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 these meters, however, is not without complications. Contrary to what the name might imply, the devices do not measure all the dissolved solids. This article describes how these devices work, what they detect and don’t detect. Additionally, it gives some tips on how to best use them.
How TDS Meters Work
TDS meters are conductivity meters. TDS meters work by utilizing a voltage 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 work of the meter here is to monitor how much current is passing between the electrodes as a gauge of how many ions are 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. Salt Build up on the operational tip will interfere with proper operation and any transfer of salts from one solution to the other can skew the readings.The buildup of salts is likely to interfere with proper operation and carrying over salts from one solution to another can distort the readings.
Clean the electrodes by soaking the tip in acid like diluted hydrochloric acid or vinegar and then rinse it well in water. If the electrodes are heavily fouled with organic material; it could help if 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. For instance when the reading of tap water is 231 ppm, RO water should be less than 230 ppm. Less of a drop than a factor of 10 shows that there is a problem with the RO membrane.
If the meter is being used to monitor the performance of an RO/DI system, the measured value should drop to near zero. If you record values higher than this, it means that something is malfunctioning or that the DI resin has become saturated and needed 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.