Over recent years, there has been a
widespread use of TDS (Total Dissolved Solids) meter for analyzing the purity of fresh water. Most aquarists use these meters to determine if tap water purification systems such reverse osmosis/deionization (RO/Dip) or reverse osmosis (RO) are working properly or if there is a need for the deionizing resins to be replaced.
The use of such devices, however, is not without complications. Contrary to what the name might imply, the devices do not measure all the dissolved solids. In this article, we will look at how these meters work, what they detect and what they don’t. Additionally, it gives some tips on how to best use them.
How they Operate
TTDS meters work as conductivity meters. They work by applying a voltage of between two or more electrodes. Ions that are positively charged will move towards the negatively charged electrode while the positively charged electrode will attract negatively charged ions. The fact that these ions are charged and moving makes them have 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. The meters are incapable of detecting macroscopic particulates as they are too large to pass through the electric fields used.
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Tips for Using TDS Meters
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 when necessary by dipping the tip in acid and then rinse them well in water. If the electrodes are heavily fouled with organic material; it could help if 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. If the drop is less than a factor of 10, then this is an indication that the RO membrane has an issue.
If the TDS or conductivity meter is being used to monitor the performance of an RO/DI system, then the measured value 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. Do not worried over 1pm because while the value of pure water is below 1ppm, there is a lot of carbon dioxide in the air that gets in the water and ionizes hence the reading on TDS may show results of 1 or 2 ppm.