TDS Meters

Reprinted from Colloidal Silver Forum with permission.

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 (conductivity) 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 and conductivity are 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. Neither electrons nor ions move at the speed of light through water. For example, when you turn on a light switch, the effect seems instantaneous. However, the electrons that went into the light bulb were not the same ones that entered the house wiring. Think of it like a pipe full of marbles; put one more marble into one end of the pipe and instantly, a marble pops out the other, but none of the marbles in the pipe moved instantly. The effect was instant, but the movement of the electrons was not. When the drag is large, slowing the motility, the ions in the vicinity of the measurement electrodes become depleted, which lowers the conductivity of the water around the electrodes.

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, while a silver ion weighs 107. So for the same number of ions in solution, the ppm of silver would be 4.65 times higher than that of sodium. To further confuse the issue, the meter reads the chemical compound and is calibrated to sodium chloride, which weighs 58 daltons. Silver oxide molecules weigh 230 daltons, so silver oxide weighs 3.97 times as much as a sodium chloride ion.

The following table demonstrates how different substances create different electrical conductivity, and the error it causes to a TDS meter calibrated for sodium chloride. Each substance is calculated for an actual 1000ppm solution (1 gram per liter)

Compound Conductivity uS TDS Error
NaCl 1997 0
Na2CO3 1664 -16.67%
NaOH 2168 +8.6%
NaHCO3 1069 -53.5%


So what does a TDS meter actually read accurately? Normally, salt water… that’s the only thing its calibrated for. This does not mean its not useful, only that whatever reading you get will be accurate only for pure salt water. Its best use is to determine how pure distilled water is. At a TDS reading of zero, its accurate for all ionized substances.

TDS meters are calibrated with a standardized solution of sodium chloride or potassium chloride depending on brand and model. Can they be recalibrated to read something other than that? Perhaps, it depends on the range of adjustment the internal circuitry offers, and of course a standard solution is necessary. Creating a standard solution of sodium chloride is easy, only requiring an accurate scale. Creating a standard solution of ionic silver is very difficult and for the most part, pointless.

1) A mho or Siemen is the reciprocal of resistance. So a resistance of 1 million ohms would equal 1/1000000 mhos or 1 micro Sieman.
2) A dalton is a unit of atomic mass equal to the atomic mass of the specific atom or the total of the atomic masses of a molecule.