How to measure TDS properly — and why your neighbour's meter reads differently.

What the meter is really measuring

A TDS meter is a conductivity meter with a conversion built in. Two metal electrodes at the tip sit in the water, the meter passes a small alternating current between them, and the ease with which the current flows depends on how many charged particles — dissolved salts — are in the way. The meter multiplies that conductivity by a factor to give a figure in parts per million. The factor is chosen at the factory to suit a typical mix of salts, which is why two meters from different makers can read the same glass of water a little differently. Neither is wrong; they are guessing the mix differently.

Conductivity also changes with temperature, so a decent meter includes automatic temperature compensation and reports what the water would read at a standard temperature. A meter without it will read a warm sample higher than a cool one. The other thing to know is what the meter is not doing: it is not identifying anything. It counts charge, and everything that carries charge counts the same. The guide on what TDS cannot tell you covers what that leaves out.

The method

1. Take the cap off and switch on. Let the display settle at zero in air.
2. Rinse the electrodes in the sample you are about to test, not in tap water from elsewhere. Then take a fresh sample.
3. Submerge to the fill line marked on the pen, no deeper. Water above the line can reach the electronics.
4. Stir gently to shake off any air bubbles clinging to the electrodes. A bubble on an electrode reads low.
5. Wait for the number to stop moving. Ten to twenty seconds is usual. Read it, or press hold if the meter has it.
6. Rinse in clean water, shake dry, cap it. Dried salt on the electrodes is the main reason old meters drift.

Write the reading down with the date, the tap it came from and the season. A number without those three is not much use a month later.

Where to test, and why it matters

Test the water you drink, at the point you drink it. That sounds obvious, but most wrong readings come from testing the wrong tap. Water changes on the way through a house. The bore outlet, the overhead tank, the kitchen tap and the purifier output can all read differently, because the tank collects dust and the tank walls give up a little mineral, because a kitchen line may be fed from a different source than the bathroom, and because the purifier changes the water on purpose.

For a purifier decision, test the feed to the purifier: the tap the inlet pipe comes from. For a membrane check, test that feed and the purifier output on the same day and compare them. The RO output guide explains what the ratio between them should be. For a general sense of the supply, test the kitchen tap. If you have both municipal and borewell lines, test each separately and label them, because a mixed reading will change every time the mix does.

Calibration

Pen meters drift. The usual causes are salt drying on the electrodes, a tired battery, and simple age. Calibration corrects the drift against a reference solution of known conductivity, sold in small sachets or bottles with the value printed on the label. Dip the meter in it, wait for the reading to settle, and adjust the meter — a screw on older pens, a button sequence on newer ones — until the display matches the label. Then rinse thoroughly, because the reference solution is far saltier than most drinking water and a trace of it will inflate the next reading.

How often depends on use. A meter used monthly on one supply can go a long time between calibrations; check it against a known sample if a reading looks surprising. A meter that has been left with wet electrodes, dropped, or stored in a hot car should be checked before it is trusted. If the meter has no way to calibrate, and the cheapest ones do not, a fresh one is the calibration.

Why readings differ between taps and seasons

Between taps: different sources, different storage. A ground-floor tap on the mains and a rooftop tank filled from the bore are not the same water. A tank that receives both will read somewhere between, and the number will move with the ratio.

Between seasons: the source itself changes. Borewell water reads highest at the end of the dry season, when the water table has dropped and the pump is drawing from deeper, more mineralised water, and lowest a few weeks after the monsoon has recharged the aquifer. Municipal supplies move less, but a utility that switches between surface and groundwater sources will show it on the meter. The TDS reference page explains why the summer number is the one to size a purifier for.

Between meters: the conversion factor and temperature compensation discussed above. If you and a neighbour disagree by a small margin on the same water, it is almost certainly the meters, not the water. If you disagree by half, one of you has a bubble on the electrode or a meter that needs calibrating.

What makes a good meter

Any pen meter will answer the basic question of which TDS band you are in. If you plan to track a membrane over time, three features earn their small extra cost: automatic temperature compensation, so morning and afternoon readings compare; a hold button, so you can read the number after lifting the pen out; and a way to calibrate, so the meter can be trusted next year. A protective cap that also serves as the fill-line guide is a small convenience. A built-in thermometer is nice but not needed. Brand matters less than the habit of rinsing and capping.