What TDS does not tell you: the four things a low reading can hide.

Why a low reading is not proof of safe water

A TDS meter measures how well water conducts electricity and converts that into an estimate of dissolved solids. It counts charged particles. It does not identify them, it does not see anything that carries no charge, and it cannot register anything present in the tiny quantities at which the dangerous contaminants do their harm. A reading of 120 ppm says the water carries little mineral. It says nothing at all about whether the water is safe to drink, and the Indian standard agrees: IS 10500:2012 has dozens of parameters, and TDS is one of them.

What follows is the list of things the meter cannot see, why each matters in India, and what the standard says about it. The TDS reference page covers what the meter can tell you, which is genuinely useful; this page is the other half.

Bacteria and viruses

Microbes do not conduct. A litre of water can carry enough E. coli to make a family ill and read exactly the same on the meter as a litre with none. IS 10500:2012 requires that E. coli and thermotolerant coliforms be undetectable in any 100 mL sample, which is a way of saying the acceptable number is zero. Contamination comes from sewage reaching a source, from a flooded borewell after the monsoon, from an overhead tank left uncleaned, and from the last metre of pipe. None of it moves the meter.

The test is bacteriological, done by a lab on a sample collected in a sterile bottle. The treatment is disinfection: UV in a domestic purifier, chlorination at the utility, boiling in an emergency. The UV vs RO guide covers which purifier that calls for.

Nitrate

Nitrate reaches groundwater from sewage and from fertiliser, and it is common in Indian borewells near fields or old septic systems. It is tasteless and colourless. IS 10500:2012 sets 45 mg/L with no relaxation, because the risk to infants is direct and there is no level the standard is prepared to tolerate as a last resort. Nitrate does conduct, so it contributes to the TDS figure, but as one indistinguishable part of the total: 45 mg/L of nitrate in 400 ppm water looks on the meter exactly like 45 mg/L of harmless bicarbonate. Only a lab can separate them. Reverse osmosis removes nitrate; UV, carbon and boiling do not.

Fluoride

Fluoride comes from the rock itself and occurs naturally in groundwater across parts of the peninsula and the north-west. It is tasteless. The acceptable limit is 1.0 mg/L and the permissible limit 1.5, and the harm above that — mottled teeth, then skeletal damage — takes years and is not reversible. One and a half milligrams per litre is far below what a TDS meter can resolve out of a total in the hundreds; a borewell at 350 ppm can be over the fluoride limit and look, on the pen, like safe soft water. This is the contaminant that most often makes a low-TDS borewell unsafe, and it is the single best reason for a borewell owner to get one lab test. RO removes it.

Heavy metals

Arsenic, lead and the other metals the standard limits are harmful at fractions of a milligram per litre: IS 10500:2012 gives 0.01 mg/L for arsenic as the acceptable limit and 0.01 mg/L for lead with no relaxation. A hundredth of a milligram does not register on a meter that rounds to the nearest part per million. Arsenic in groundwater is a known problem in parts of the Ganga basin; lead comes from old plumbing and fittings rather than from the source. Neither tastes of anything. Both need a laboratory, and both are removed by RO, which is one reason RO is fitted even on water whose TDS did not require it.

Chlorine, pesticides, taste and smell

Residual chlorine from municipal treatment, the earthy taste of a reservoir in summer, the smell of a tank that needs cleaning, traces of pesticide in run-off: none of these are dissolved salts and none of them show on the meter. They are what the carbon stage of a purifier is for. Activated carbon adsorbs chlorine and organic taste and odour compounds, which is why every purifier has one ahead of the membrane and why it is changed on a schedule. In the LUXGEN range that stage is the 10″ coconut-shell carbon cartridge for standard housings, rated up to twelve months. Cloudiness, for what it is worth, does not show on the meter either: turbidity is suspended, not dissolved, and the standard limits it separately at 1 NTU acceptable and 5 permissible.

What the meter is still good for

None of this makes the TDS meter useless. It answers three questions well: whether the water needs RO at all, whether a membrane is still working, and whether a borewell is drifting saltier through the year. For all three, the measuring guide gives the method. What the meter cannot do is stand in for the lab, and the site's purifiers reflect that: the Royale Copper, like the Aura and the Curve, runs UV after the membrane precisely because RO output that has been sitting in a tank still needs disinfecting, whatever its TDS.

What to do once

Take one sample to a water testing laboratory and ask for bacteriological count, nitrate, fluoride and, if you are in an arsenic-affected district or have old plumbing, arsenic and lead. Keep the report. Repeat the bacteriological test after a monsoon that flooded the bore. Everything else on this site — the TDS bands, the membrane choice, the cartridge schedule — assumes that report is clean, and says so.