How to Use a TDS Meter Correctly

Last updated: 21 August 2026.

Submerge the probe between two to three centimeters into still water at room temperature, observe the number stop changing, and note the reading in ppm.

The next point is one that is been confused by just about everyone. A TDS meter does not measure hardness; it measures the conductivity of water and translates that into an estimate about the total dissolved solids, of which calcium is just one component.

That distinction decides whether your reading is useful or misleading. Care Dale's filter is dermatologist-certified, clinically tested, and even so the honest answer is that a TDS number tells you to investigate, not what to buy.

What is a TDS meter actually measuring?

Conductivity, converted. Pure water conducts electricity poorly, and every dissolved salt improves it, so the meter infers total dissolved solids from resistance between two probes.

The overall quantity consists of calcium and magnesium minerals making the water hard. It also combines sodium, cholorides, sulphate and nitrates, which do not create any scaling or affect the process of soap lathering.

Therefore, two taps can read 400 ppm but act entirely differently when used in the shower. One may be high in calcium and leave scale behind. The other would be high in sodium but leave behind virtually nothing.

Scale-forming tendency depends on more than the total. The Chartered Institution of Building Services Engineers (CIBSE) notes, in its technical module on hot water, that the Langelier Saturation Index, the standard measure of whether water will deposit or dissolve calcium carbonate, combines pH, calcium concentration, dissolved solids and temperature.

To put this simply, a single number could never give you the answer to the question alone. This limitation stems from the instrument itself, rather than a flaw specific to your device. It is always wise to be aware of what hardness actually is before making assumptions based on number alone.

Hardness is measured separately, as milligrams per litre of calcium carbonate. The Maryland Geological Survey classifies water as soft below 60 mg/L and very hard above 180 mg/L, and that scale is about calcium and magnesium alone.

If the difference between the two numbers matters for your decision, it is worth reading which one you should actually be tracking.

How do you take an accurate reading?

Five steps, in order, and the whole thing takes under a minute.

1. Rinse the probe in clean water and shake it dry. Residue from the last sample is the most common source of a wrong first reading.
2. Fill a clean glass with water from the tap you actually use. A used cup or a plastic bottle contributes its own dissolved solids.
3. Let the water sit two to three minutes so it reaches room temperature and stops moving.
4. Dip to the marked line, usually two to three centimetres, not to the display housing.
5. Wait for the number to settle before reading it. Meters drift for several seconds and the first figure shown is rarely the final one.

Make three measurements using the same glass and take the middle one. If the readings show more than a ten percent variation, it means the sensor must be cleaned.

What do the numbers actually mean?

Refer to Indian standards instead of referencing a chart based on another country. IS 10500:2012 establishes both a limit and a maximum for all parameters, and the two values differ significantly.

Parameter	Acceptable	Permissible maximum
Total dissolved solids	500 mg/L	2,000 mg/L
Total hardness (as CaCO3)	200 mg/L	600 mg/L
Chloride	250 mg/L	1,000 mg/L
Iron	1.0 mg/L	No relaxation

Those figures come from the IS 10500:2012 drinking water table published by IIT Kanpur, and they are drinking water standards, not bathing standards.

Indian groundwater frequently sits above them. A 2025 survey of a southern Tamil Nadu block recorded total dissolved solids ranging from 96 to 7,334 mg/L across its samples, with hardness above the standard in roughly half.

A reading over 500 ppm is worth acting on. A reading over 1,000 ppm in a borewell-fed building is common in parts of India and tells you the supply is groundwater rather than treated surface water.

The sort of building is as significant as the city. Apartment buildings which replenish from tankers in summer exhibit readings varying by several hundred parts per million at the same time, which is why borewell-fed buildings behave differently from piped ones nevertheless appears in street.

Where do most people go wrong?

Four mistakes, and three of them happen before the probe touches water.

• Testing filtered drinking water instead of the bathroom tap. The two are usually different supplies, and only one of them touches your hair.
• Reading hot water. Conductivity rises with temperature, so a hot tap reads higher than the same water cold and the difference is the thermometer, not the mineral content.
• Assuming TDS equals hardness. It does not, and a water softener salesman quoting your TDS number is quoting the wrong figure.
• Testing once. Supply changes when a building switches from municipal to tanker or borewell, which in most Indian cities happens seasonally.

Twice a year and any time there is a change in supply, test the bathroom cold tap in the same glass.

One more habit worth building. Write the figure and the date somewhere you will find it again, because a single reading has no context and two readings six months apart tell you whether the supply is drifting.

For a cross-check that needs no device at all, the soap lather test still works.

What should you do with a high reading?

Match the response to the number, and be clear about what a reading can and cannot justify.

A high reading is a signal to examine what is in the water, and not sufficient proof of any one mineral. Lab tests identifying amounts of calcium and magnesium, iron and chloride, cost only several hundred rupees and provide answers not achievable by meters.

For hair and skin specifically, the mechanism is well documented. Hard water reacts with soap to form an insoluble residue, and research on that residue found it aggravates skin conditions in patients with atopic dermatitis. A UK Biobank analysis of 306,531 adults found higher odds of eczema where domestic water exceeded 200 mg/L of calcium carbonate, though it recorded no link with new-onset cases.

At the outlet, Care Dale runs three layers, pre-filtration for sediment, CareTec ultrafiltration for hardness, then a chlorine and impurities stage. You can match a filter to your supply type at ₹1,499 for municipal water, or ₹1,899 for the borewell and tanker version.

Now the limit that matters most on this page. The Care Dale filter is not a purification system, so it does not reduce total dissolved solids, nor does it separate out iron or heavy metals. No shower filter, Care Dale included, addresses iron, heavy metals, chloramine, TDS or bacteria.

That is worth saying plainly because this article is about a TDS meter. If you fit a filter and then re-measure TDS expecting the number to drop, it will not, and nothing has gone wrong.

What four weeks usually look like

The first week can be taken as baseline. You take three readings from the bathroom tap and record them along with the date, which is by far more than what most households do.

Weeks two and three are where the pattern starts appearing. If the building switches to tanker supply mid-month, the number shifts, and you can now know the reason why hair feels different some weeks.

At the end of week four it becomes clear if your supply is either stable or unstable. And this information will determine which type of filter you choose for your building.

The realistic outcome is a decision made on data, not a lower number on the meter.

Frequently asked questions

Does a TDS meter need calibration? Most consumer meters ship pre-calibrated and hold that setting for a year or more. If readings drift or disagree across three attempts, calibration solution is inexpensive and the procedure is usually a single screw adjustment against a known reference fluid.

Is low TDS water better? Not automatically. Very low readings indicate demineralised water, which some people find flat to drink, and the figure says nothing about bacteria or organic contamination. Extremely low and extremely high readings both warrant a proper laboratory analysis rather than a purchase.

Can I test water with a multimeter instead? Technically yes, since both devices measure conductivity, but a multimeter reports resistance in ohms and gives you no conversion to parts per million. For a few hundred rupees a dedicated meter removes the arithmetic and the error that comes with it.

Why does my reading change through the day? Because the supply itself changes through the day. Buildings on mixed sources switch between piped, borewell and tanker water depending on availability, and each source carries a different mineral load, so morning and evening readings often differ for exactly that reason rather than any meter fault.

Does boiling water lower the reading? Boiling drops out some of the temporary hardness as scale in the kettle, so a settled sample may read slightly lower afterwards. It does nothing to permanent hardness or to sodium and chloride, and it is not a treatment method for bathing water.

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This article was produced as commissioned content for Care Dale.