User Guide

What is TDS?
TDS stands for Total Dissolved Solids. It is the measurement of all the dissolved substances in a
water sample. These “dissolved solids” are mostly inorganic salts like sodium, calcium,
magnesium, and potassium cations along with chloride, bicarbonate, carbonate, phosphate,
sulfate, and nitrate anions. TDS also includes dissolved organic compounds. Truly accurate TDS
measurements must be performed in an analytical laboratory. The water sample is weighed
and then heated under tightly controlled conditions to evaporate the water. The remaining
weight of the leftover “solids” is used to calculate the TDS in the water sample. The unit of
measurement is parts per million (ppm), which is the same as milligrams per liter (mg/l).
However, this level of accuracy is not necessary for most at-home testing applications. It is
expensive and must be performed in a qualified laboratory. Using a TDS meter is a much faster
method.
How TDS meters work
TDS meters do not directly measure the total dissolved solids. Remember, the TDS is
determined by evaporating the water and weighing the remaining solids. Hand-held TDS meters
measure electrical conductivity (EC). The EC unit of measurement is a microsiemens (µS).
Dissolved inorganic substances like salts, minerals, nitrate, phosphate, and metals conduct
electricity. The higher the concentration of these substances in the water sample, the higher
the electrical conductivity. The TDS meter converts the EC reading into TDS as parts per million
(ppm). Dissolved organic compounds do not conduct electricity and are therefore not detected
by TDS meters. TDS meters cannot be used to test for organic substances like pesticides,
gasoline, and solvents.
Why is the TDS important?
TDS is not a measurement of harmful substances or pollutants. TDS is simply a measurement of
all substances dissolved in water. Distilled water has a TDS of 0.0 ppm, while the ocean has a
TDS of 10,000 ppm. Both water sources are safe. An acceptable TDS is relative to the water
being tested and what the water is used for. Here is an example: If you tested the TDS of
distilled water and found it to be 300 ppm, you would know that the water was not distilled.