Two names for nearly the same thing
Total dissolved solids (TDS) is the mass of everything dissolved in the water — salts, minerals, and a little organic matter — expressed in mg/L (equivalent to parts per million). Electrical conductivity (EC) is how easily the water conducts a current, in microsiemens per centimetre (µS/cm). Dissolved salts exist in water as charged ions, and ions carry current, so the two numbers track each other closely.
Handheld meters do not measure TDS directly. Every "TDS meter" is an EC meter multiplying by a conversion factor, typically between 0.5 and 0.7 depending on which salts dominate. That is why two meters can disagree on the same glass of water while both being "right": they are applying different factors.
Typical values to calibrate your intuition
| Water | TDS (mg/L, approx.) |
|---|---|
| RO permeate | 5 – 50 |
| Cape Town municipal water | 100 – 250 |
| Typical inland municipal water | 200 – 500 |
| Brackish borehole | 1 000 – 8 000 |
| Seawater | ~35 000 |
The SANS 241 drinking-water standard allows up to 1 200 mg/L TDS, though most people taste saltiness well before that.
What the number is used for
- Choosing a treatment route. Filtration and softening do not reduce TDS meaningfully; once TDS itself is the problem, you are in reverse osmosis territory. Feed TDS is the first input in membrane selection — it sets brackish versus seawater elements and the operating pressure.
- Monitoring an RO plant. Comparing feed EC and permeate EC gives salt rejection in seconds; a drifting permeate EC is the earliest warning of membrane trouble.
- Checking a softener is not the answer. A softener swaps calcium and magnesium for sodium — TDS stays roughly the same. If the complaint is "salty water", a softener will not fix it and can make the taste slightly worse.
What it cannot tell you
Conductivity is a bulk measurement. It says nothing about which ions are present — 500 mg/L of mostly sodium chloride behaves very differently from 500 mg/L dominated by calcium bicarbonate (the second one scales). It also misses things that matter and do not conduct: bacteria, silica in its unionised form, dissolved organics and non-ionised gases barely register. A low TDS reading is not evidence that water is safe or non-scaling; it is evidence that it is not salty.
Rule of thumb: treat TDS/EC as the headline of the water analysis, not the whole story. It tells you whether RO is on the table and it makes an excellent day-to-day monitoring number, but every sizing decision beyond that needs the full ionic breakdown.