Start from the decision, not the sensor
It's tempting to buy a multi-parameter probe and measure everything. But every online water sensor is a thing to calibrate, clean and maintain — and a reading nobody acts on is just cost. The right approach is the same as all good monitoring: decide what question you're answering (Is this water safe to drink — a water safety question? Is my discharge causing water pollution? Is my treatment process healthy?), then measure the parameters that answer it. Here's what the common ones actually tell you.
The core parameters
- pH — how acidic or alkaline the water is. Drives corrosion, disinfection effectiveness and process chemistry; a licence limit for most discharges.
- Turbidity — cloudiness from suspended particles. A key potable-water safety and treatment indicator, and a discharge parameter; a turbidity spike often flags a process problem upstream.
- Conductivity / TDS — the level of dissolved salts. Indicates salinity, contamination or ingress, and matters for process water and irrigation suitability.
- Dissolved oxygen (DO) — oxygen available in the water. The heartbeat of biological wastewater treatment (too low and treatment fails) and a measure of river health.
- COD (chemical oxygen demand) — the organic pollutant load. COD in water treatment is a central effluent-compliance and treatment-performance parameter, and a key input to water treatment plant monitoring.
- Residual chlorine — disinfectant left in potable water. Confirms the water stays safe through the network; too little risks pathogens, too much causes taste and by-product issues.
- Temperature — cheap to measure and influential: it affects DO, reaction rates, disinfection and many sensor readings, so it's often measured alongside the rest.
Which to monitor, by water type
| Water | Usually most useful | Why |
|---|---|---|
| Potable / drinking | Turbidity, residual chlorine, pH, conductivity | Safety and disinfection through the network |
| Wastewater process | Dissolved oxygen, pH, flow, level | Keeping the biological process healthy |
| Final effluent / discharge | COD, pH, turbidity, conductivity (per licence) | Proving compliance with your water-use licence |
| Process / industrial | Conductivity, pH, temperature | Protecting equipment and product quality |
The catch: calibration and maintenance
Online water-quality sensors are only as trustworthy as their upkeep. Probes drift, foul and age — an uncalibrated pH or DO sensor can quietly mislead you, which is worse than no sensor. So a real programme budgets for calibration schedules, cleaning (especially in dirty water and wastewater), and sensor replacement, and ideally cross-checks against periodic lab samples. This is why "measure everything" backfires: each added parameter multiplies maintenance. Scope to what drives a decision, and the upkeep stays manageable.