Water Quality
TDS Meters: How to Use One, and What It Cannot Tell You
A digital TDS meter is a small probe with two electrodes. It passes a current between them, measures the water's electrical conductivity, and converts that to an estimated total dissolved solids figure in parts per million. Dissolved salts conduct; pure water barely does.
For roughly ₹300 it answers the question that determines everything else about a water purifier purchase: do you need reverse osmosis, or would a UV+UF unit serve you better at a third of the price and none of the water wastage? Almost every regrettable purifier purchase in India traces back to skipping this step.
It is equally important to understand what the reading cannot tell you, because that misunderstanding is genuinely dangerous.
What the number means
| TDS (ppm) | Classification | Taste | What to buy |
|---|---|---|---|
| 0 – 50 | Very low / demineralised | Flat, insipid | Too low — blend minerals back via TDS controller |
| 50 – 150 | Ideal | Clean and full | This is the target for purifier OUTPUT |
| 150 – 300 | Good | Pleasant | UV + UF is sufficient |
| 300 – 500 | Acceptable (BIS limit 500) | Noticeable minerality | RO recommended |
| 500 – 1,000 | High | Hard, slightly brackish | RO required |
| 1,000 – 2,000 | Very high | Brackish | RO rated to 2,000 ppm input |
| Above 2,000 | Brackish | Salty | RO rated 3,000 ppm + full lab analysis |
The limitation that matters most
A TDS meter measures conductivity. It cannot distinguish harmless dissolved calcium from dissolved arsenic, and it cannot detect anything that does not conduct. That has two serious consequences.
First, dangerous water can read as excellent. Arsenic and fluoride at harmful concentrations barely move a TDS reading, so water at 120 ppm in an arsenic belt can be simultaneously 'good TDS' and genuinely unsafe. Second, TDS tells you nothing about microbiological quality — bacteria and viruses do not register at all.
So the correct use is narrow and valuable: use TDS to select purification technology, and use a NABL-accredited laboratory test to establish whether your water is safe.
Important
How to measure correctly
- Remove the cap and switch on. Wait for the display to settle before dipping.
- Dip only to the marked immersion line — submerging the body damages the meter.
- Stir gently and wait for the reading to stabilise, usually 10–20 seconds.
- Use the HOLD button to freeze the reading before lifting the probe out.
- Rinse the electrodes in clean water after each use and dry them. Deposits on the electrodes cause drift.
- Take readings at similar water temperature. Conductivity rises with temperature; most meters compensate automatically, but large differences still affect comparability.
- Measure both input (tap) and output (purifier) each time, and write both down with the date.
Calibration and buying
- Most ₹300 pen meters are factory-calibrated and not user-adjustable. That is adequate for the decisions you are making.
- Better meters (₹800–₹2,500) support calibration against a standard solution, typically 342 or 1,413 µS/cm.
- Sanity-check a cheap meter against a known reference: distilled water should read close to 0, and a fresh RO output should read low.
- If two meters disagree substantially, the older one with deposits on the electrodes is usually the wrong one.
- An EC (electrical conductivity) meter measures the same underlying property; TDS is simply EC multiplied by a conversion factor, usually 0.5 or 0.7. Meters using different factors will disagree slightly on the same water — which is normal, not a fault.
Using a meter to monitor your purifier
- Record input and output TDS immediately after installation. That is your baseline.
- Repeat monthly. It takes ten seconds and creates the log that protects you from unnecessary part replacements.
- A rising output TDS with unchanged input is the earliest reliable sign of membrane degradation — well before any taste change.
- Calculate rejection: (input − output) ÷ input × 100. A healthy domestic RO typically achieves 85–95%.
- Take the meter to a technician's visit. Households that can produce a log get measurably better service outcomes, because vague diagnoses become hard to sustain.
Where to buy
We have not individually tested products in this category, so we are not going to rank them as though we had. Use the buying criteria above to judge listings yourself, and these searches as a starting point.
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