First-hand evidence
Long-run diaries
What happens when a purifier runs in a normal occupied Indian home for thirty days, or six months — logged daily against a published protocol, and reported including the days nobody switched it on.
Important
Reviews in this category are written in the first week of ownership, which is the least informative week there is. Everything works, the filter is new, the novelty has not worn off, and nobody has yet discovered what the machine sounds like at 2 a.m. or what the replacement cartridge costs.
A long-run diary is built to capture the rest. Thirty days for an air purifier through a real pollution period; six months for an RO system on a real Indian supply. Logged daily, with the same instrument, in a home where people cook, open doors and occasionally switch things off.
That last part matters and is usually edited out of published testing. A unit that is switched off at night because it is too loud delivers nothing during the hours it was bought for, and that fact belongs in the results rather than in a footnote about user behaviour.
Air: the 30-day protocol
Run in a normal occupied bedroom through a real pollution period, at whatever speed the household actually tolerates.
| What is logged | How | How often |
|---|---|---|
| Indoor PM2.5 | Laser particle counter, fixed position, away from windows and cooking | Continuous, 1-minute intervals |
| Outdoor PM2.5 | Nearest CPCB monitoring station reading | Hourly, for the indoor/outdoor ratio |
| Fan speed and mode | Recorded manually, including any change and why | On every change |
| Operating hours | Unit's own counter, plus a plug-in energy meter's runtime | Daily |
| Electricity consumed | Plug-in energy meter, cumulative kWh | Daily |
| Filter indicator state | Photographed, so the unit's claim is on record against the measurements | Weekly |
| Pre-filter condition | Photographed before and after each cleaning | On every clean |
| Noise at the running speed | Sound level meter at 1 m, with background stated | Weekly |
| Household observations | Written note — smell, draught, whether anyone switched it off and why | Daily |
Water: the 6-month protocol
Run on a real Indian supply, with feed and product TDS logged weekly and anchored to a laboratory result quarterly.
| What is logged | How | How often |
|---|---|---|
| Feed TDS | Same calibrated handheld meter, same tap, same time of day | Weekly |
| Product TDS | Same meter, after a 30-second flush | Weekly |
| Rejection rate | Derived: (feed − product) ÷ feed | Weekly, computed |
| Water temperature | At every TDS reading, because conductivity rises ~2% per °C | Weekly |
| Product and reject volume | Measured over one full tank-fill cycle | Monthly |
| Litres consumed | Household estimate, recorded honestly including gaps | Weekly |
| Service events | Part, date, who fitted it, and whether OEM or compatible | As they happen |
| Laboratory cross-check | Gravimetric TDS on a product sample, anchoring the meter series | Quarterly |
| Faults and oddities | Written note, including anything that resolved itself | As they happen |
Rejection rate is the figure this protocol exists to capture. It falls silently on every domestic RO system — output volume stays normal while treated-water TDS climbs — and nothing on the unit reports it. A weekly reading with the same meter is the only way to see it happen.
What a single diary cannot establish
- It is not a comparison. One unit in one home tells you how that unit behaved there. Ranking two units would require running both simultaneously in comparable conditions.
- It is not a reliability statistic. One machine surviving six months says nothing useful about failure rates, and one failing says nothing about a brand.
- The environment is not controlled. That is the point of the exercise and it is also its limitation — a Delhi winter and a Bengaluru monsoon produce completely different filter loading from the same unit.
- Consumer instruments are indicative. Optical particle counters and handheld TDS meters are repeatable and internally comparable, not reference-grade. The quarterly laboratory cross-check exists for exactly that reason.
Frequently asked questions
Why run a purifier for months instead of doing a chamber test?
What can a single long-run diary establish?
Do you accept units from manufacturers for long-run testing?
What is the most useful thing a long-run test reveals?
Any unit tested here is bought at retail. If that ever changes it will be stated in the first paragraph of the diary rather than in a footnote, and no manufacturer sees a result before publication. Last reviewed 2026-08-08.