onto the electrodes. With AC, anything that plates onto the negative electrode will in theory come off when the polarity reverses. AC also keeps the ions from migrating from one electrode to the other, thereby keeping the solution homogeneous.
Another thing which must be understood is the actual meaning of parts per million (ppm). PPM means the weight of one substance contained in a given weight of another different substance. For example, if a million pounds of bread flour contained 1 pound of dirt, it would be 1 ppm dirt in the flour. We are not interested in flour though, we are interested in silver ions in water. For silver, one PPM means 1 gram of silver in 1 million grams of water, which is equivalent to 1 milligram of silver in 1 kilogram of water. Since water weighs 1 gram per milliliter, this is equivalent to 1 milligram of silver per liter of water.
The problem with measuring silver ions goes back to the what the TDS meter actually measures – electrical current (conductivity). In a solution, electrical current (charge) is carried by ions moving through the solution, and current is actually ions moving per time. It doesn’t matter whether it is a silver ion, or a sodium ion etc. However, not all ions are equal, each kind of ion has a different weight. For example, a single sodium ion weighs 23, while a silver ion weighs 108, or 4.7 times as much as a sodium ion. Yet, the TDS meter cannot distinguish between the two.
Another source of error is that ionic silver is not just silver, it is silver oxide, or silver chloride or some other silver salt. Therefore, the ppm of the colloidal silver will have a different value for the same amount of silver depending on the silver salt present. It is not possible to just have silver ions, there will always be a matching anion present, like hydroxide, chloride, nitrate, etc.
Most TDS meters are calibrated to read a mix of ions normally found in ground and drinking water. This is done by adjusting the circuitry to get a specific reading while the TDS meter electrodes are immersed in a standardized solution of salt. The salt solution is just used as a repeatable reference, and the TDS meter is not calibrated to read the actual ppm of the salt solution.
The result of these considerations is that the only TDS reading that is accurate for silver is ZERO, and any other reading will be incorrect. A TDS meter is not very useful for measuring silver, but is useful for measuring the purity of the water used to make colloidal silver.