The use of TDS meters for determining the purity of fresh water has become widespread over the recent years. Many aquarists use them to determine if tap water purification systems such as reverse osmosis (RO) or reverse osmosis/deionization (RO/DI) are in optimal working condition, or if deionizing resins need to be replaced.
Using these devices, however, does not come with complications. Contrary to what the name might suggest, the devices are incapable of measuring all the dissolved solids. This article describes how these devices work, what they detect and don’t detect. It also offers some tips on how to best use the TDS meters.
How TDS Meters Work
TDS meters are conductivity meters. TDS meters work by utilizing a voltage between two or more electrodes. Positively charged ions move toward the negatively charged electrode while the negatively charged ions move toward the positively charged electrode. These ions have an electric current because they are charged and are moving. The the meter then monitors the amount of current passing between the electrodes hence gauging the number of ions in the solution.
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TDS meters only detect mobile charged ions and will not detect any neutral compounds such as alcohol, sugar, and unionized forms of silica, ammonia and carbon dioxide. The meters are incapable of detecting macroscopic particulates as they are too large to pass through the electric fields used.
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TDS Meters Using Tips
Make sure that you rinse the business end of the TDS meter prior and after each use with clean, fresh water. If there is built-up salt, then this will interfere with the readings, and the carryover of salts from one solution to another can alter the readings.
Clean the electrodes when necessary by dipping the tip in acid and then rinse them well in water. In case it is heavily covered with organic material, it is appropriate that you soak the tip in alcohol or bleach.
If you are using the TDS meter to monitor the performance of an RO membrane, then the measured value should drop by at least a factor of 10 from the starting tap water. For instance when the reading of tap water is 231 ppm, RO water should be less than 230 ppm. Less of a drop than the factor of 10 shows that the RO membrane has a problem.
If the meter is being used to monitor the performance of an RO/DI system, the measured value should drop to near zero. Higher values are an indication that something is amiss or that the DI resin is saturated and needs replacement. Do not agonize over a 1ppm reading from pure water since the air has some elements of carbon dioxide which get in the water and ionizes it causing a higher meter reading.