Where to fit pressure gauges — the cheapest diagnostics you can buy
Short answer: Fit gauges before and after every filter and across the membrane housing. The difference between two gauges tells you what is choking, what is worn and what is working — for about ₹315 each.
A plant with gauges can be diagnosed in fifteen minutes. Without them, the same fault takes hours of guesswork and often ends with a part being replaced unnecessarily.
Where they belong
| Position | What it tells you |
|---|---|
| Feed pump outlet | Pump is delivering, or is worn |
| Before and after cartridge filter | When to change the cartridge — the most useful pair |
| Before and after media filter | When to backwash |
| Before and after softener | Bed compaction or channelling |
| HP pump outlet (membrane feed) | Operating pressure |
| Membrane reject line | Pressure drop across the elements |
Reading pressure drop
Pressure drop (ΔP) is inlet minus outlet:
| ΔP across cartridge | Meaning |
|---|---|
| 0.2 – 0.3 kg/cm² | New filter, normal |
| 0.5 kg/cm² | Roughly half life |
| 0.8 kg/cm² | Prepare to change |
| Above 1.0 kg/cm² | Change now |
The simple rule: change when ΔP reaches double the new-filter reading.
Reading the membrane
Across the membrane housing, ΔP tells you about fouling:
- ΔP rising with flow falling → fouling or scaling, time to clean
- ΔP normal but permeate TDS rising → membrane degraded, not fouled
- ΔP rising sharply → heavy fouling, clean before it becomes irreversible
See our membrane monitoring guide.
Choosing the range
The gauge should read your normal pressure at roughly the middle of its scale. Too large a range and you cannot see small changes; too small and the gauge will be damaged.
| Application | Normal pressure | Gauge range |
|---|---|---|
| Feed line, pre-treatment | 2 – 4 kg/cm² | 0 – 7 kg/cm² |
| Cartridge filter | 2 – 4 kg/cm² | 0 – 7 kg/cm² |
| RO membrane feed | 10 – 15 kg/cm² | 0 – 25 kg/cm² |
| Softener or media filter | 2 – 4 kg/cm² | 0 – 7 kg/cm² |
Glycerine filled gauges
Where a pump causes vibration or pressure pulsation, the needle flickers constantly and the mechanism wears. A glycerine filled gauge damps this:
- The needle is steady and readable
- The mechanism lasts far longer
- Slightly more expensive, but worth it near pumps
Use glycerine filled at the pump outlet and on dosing lines. A dry gauge is fine on quiet parts of the line.
Fitting them
- Fit an isolation cock under each gauge, so it can be replaced without draining the line
- Mount at eye level where possible — a gauge nobody can read gets ignored
- Use PTFE tape, four to six turns, and do not over-tighten
- On a dosing line, fit a diaphragm seal if the chemical would attack the gauge
Record the readings
Gauges only help if somebody looks at them. Keep a simple weekly record:
| Date | Cart in | Cart out | ΔP | Membrane feed | Note |
|---|---|---|---|---|---|
| 01/09 | 3.5 | 3.2 | 0.3 | 12 | New cartridge |
| 15/09 | 3.5 | 3.0 | 0.5 | 12 | Normal |
| 01/10 | 3.5 | 2.6 | 0.9 | 12.5 | Change cartridge soon |
This record shows you the trend, so cartridge changes and cleanings can be planned rather than forced.
What gauges tell you that nothing else does
Consider a plant with reduced output. Without gauges you might replace the membrane, the pump, or both. With gauges:
- ΔP high across the cartridge → it is just a choked filter, ₹105
- Pump outlet pressure low → the pump is worn
- Both normal, membrane ΔP high → the membrane needs cleaning
Full diagnosis is in our RO troubleshooting guide.
What to order
See our pressure gauges range. Tell us:
- Where it will be fitted
- Normal operating pressure
- Connection size and bottom or back entry
- Whether the line has vibration (for glycerine filled)
Order pressure gauges
Tell us where they will be fitted and your operating pressure, and we will supply the right range and type.
💬 WhatsApp par poochhein 📞 9910646957Frequently asked questions
Where should pressure gauges be fitted on an RO plant?
Before and after every filter, and across the membrane housing. If you only fit one pair, put it across the cartridge filter, since that single measurement prevents more problems than any other instrument.
What pressure drop means the cartridge needs changing?
About zero point two to zero point three kg per cm2 on a new filter, rising to above one kg when it needs changing. The simple rule is to change when the drop reaches double the new-filter reading.
What gauge range should I choose?
One where your normal pressure sits around the middle of the scale. Too large a range and you cannot see small changes, too small and the gauge will be damaged. Pre-treatment usually uses zero to seven kg, membrane feed zero to twenty five.
When is a glycerine filled gauge worth using?
Where a pump causes vibration or pressure pulsation. The glycerine damps the needle so it is readable and the mechanism lasts much longer. Use them at pump outlets and on dosing lines.
Why fit an isolation cock under each gauge?
So the gauge can be replaced or checked without draining the line. It is a small addition that makes maintenance considerably easier.