SpecificationUpdated 6 November 2026

Specify the optical method before ordering a turbidimeter

Choosing between the PTV 1000 infrared and white-light models affects the instrument order, the historian and how operators compare new readings with the existing record.

By Fowzie Smith-Tahir LinkedIn

A replacement analyser can fit the backboard, connect to the plant system and still raise a question at commissioning. The new display and the old historian tag both show a value called “turbidity”, yet the two instruments used different light sources. The tag alone cannot explain the change. Settling the optical method at specification stage gives everyone a clear basis for interpreting the new measurements.

Find out what the measurement has to support

Begin with the monitoring point and the decision the reading supports. The team replacing an instrument on an established filter outlet may need to preserve continuity with years of plant data. On a new installation, the method may already be specified in the project documents or the reporting procedure. Find the named method and check it against the existing instrument records, rather than relying on the shorthand in a screen label.

The infrared PTV 1000 uses the infrared nephelometric approach described by ISO 7027-1 and displays FNU. The white-light model uses Lovibond’s white-light LED method and displays NTU. Both use scattered light to measure turbidity, but the wavelength affects the response of real water. PMA’s interview with Lovibond instrument developer Mike Sadar explains the two light-source options and the reasoning behind them.

Plan how you will compare the old and new readings

In a handover, it is tempting to compare two numbers and move straight to a conclusion. The optical method is one piece of the comparison. The other is the water that actually reached each instrument. A sample taken at a sink after a delay can behave differently from water that is passing through the online flow body. Particles may settle in a container and bubbles can affect optical readings.

If the site needs a side-by-side comparison, agree the sampling point, timing and reference method before commissioning. Keep the result alongside the method and units used by each instrument. Even where the readings agree closely in a particular set of samples, record the method change. Other samples may contain a different mix of particles or colour and produce a different relationship. Our optical methods guide covers how to approach those comparisons.

Make the historian part of the specification

Confirm the instrument’s local units, the plant signal mapping and the label that operators will see. Check the same items on trends, reports and exports. A commissioning team can test a reading all the way from the instrument display to the historian, then hand over a record of the exact model, optical method and date the tag changed.

For a replacement that uses a different method, agree how to mark the handover in the trend. Keeping the old and new method information visible can prevent a future reviewer from treating the transition as a change in water quality. The PTV 1000 technical documents give engineering teams a starting point for confirming the instrument and plant interface they intend to use.

Resolve any gaps before placing the order

When a specification only says “turbidity in NTU”, ask the person responsible for the measurement which method the site intends to use. A short conversation with operations, instrumentation and the reporting owner can reveal whether the requirement is to preserve a historical trend, match a reference method or meet a separately documented obligation. Capture the answer in the equipment specification and in the commissioning plan.

Once the method is agreed, use the PTV 1000 model comparison to review the infrared and white-light options. Bring the sample point, the intended reporting method and any existing instrument details into that discussion. Those three pieces of information make it much easier to specify a measurement the site can use and explain.

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