Turbidity
Turbidity — The reduction in transparency of a liquid caused by suspended material, measured here as the light scattered out of a beam at a defined angle.
- Window coating drift
- 0.4NTU per week
- Source: Illustrative sample value for a demonstration fixture, not a measured field figure.
Why does a laboratory sample disagree with the online reading?
Usually because the two answer different questions: the online value reports the moment the light crossed the window, while the laboratory method reports one grabbed volume after homogenisation. Check the sampling point and the holding time before checking the calibration.
Where the number comes from
An inline turbidity transmitter illuminates the process stream and measures part of the light deflected by what is suspended in it. That is the whole measurement chain, and it explains both what the value is good at and where it stops being a laboratory result.
The reading updates as material passes the window, which is what an operator needs between two grab samples. It is not a mass balance, and it does not distinguish a clay platelet from a droplet of the same scattering strength.
The three things it cannot see
Colour dissolved in the liquid absorbs part of the beam and is read as scattering by a single-beam instrument. Large, fast-settling particles leave the sensing volume before the light reaches them. And a coated window scatters light of its own, which is indistinguishable from a real solids increase until someone wipes it.
None of these are instrument faults. They are the boundary of an optical method, and a selection that ignores the boundary produces an instrument that looks fine on paper and argues with the laboratory online.