The use of Total Dissolved Solids (TDS) meters for analyzing the purity of fresh water has become popular in the past few years. 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 these meters, however, is not without complications. Far from the name might suggests, these meters are not able to measure all the dissolved solids. This article describes how these devices work, what they detect and don’t detect. It also gives some advice on how to best use them.
How TDS Meters Work
TDS meters are, in reality, conductivity meters. TDS meters work by utilizing a voltage 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. These ions have an electric current because they are charged and are moving. 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.
Overwhelmed by the Complexity of Tips? This May Help
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.
5 Key Takeaways on the Road to Dominating Health
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 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.
If the TDS meters are being used to measure RO membrane performance; the measured value should drop by at least a factor of 10 from the starting tap water. If, for example, the tap water reads 231 ppm, then the RO water should be less than this. 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. 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.