Why this measurement is difficult to get right
Water quality monitoring has two distinct modes that are frequently confused. Online analysers run continuously on a process stream and are judged on drift and maintenance interval, because the value has to be trustworthy between visits. Laboratory measurements are made on discrete samples under controlled conditions and are judged on accuracy and documentation. The same parameter, measured two ways, is two different instruments with two different service lives.
Where a discharge consent or an internal specification applies, the parameter and the reporting threshold are already fixed by somebody else. That turns the instrument into a compliance component rather than a free technical choice: the question stops being which measurement is best and becomes which instrument can hold the required performance unattended until the next maintenance window.
The third factor is what the sample does to the sensor. Fouling, scaling, biological growth and reagent consumption all set how often somebody has to attend to the instrument, and that interval determines the real running cost far more than the purchase price does. Access for cleaning is therefore a design decision, not an afterthought.
What has to be measured
- pH and ORP for reaction, neutralisation and corrosion control.
- Conductivity and total dissolved solids for demineralisation and rinse quality.
- Turbidity and suspended solids for filtration performance and final effluent.
- Dissolved oxygen and residual chlorine for biological treatment and disinfection.
- Temperature, because pH and conductivity both depend on it and compensation has to be applied somewhere.
Which instrument measures what
| Measurement | What the reading has to establish | Instruments commonly specified |
| Continuous pH, ORP and conductivity | That a process is being held inside its control band between maintenance visits, which is a drift requirement rather than a bench accuracy one | Process analysis instruments |
| Discrete sample measurement in controlled conditions | An accurate figure for a sample that can be prepared, documented and repeated - suitable for compliance records rather than control | Laboratory instruments |
| Turbidity and suspended solids online | Whether filtration is performing, which depends on stability at low values rather than on range | Turbidity analysers |
| Disinfection residual | That the required residual is present at the point of measurement, where it decays quickly and cannot be sampled and carried away | Chlorine analysers |
| Sample composition and elemental content | What the other measurements can only indicate indirectly, and often the parameter a consent actually limits | Analytical instruments |
A consent or specification usually names the parameter and the threshold, which fixes the measurement before the instrument is chosen.
Narrowing the choice for a water quality programme
- Establish whether the value is used to control a process or to report compliance, because that decides between an online analyser and a laboratory meter.
- Find the consent, specification or internal standard that names the parameter and the threshold, if one exists.
- Confirm the measurement range needed around the threshold, since a wide range often costs resolution where it matters.
- Establish the sample condition - temperature, pressure, solids, bubbles - and whether conditioning is needed upstream.
- Decide how often somebody can reasonably attend the instrument, because that sets the drift and cleaning requirement.
- Check that the sensor can be reached and removed for cleaning without shutting the process down.
- Confirm how temperature compensation is applied, since pH and conductivity both depend on it.
- Decide how the reading is recorded and how long it has to be kept, which follows from whether it is evidence.
Where this application ends and the next begins
| Adjacent application | What separates it from water quality monitoring |
| Water flow measurement | That work reports how much water passed, while this one reports what was in it, and the two are recorded by entirely different instrument families. |
| Tank & silo level monitoring | Level gives a quantity in a vessel; it says nothing about composition, and it is judged on headspace conditions rather than on sample condition. |
| Product quality inspection | Inspection judges a manufactured article, whereas this work judges a stream whose composition changes with time and with upstream events. |
| Energy efficiency testing | That work accumulates consumption; water quality is a concentration, and it is measured continuously for compliance rather than summed. |
| Industrial test bench & calibration | A calibration bench verifies the analyser itself, which is a separate activity from using it to monitor a process. |
What to send with the enquiry
- The parameters to be measured and the threshold for each.
- Whether the result controls a process or reports compliance.
- The source - process water, effluent, potable or a treatment stage.
- Expected range, and the condition of the sample at the measuring point.
- Temperature and pressure at the point of measurement.
- Whether sample conditioning is available or has to be included.
- How often the instrument can be attended for cleaning and calibration.
- Whether readings must be logged, and for how long they must be kept.
- Quantity required and the date it has to be in use.
- Whether the customer expects calibration support or consumables as part of the supply.
