TDS Meters
A TDS meter measures total dissolved solids in water, in parts per million, by reading electrical conductivity. It is useful for checking whether a reverse osmosis membrane is still working and when a ZeroWater filter needs changing. It does not tell you whether water is safe, because many harmful contaminants, such as lead or bacteria, barely affect TDS.
How to use a TDS meter
- RO check: compare tap TDS with filtered TDS. RO systems typically reduce TDS by 90% or more, and a falling percentage can signal a worn membrane.
- ZeroWater: replace the filter at a reading of 006.
- Reference: EPA's secondary, non-enforceable guideline for TDS is 500 mg/L, based on taste.
- Temperature: choose a meter with automatic temperature compensation.
- Calibration: check against a calibration solution occasionally.
For real safety testing, use Well Water Test Kits or Lead Water Test Kits. To reduce TDS, compare Reverse Osmosis Systems.
FAQs
What does a TDS meter tell you?
It shows the total amount of dissolved minerals, salts and other substances in water, in parts per million, based on conductivity. It is useful for comparing water before and after an RO system or ZeroWater filter. It cannot identify specific contaminants and does not indicate whether water is safe to drink.
What is a good TDS level for drinking water?
EPA's secondary guideline for TDS is 500 mg/L, a non-enforceable level based on taste and appearance rather than health. Many tap waters range from 50 to 400 ppm. RO water often reads under 50 ppm. Low TDS is not proof of safety, and higher TDS is not necessarily unsafe.
How do I check if my RO membrane is working?
Measure TDS in tap water and in filtered water, then calculate the reduction. A healthy membrane typically reduces TDS by 90% or more. If filtered TDS rises steadily or the reduction falls well below the system's original performance, the membrane or post-filters may need replacing.