Water turbidity - how to measure it and what it means (NTU/FNU)
One downpour is enough to turn a previously clear stream milky or brown. That change, visible to the naked eye, has its number - and people who measure it regularly read the catchment’s story from it: when soil washed off the fields, when a bloom started, when someone stirred up the water upstream. Turbidity is one of the most intuitive and at the same time useful indicators: simple to capture, yet telling.
It is measured by students, monitoring staff, river keepers and citizen-science groups - from a professional turbidimeter to a plain turbidity tube that even children can use in a workshop. It is a measurement that combines simplicity with real value: it shows the water’s response to what is happening in the catchment, often faster than any other parameter.
What turbidity is
Turbidity is the optical “haziness” of water caused by suspended matter - particles of soil, silt, plankton, organic matter. It is not the same as the concentration of suspended solids, but a good, quick indicator of it. The more particles scatter light, the higher the turbidity.
It is given in NTU or FNU units (the infrared-beam method per ISO 7027 gives FNU). High turbidity after rain signals erosion and runoff; persistent turbidity - a bloom or a steady inflow of suspended matter. Turbidity also limits the access of light in the water column, so it matters for life.
How to measure it - two routes
A turbidimeter (nephelometer) - an instrument measuring light scattered at 90°.
- Calibrate the meter with a standard (formazin/standard).
- Take a sample into a clean cuvette, without bubbles; wipe the outside of the wall.
- Mix gently and measure right away - suspended matter settles, so a delay underestimates the result.
A turbidity tube - a field, simple equivalent of the Secchi disk for shallow watercourses: you pour water into a transparent tube with a marker at the bottom and drain the water until the marker becomes visible; you read the height of the column from the scale. Cheap, great for education and citizen science, though less accurate.
What the result means
What says the most is the change relative to the baseline of a given water: you know your watercourse’s typical turbidity, so a sudden jump after rain or a long-persisting haze stands out at once. A single reading is worth less than its trend - and than its link to rainfall or runoff.
The most common mistakes
- Air bubbles in the cuvette - they overestimate the reading.
- A scratched or dirty cuvette - it falsifies the measurement; keep it clean.
- Delay and settling - measure a fresh, gently mixed sample.
- No calibration or strong water colouration (interference) not accounted for.
How often
The value lies in a steady series plus measurements at events (after downpours, during blooms). That is when turbidity carries the most information about the catchment. A fixed site and time - as always - make the difference.
Sources and further reading
The methodology is described in the standard EN ISO 7027 (determination of turbidity) and in GIOŚ materials on water monitoring. Turbidity pairs well with transparency (the Secchi disk) and total suspended solids - together they give a fuller picture of water clarity.
How to make it easier
Turbidity means the most in connection with time and weather - and that relationship is easy to lose when readings scatter across notes and files, detached from the rest of the measurements.
LimnoLog gathers this in one place:
- you record turbidity right in the field - also offline, straight after a downpour;
- the reading goes to a station and a session, so “the same place, the same time” holds on its own;
- a chart emerges from the series - responses to rain and blooms are visible at a glance;
- you can share the results via a link or export them.
And, as always - alongside turbidity, with the same system you record transparency, discharge, oxygen or any other indicator. One coherent picture of the water, not a handful of separate spreadsheets.
← All methodsEarly users get free access for now and keep their features for good. If you watch over your water - take a look and try it.