Over recent years, there has been a
widespread use of TDS (Total Dissolved Solids) meter for analyzing the purity of fresh water. Many aquarists use the TDS meters to determine if the processes used to purify tap water like reverse osmosis or reverse osmosis/deionization are working properly or if deionizing resins require replacement.
The use of such devices, however, is not without complications. Contrary to what the name might suggest, the devices are incapable of measuring all the dissolved solids. Here, the working mechanism of these meters is discussed, along with what they can and cannot detect. Additionally, it gives some tips on how to best use them.
How TDS Meters Work
TDS meters are conductivity meters. The meters work by using a voltage of between two or more electrodes. Positively charged ions will get attracted to the negatively charged electrode while the negatively charged ones will move to the positively charged electrode. The ions are charged and moving hence they constitute an electrical current. 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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The TDS meter will detect mobile ions that are charged and not detect any uncharged or neutral compounds like sugar, unionized forms of silica, carbon dioxide, and alcohol. These meters do not also detect macroscopic particles as they are too large to go in the electric fields applied.
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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.
Ensure that the electrodes are cleaned whenever necessary by dipping the tip in acid like vinegar or diluted hydrochloric acid and then rinsing it well in water. In case it is heavily covered with organic material, it is appropriate that you soak the tip in alcohol or bleach.
When using the TDS meter to measure the performance of Reverse Osmosis membrane, then the measured value should drop by at least a factor of 10 from the staring tap water. If, for example, the tap water reads 231 ppm, then the RO water should be less than this. Less of a drop than a factor of 10 shows that there is a problem with the RO membrane.
When the TDS meter is being used to measure the performance of the RO/DI system, the value measured should drop to near zero. If the values are higher, it only means that there is something that is not operating well or the DI resin is becoming saturated and requires replacement. Readings of 1ppm should not worry you as there is carbon dioxide in the air that gets in the water and ionizes it causing the TDS to yield results of 1 or 2 ppm even in pure water.