When to replace an RO membrane: the symptoms, the meter test, and what shortens its life.

The symptoms

Four things happen as a membrane wears, and they usually arrive in this order.

1. The output TDS creeps up. Slowly, over months, with the feed unchanged. This is the first and the most reliable sign, and the only one you can measure.
2. The tank fills slower. Scale and fouling on the membrane surface cut the flow through it. If the sediment cartridge is fresh and the pump sounds normal, the membrane is the bottleneck.
3. The taste changes. Purified water that starts to taste of the feed, salty or chalky, is a membrane letting more through.
4. More water to drain. As the membrane passes less, more of the feed goes to reject. Hard to notice without measuring, but it is there.

None of these on its own means a new membrane. A clogged sediment cartridge causes the second and a tired pump causes the first, and both cost far less than ₹650. Rule them out before you order.

Confirm it with a TDS meter, not a guess

The test is a ratio. Read the feed water and the output, and divide output by feed. On a healthy membrane with the TDS controller shut, the ratio is a few percent; the LUXGEN molded membrane is rated 96 to 98% rejection, so 2 to 4 ppm gets through for every 100 in the feed. Write both readings down on the day a new membrane goes in. That day-one figure is the baseline that every later reading is measured against, and without it the meter cannot tell you much.

Then read the output once a month. When it has roughly doubled from the day-one figure with the feed unchanged, the membrane is done: 30 to 60, or 60 to 120. That is the rule the filter change schedule uses, and the RO output TDS guide goes through the reading in detail, including why summer raises the feed and the output together without anything being wrong. Two cautions: run the purifier for a minute before sampling so you read fresh membrane water, and leave the TDS controller at the same setting for every reading, or the numbers mean nothing. The how to measure TDS guide covers the meter itself.

A sudden jump rather than a creep points somewhere else. A blocked sediment cartridge or a weakening pump drops the pressure, and rejection falls with it. Change the ₹190 cartridge first; if the reading recovers, the membrane was never the problem.

What shortens its life

A membrane is rated for 18 to 24 months in the schedule. What decides which end you reach:

• Chlorine. The membrane is a thin polyamide film, and chlorine attacks it. Municipal supply is chlorinated by design, and the carbon cartridge ahead of the membrane is the only thing stopping it. A spent carbon cartridge lets chlorine through, and the damage is permanent: rejection falls and never comes back.
• Silt. Sand, rust and fine particles that get past a spent or bypassed sediment cartridge settle on the membrane surface and block it. Flow falls first, rejection later.
• Hardness and scale. As the membrane concentrates the feed, calcium comes out of solution on its surface. On feed above 2000 ppm, or on hard water at any TDS, this can finish a membrane in months; the hard water and appliances guide explains, and a softener ahead of the purifier is the fix.
• Low pressure. Rejection depends on pressure. A weak pump or a clogged pre-filter lets more salt through and makes the membrane look older than it is.
• High feed TDS. Every membrane lives shorter on 1500 ppm than on 300. On feed above 2000 ppm the standard membranes are the wrong part; the molded high-TDS membrane, rated to 3000 ppm, is what the high TDS guide points to.

Why the ₹190 cartridge protects the ₹900 membrane

Look at the list above. Three of the five killers come through the pre-filters, and the pre-filters are the cheapest parts on the machine. A sediment cartridge is ₹190 and a carbon cartridge ₹220. Changed on time, together they cost ₹410 a year. Skipped, they cost a membrane a year early, which is ₹650 to ₹900, plus the months of drinking water the membrane was no longer treating properly. That is the whole economics of an RO purifier in one sentence: the cheap parts exist to protect the dear one. The replacement cost guide lays out every part and its price.

Which membrane to buy

Three, all in the 1812 size that fits most domestic purifiers. The 80 GPD at ₹650 for a standard pump and ordinary supply. The 100 GPD at ₹800 where the pump can drive it; on a weak pump it performs worse than the 80 it replaced, which the 80 vs 100 GPD guide explains. The molded high-TDS membrane at ₹900, one-piece with no glued seams, for feed up to 3000 ppm, behind the 140 PSI pump. Fit the new one the same way round as the old one, and write the date and the day-one readings on the housing. That is the membrane's whole maintenance schedule.