Whether you run a packaged drinking water unit, a process RO plant in a pharma or food factory, a hospital supply or a housing society system, two numbers tell you most about how your plant is performing: TDS and pH. Checking them once a shift with a handheld meter is better than nothing, but membranes foul, dosing pumps stop and bypass valves get left open between checks. Online monitoring closes that gap.
What TDS and pH tell you
Total Dissolved Solids (TDS) is the total amount of dissolved salts and minerals, expressed in mg/L (roughly equal to ppm). Online instruments usually measure electrical conductivity (µS/cm) and convert it to TDS using a factor, typically between 0.5 and 0.7 depending on the water. Rising permeate TDS is the earliest sign of membrane damage, O-ring leaks or a failing RO stage.
pH measures how acidic or alkaline the water is on a 0–14 scale. It matters for taste, for corrosion of pipes and equipment, for the effectiveness of disinfection, and for membrane life (antiscalant and acid dosing are often controlled by pH).
The limits: BIS IS 10500 for drinking water
In India, drinking water quality is specified by BIS IS 10500. For the two parameters in this article, it gives an acceptable limit and a permissible limit in the absence of an alternate source:
| Parameter | Acceptable limit | Permissible limit (no alternate source) |
|---|---|---|
| TDS | 500 mg/L | 2,000 mg/L |
| pH | 6.5 – 8.5 | No relaxation |
Packaged drinking water, pharmaceutical purified water and process water have their own, stricter specifications, and many plants set internal targets well below the BIS acceptable limit — for example permeate TDS under 100 mg/L. Very low TDS is not automatically "better" for drinking water either; many plants blend or remineralise. Set your limits according to your application and the standard that applies to you.
Where to measure
- Raw / feed water — TDS and pH, to know what your plant is dealing with and to calculate salt rejection.
- RO permeate — TDS (conductivity), the key indicator of membrane health.
- Product / storage tank — TDS and pH after blending or dosing, which is what users actually get.
- Reject line and flows — flow meters on feed, permeate and reject let you calculate recovery.
With feed and permeate TDS you can compute salt rejection = (1 − permeate TDS ÷ feed TDS) × 100. A steady fall in rejection over weeks is the clearest signal that membranes need cleaning or replacement.
Choosing online sensors
- Output: choose transmitters with Modbus RTU (RS-485) or 4–20 mA output so they can be read by a gateway. Modbus gives you several values (TDS, conductivity, temperature) on one cable.
- Temperature compensation: conductivity changes about 2% per °C. Make sure the sensor compensates to 25 °C.
- Range and cell constant: pick a conductivity cell suited to your range — permeate and raw water may need different cells.
- pH electrodes need periodic calibration with buffer solutions and replacement every year or so; plan for access.
- Installation: mount in a flow cell or pipe section that is always full, with no air bubbles, and with isolation valves for maintenance.
Setting alerts that people trust
Alerts that fire all the time get ignored. A few principles help:
- Use bands, not just a single limit. For example: Excellent, Good, Fair and Poor bands for TDS, with a warning when water moves from Good to Fair and an alarm when it crosses your limit.
- Add a delay. A value must stay out of range for a few minutes before alerting, so start-up spikes do not trigger messages.
- Alert on trends too. Permeate TDS rising steadily over a day matters even if it is still below the limit.
- Send to the right person. The operator gets the first alert; the plant manager gets it if the problem continues.
- Alert when data stops. A sensor or plant that goes offline is a problem in itself.
Reporting compliance
For audits, customers and your own peace of mind, a daily figure is more useful than thousands of readings: compliance % = time within limits ÷ total time. Combine it with daily minimum, average and maximum TDS and pH, and a 30-day trend, and you have a report anyone can understand.
A practical starting setup
- Permeate TDS/conductivity and product-tank pH as the minimum.
- Feed TDS and flow meters when you want rejection and recovery.
- Pump energy meters if you also want cost per kilolitre.
How SensorLoop helps
The SensorLoop water quality dashboard was built for exactly this: live TDS, pH, EC, temperature and flow from Modbus or 4–20 mA sensors through a ModbusSudo gateway, with BIS IS 10500 bands, 24-hour charts with limit lines, daily compliance % and WhatsApp alerts. Each tank or plant gets a clear name — like "Tank 01 TDS" — so operators know immediately where to look.