Measurement of Turbidity in Water

Turbidity is a measure of cloudiness in liquids, has been recognized as a simple and basic indicator of water quality.

Turbidity measurement involves the use of a light beam, with defined characteristics, to determine the semi-quantitative presence of particulate material present in the water or other fluid sample. The light beam is referred to as the incident light beam. Material present in the water causes the incident light beam to scatter and this scattered light is detected. The higher the quantity of the particulate material contained in a sample, the greater the scattering of the incident light beam and the higher the resulting turbidity.


Following are challenges faced in Turbidity measurement

Bubbles

Bubbles are the bane of turbidity measurement and hence need to eliminate bubbles from entering the measurement chamber.

Sample cell

Glass cells are prone to scratching and fouling. Added to this is the issue of condensation which needs temperature controlled light source to prevent condensation on the optics.

Stray light

Stray light is another problem which isn’t associated with the scattering by the particles in the sample. The stray light needs to be diverted away from detectors thereby preventing false signal being emitted.

Contamination

Wetted parts to be designed in a way to minimise sediment build up thereby preventing contamination issues.



Instrument optical aspects


Turbidity calibration is based on instrument response to formazin which is polymer and has relatively consistent light scattering properties but is difficult to handle and is not stable in dilute solution. Given that drinking water works are working below 1NTU then the solutions you would use to calibrate would have to be made up fresh before use.


Sample colour/coloured absorbing particles


Light absorption by colour affect measurements in instruments using a light wavelength below 800nm.


Particle Size / density


instruments handling of the different scattering properties of particles of different size and density is also an issue as it has to be decided whether to detect smaller particles by using a shorter wavelength (and hence run the risk of colour interference) or use the longer wavelength



 

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