What are TDS Meters and What are their Use?

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. In this article, we will look at how these meters work, what they detect and what they don’t. It also gives some advice on how to best use them.

How TDS Meters Work
TDS meters are, in reality, 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 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. The meters do not also detect macroscopic particulates as they are too large to move in the electric fields applied.
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Tips for Using TDS Meters
Always ensure that to rinse the using end of the TDS meter before 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 is heavily covered in organic material, soaking the tip in bleach or alcohol may help.

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.

When the TDS meter is being used to measure the performance of the RO/DI system, the value measured 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.