Understanding water tests
Reviewed: September 2026 · General information, not a diagnosis of a water source
TDS means total dissolved solids. A handheld TDS meter estimates the concentration of dissolved charged material by measuring electrical conductivity. It is useful for comparison, but one TDS number cannot tell you whether water is microbiologically safe or identify every dissolved substance.
Low TDS does not automatically mean safe water, and high TDS does not identify the cause. Use the result as one part of a wider source-water assessment.
What can contribute to TDS?
Dissolved minerals and salts can enter water naturally from soil and rock or through human activity. Calcium, magnesium, sodium, bicarbonate, chloride, and sulphate are examples of ions that may affect conductivity. The meter combines their electrical effect into an estimate; it does not give a separate reading for each one.
What a TDS meter can and cannot do
| A TDS meter can help with | A TDS meter cannot confirm by itself |
|---|---|
| Comparing inlet and RO outlet readings under similar conditions | Absence of bacteria, viruses, or parasites |
| Watching for a major change over time | The identity and concentration of every chemical |
| Supporting membrane-performance checks | Whether a well or tank is protected from contamination |
| Recording a quick installation reference | A complete potability or health-safety decision |
Temperature, calibration, meter quality, and the composition of the dissolved material influence the reading. Compare measurements with the same meter and method where possible, and treat unexpected changes as a reason to investigate rather than as proof of one cause.
How is TDS relevant to an RO filter?
The RO membrane can reduce many dissolved ions, so the outlet reading is normally expected to differ from the inlet reading when the system is functioning under suitable conditions. The reduction is not a promise of zero TDS. Final mineral conditioning may also influence the outlet value.
If the inlet pressure is low, pre-filters are blocked, the membrane is aged, or flow is unusual, performance may change. A technician should assess the complete system rather than replacing a part based on one isolated reading.
When is laboratory testing more appropriate?
Laboratory analysis is more appropriate when there is illness, flooding, a suspected contamination event, unusual colour or odour, a vulnerable household member, or a need to identify a specific chemical or microorganism. Select tests based on the source and suspected risk. A laboratory result for one submitted sample represents that sample and test conditions; it should not be turned into a broad certification claim for every unit or every source.
A practical home checklist
- Record whether the source is mains, well, bowser, spring, or stored water.
- Note changes after rain, drought, cleaning, repairs, or nearby activity.
- Measure inlet and outlet water using a consistent method.
- Keep cartridge and service dates with the readings.
- Seek an appropriate water test when the risk cannot be answered by TDS.
During Aqualife installation: a basic water test is completed during the installation visit. It is not a complete laboratory safety assessment and is not promised as a separate pre-installation visit.
