An online analyser can only report on the sample it receives. That sounds obvious, but sample-point decisions are often made late in a project, after a measurement objective and an instrument have already been discussed. The result can be a well-installed analyser answering a different question from the one the operations team actually needs answered.
For UK water teams, the distinction is increasingly important. The Drinking Water Inspectorate’s 2025 learning from chemical failures stresses the need to use monitoring data to identify deteriorating trends and manage source, treatment and network risks. The Environment Agency’s wastewater guidance separately defines permit sampling and laboratory arrangements. Both are useful context, but neither creates a universal rule for an online sample point. The correct location is the one that represents the operational decision, the relevant treatment stage and the water conditions the team needs to understand.
This guide is about choosing that location. It does not replace a site risk assessment, permit condition, sampling plan or laboratory method.
Start with the decision, not the pipe
Before walking a site, write one sentence that completes: “We need this online result so that we can…” The ending might be to spot a source-water change, confirm whether a treatment stage is responding as expected, investigate a trend, protect a downstream process or decide when to take a confirmatory sample.
Those are different decisions. A raw-water take-off may be useful for seeing incoming variation; a post-treatment point may be useful for observing a treatment response; a final-water or discharge point may serve another operational purpose. Selecting a point because it is close to power and drainage can be sensible only after it has passed the more important test: does its water represent the decision?
For an online colourimetric application, the supplied 3S-CL brochure describes a configured reagent-based analysis sequence and lists potable, surface, process, boiler, cooling and wastewater among the sample types. It does not make every sample point automatically suitable. The parameter, water matrix, expected range and sample presentation still need an application review.
Map the water journey through the relevant process
Draw the treatment or process route on one page. Mark the candidate take-offs, returns, blend points, dosing points, filters, storage and final outlet. Then ask four practical questions at each candidate point.
Is this water representative?
Consider whether the point is affected by dead legs, intermittent flow, stratification, a local return, a dosing injection or an upstream change that has not mixed fully. “Representative” does not mean identical to every other sample on site; it means representative of the defined monitoring objective.
For comparison with laboratory work, write down where and when the laboratory sample is collected, how it is prepared and what result it reports. The recently published 3S article Online Water Analyser vs Laboratory Results explains why unlike sampling windows, methods and units can produce a misleading comparison.
What treatment stage does it describe?
State whether the sample is raw water, a treatment-stage outlet, a blend, a process return, boiler feed, cooling water, final water or effluent. A label such as “filter outlet” is not enough on its own if there are parallel trains, recycle flows or chemical additions nearby.
This is especially useful during handover. A future operator should be able to understand what a trend means without relying on the installer’s memory.
Can the sample be delivered consistently?
The 3S-CL brochure specifies that the analyser receives its sample through a pressure-free reservoir with overflow to drain, and gives the platform sample-flow and temperature conditions. Those values are installation information, not proof that an upstream process line is ready to connect. The application review should establish whether a suitable take-off, flow, pressure reduction or reservoir arrangement is needed.
What could change before analysis?
Temperature, pressure, suspended solids, colour, air entrainment and the time between take-off and analysis can all matter. Document likely operating extremes, not merely normal conditions. This creates a better brief for the project team and avoids treating a sample-system question as an analyser fault later.
Treat sample conditioning as part of the measurement design
If solids or floating material could affect the sample supplied to the analyser, discuss the sample-conditioning arrangement as part of the measurement design. The supplied 3S filtration-unit data sheet describes a system intended to filter suspended solids and provide a cleaner sample to a connected 3S analyser, with programmable automated backflush. It offers a range of filter meshes; that range is not a recommendation to choose a mesh without understanding the process.
The useful questions are: what material is present, what needs to remain in the reported sample, what must be prevented from reaching the analyser, how fast does the sample change and how will the unit be checked and maintained? The separate sample-conditioning resource is the right supporting read for that discussion.
Keep operational trends separate from statutory evidence
An online result can be valuable operational information. It can help a team notice change, investigate a treatment response or decide that a confirmatory sample is needed. It should not be described as a substitute for permit-defined sampling, statutory monitoring or accredited laboratory analysis.
The Environment Agency explains that wastewater monitoring requirements, parameters, locations and reporting are set through the relevant framework and permit. In drinking water, the DWI expects companies to manage risks through their own risk-assessment and control arrangements. The practical rule is simple: define in writing which job the online result is intended to do, and keep it distinct from any regulated evidence route unless that route has been explicitly approved.
A short sample-point brief for an application review
Bring these details to a discussion with PMA:
- the parameter and reporting basis required;
- the operational decision the result will support;
- a process drawing with proposed take-off and drain locations;
- normal and upset sample conditions, including solids, colour, temperature, pressure and flow;
- expected range and how quickly the value may change;
- any existing manual or laboratory comparison method;
- available utilities, drainage, access and maintenance constraints; and
- the proposed response when an unexpected trend appears.
That brief does not lock a project into a configuration. It lets PMA assess whether a 3S application, sample reservoir, filtration or another sample arrangement is appropriate.
Contact PMA about an online monitoring sample point
If you are planning an online monitoring installation, send the proposed sample location with the treatment objective and operating conditions. PMA can review the configured 3S online colorimetric analyser application alongside the analyser-selection guide. Contact PMA to start the discussion.
Frequently asked questions
Is the final outlet always the best sample point?
No. It may be appropriate for a final-water or effluent operational question, but a raw-water, intermediate or blend point may be more useful for a different decision. Choose the location against the defined purpose.
Can an online analyser replace compliance sampling?
Not by assumption. Compliance methods, sample locations and reporting requirements are site- and permit-specific. Online data should be treated as operational information unless the required approval route says otherwise.
When should filtration be considered?
Consider it when suspended solids or floating material could make sample presentation unsuitable. The right arrangement depends on the parameter, matrix and monitoring objective.