TDS meters are real popular among home brew colloidal silver makers. But how do they work, and what exactly do they measure?
TDS meters are basically AC ohm meters. They measure the electrical resistance between a pair of electrodes. Thats all they do. The meters have some logic circuitry which either converts the readings to micro Siemans (also called a mho)1, or to a number which supposedly represents the ppm of dissolved solids in the water.
Neither of these measurements is a true reading of what is in the water and how much.
Since electrical resistance is computed by measuring the current flow between the two electrodes, it can only measure substances which have an electrical charge and can move that charge from one electrode to the other.
But not all substances carry electrical charge. Only ions carry charge, and many substances do not produce ions in water. Ordinary table sugar is an example of a substance which does not ionize, and does not carry current. However, anyone can plainly see that table sugar does indeed dissolve. So a TDS meter cannot read all things that are dissolved in the water, only ionized substances. Ergo, the words TOTAL dissolved solids a complete and TOTAL misnomer.
Another problem with measuring even the ionic portion of the water is that the reading depends on how mobile the particular ions are… how fast can they move. Large ions tend to move slower than smaller ones just because of the drag through the water. This causes higher resistance and a smaller TDS reading.
And yet another factor is the actual charge of the ion. For instance, a calcium ion is more positive than a sodium ion, because a sodium ion is only missing one electron, whereas a calcium ion is missing two. So a calcium ion carries twice as much charge as a sodium ion.
Continuing with problems, the TDS meter or its cousin the microSiemen meter are both blind to what the actual ions are. Since ppm is defined as the weight of one substance present in another substance, and since different ions have different weights, it is impossible for a TDS meter to read actual ppm except for the ion it is calibrated to. For instance, a sodium ion weighs 23 daltons2