A total dissolved solids meter works by adding a voltage between two or more electrodes, according to Reefkeeping. Positively charged ions tend to move toward the negatively charged electrode, while negatively charged ions move toward the electrode with a positive charge.
Reefkeeping states that TDS meters are used to analyze the purity of freshwater. Aquarists typically use these devices to find out if deionizing resins need to be replaced or if tap water purification systems, such as reverse osmosis, are working well. However, they do not measure all dissolved solids. They use various units of measure, but all measurements are taken as parts per million.
TDS meters are conductivity meters, according to Reefkeeping. When voltage is applied between electrodes, the ions become charged and moving, thus constituting an electrical current. TDS meters monitor the amount of current that passes between the electrodes as a measure of the number of ions in a solution. They detect only mobile charged ions and not neutral compounds, such as sugar, alcohol, pesticides and unionized forms of silica, ammonia and carbon dioxide. They also do not detect bacteria, viruses and macroscopic particles, which are too big to move in the applied electric fields. TDS meters are generally useful for testing the purity of water in aquariums.