How to actually test your water

You buy a TDS meter for a few hundred rupees, dip it in a glass of water, get a number like "87 ppm," and feel reassured. Or you get "340 ppm," panic, and buy a purifier. Neither reaction is really justified, because a TDS number was never designed to answer the question you're actually asking: is this water safe to drink?

Three tools get used, loosely and interchangeably, for "testing water" in Indian households: the handheld TDS meter, home test strips, and a proper lab test against the BIS standard IS 10500:2012. They measure different things at different confidence levels, and mixing them up leaves people either falsely reassured or needlessly alarmed. Here's what each actually tells you, what it can't, and which one your situation calls for.

What a TDS meter actually measures

A TDS meter doesn't measure dissolved solids directly — it measures electrical conductivity and estimates TDS from that. Dissolved ions — mostly calcium, magnesium, sodium, chloride, carbonate and similar salts — conduct electricity in proportion to their concentration, so the meter reads conductivity and multiplies by a conversion factor to output a ppm figure. This is reasonably consistent under normal conditions, with roughly 5% error on decent handheld units, but the conversion factor assumes a typical ion mix, so accuracy drops in unusual water chemistry or when the meter isn't temperature-compensated.

More important than the error margin is what the number can't tell you. A reading of 300 ppm could be mostly harmless calcium and magnesium, or it could include nitrate or fluoride at concerning levels — conductivity doesn't care what's carrying the charge. It also says nothing about anything that doesn't ionise: bacteria, viruses, pesticides and most organic contaminants pass through a TDS reading completely invisible. A borewell can read a clean 150 ppm and still be microbiologically unsafe, or carrying arsenic at multiples of the permissible limit.

BIS's drinking water standard, IS 10500:2012, sets an acceptable TDS limit of 500 mg/L and a permissible limit of 2000 mg/L where no alternative source exists. WHO takes a different position — it sets no health-based limit for TDS at all, treating it purely as a taste issue: palatability declines above roughly 600 mg/L and water turns objectionable past about 1000 mg/L. BIS is regulating an upper bound; WHO is describing when water starts tasting off. Neither treats it as a safety marker. What a TDS meter is genuinely useful for is trend-tracking — confirming your RO membrane is still rejecting salts, or noticing your source water has changed.

What a certified lab test actually covers

A full test against IS 10500:2012 checks over 80 parameters across three categories. Physical (turbidity, colour, odour, pH, TDS), chemical (heavy metals including arsenic, lead and mercury; fluoride; nitrate; residual chlorine; pesticides in the extended panel), and microbiological (total coliform, faecal coliform, E. coli). It's the only method of the three that can actually confirm or rule out the contaminants people worry about most, because it uses laboratory instrumentation and wet-chemistry methods rather than a proxy reading.

Accreditation is what makes the report count. Look for NABL accreditation (ISO/IEC 17025) — it's what makes results defensible to regulators; find accredited labs via BIS's LIMS portal (lims.bis.gov.in) or NABL's directory. There's also a government network of roughly 2,870 water-testing labs run by state Public Health Engineering departments and the Jal Shakti ministry, of which 1,707 are accredited and open to the public at nominal rates. Private labs tend to be faster, with wider panels where you need them.

A basic potability package — core physical, chemical and microbiological parameters — runs roughly ₹1,500–2,500 per sample at a private NABL lab; a comprehensive panel with heavy metals and pesticides can run ₹10,000 or more. Turnaround for a full panel is typically 5–7 working days; microbiology-only results can come back in 3–4 days, heavy-metals-only in 2–3.

It's worth the cost in specific situations: moving into a home with unfamiliar source water, after a sudden change in taste, colour or smell, if someone develops unexplained gastrointestinal illness, and — the one people skip — annually if you depend on a borewell or groundwater. The Central Ground Water Board's 2023 sampling, released as the Annual Groundwater Quality Report 2024 and based on over 15,000 monitoring locations, found 19.8% of samples over the permissible nitrate limit, 9% over fluoride limits, and 3.5% over arsenic limits, with nitrate above safe levels in roughly 440 districts — 56% of the country. These are geogenic contaminants: naturally occurring, geographically clustered, and undetectable by taste, smell, or a TDS meter.

Home test strips: fast screening, not lab-grade certainty

Dip-strip kits test one parameter at a time against a colour chart — chlorine, hardness, nitrate, pH — and give a result in under a minute. They're useful for routine checks: confirming a softener is still working, checking for municipal chlorine residual, getting a rough hardness estimate. What they don't give you is a precise number — most report a range ("0–250 ppm hardness"), fine for a sanity check, useless for calibrating a treatment system.

Reliability varies by parameter and brand. Manufacturer and field data put strip error generally in the 10–20% range, tightest for nitrate, nitrite, iron and coliform presence, loosest for pH and chlorine, since reading a colour chart is inherently subjective. Treat a strip result as a screening signal, not a number to act on.

The H2S vial test deserves a specific mention — it's an India-specific, genuinely useful tool. Licensed from NRDC and produced domestically, including by DRDO-linked facilities, it costs roughly ₹15–25 a vial at bulk rates (retail listings run higher) and detects fecal bacteria: the sample turns black within 24–48 hours at room temperature if contamination is present, stays yellow if not. It's the basis of the Field Test Kits used under the Jal Jeevan Mission, where over 24 lakh trained volunteers screen village water sources. It only shows presence or absence, not concentration or organism identity — a positive is meant to trigger a lab test, not stand in for one, which is exactly how the Mission's own protocol treats it.

What an ordinary household should actually do

• On municipal supply with no complaints: an occasional TDS check or strip test is enough. Save the lab test for when something changes.
• Moving into a new home, or connecting a new borewell: get one full IS 10500 lab test before treating it as your primary source — you're establishing a baseline you don't otherwise have.
• Dependent on a borewell year-round: get a full lab test annually — the single highest-value test for the largest number of Indian households, given how widespread fluoride, nitrate and arsenic are in groundwater.
• Sudden change in taste, colour or odour, or unexplained illness at home: an H2S vial or strip check can flag a problem fast, but follow up with a lab test rather than trusting the screening result alone.
• Running an RO or other purifier: a TDS meter is the right tool to confirm the membrane is still rejecting salts — that's what it was built to check, not overall water safety.