A Total Dissolved Solids (TDS) tester is a common, handheld device used for a rapid assessment of water quality. It provides a simple numerical measurement representing the concentration of non-water elements dissolved in the liquid. The TDS value offers an immediate, though limited, snapshot of the water’s composition. Interpreting this reading accurately requires understanding what the metric represents and how the device functions.
Defining Total Dissolved Solids
Total Dissolved Solids (TDS) refers to the combined mass of all inorganic and organic substances dissolved in water, excluding the water molecules themselves. These solids are small enough to pass through a filter with pores around two micrometers in size. The composition of TDS is highly variable and includes a wide array of constituents.
Common inorganic constituents include minerals and salts, such as positively charged ions like calcium, magnesium, sodium, and potassium. Negatively charged ions like chloride, sulfates, carbonates, and nitrates are also major contributors. TDS concentration is typically reported in parts per million (ppm) or milligrams per liter (mg/L), which are functionally equivalent units.
How the Tester Measures TDS
The handheld TDS meter does not measure the physical mass of dissolved solids, which is a time-consuming process. Instead, the device relies on an indirect measurement of the water’s electrical conductivity (EC). Dissolved inorganic solids, particularly salts and minerals, dissociate into ions in water, allowing the liquid to conduct an electrical current. The higher the concentration of ions, the higher the electrical conductivity.
The meter utilizes two electrodes to pass a small electrical current through the water sample and measures the resulting conductance. This EC reading is typically measured in micro-siemens per centimeter (uS/cm). The meter then uses a pre-programmed conversion factor to translate the EC measurement into the TDS reading, displayed in parts per million. This conversion factor often ranges between 0.5 and 0.7, meaning the TDS value is an estimate based on the assumption that the dissolved substances are primarily common inorganic salts.
Interpreting TDS Readings and Water Quality
The primary application of a TDS tester is monitoring changes in water quality, especially tracking the efficiency of purification systems like reverse osmosis (RO) units. A well-functioning RO system should show a significant reduction in the TDS level from the source water, typically achieving a rejection rate above 90%. A sudden increase in the TDS reading from a filtered source signals that the system’s membrane needs replacement or maintenance.
TDS also influences the aesthetic qualities of water, particularly its taste. Water with extremely low TDS may taste flat or insipid, while levels above 500 ppm can result in a metallic or salty flavor and may cause scale buildup in pipes and appliances. The World Health Organization suggests that water with a TDS level below 300 mg/L is considered excellent in terms of palatability.
It is important to recognize the distinction between a TDS reading and water safety. A low TDS reading does not guarantee the water is free of harmful contaminants like bacteria, pesticides, or certain heavy metals. Since these substances often do not ionize or significantly alter conductivity, the meter cannot detect them. Conversely, high TDS levels are often due to harmless minerals like calcium and magnesium, meaning high numbers do not automatically indicate dangerous water. The TDS meter is best understood as a tool for measuring mineral content and filtration system performance, not as a comprehensive safety test.