Turbidity measurement

What is turbidity?

Turbidity is an optical parameter characterizing the opacity of a fluid. It is generally induced by the presence of suspended particles whose origin is quite variable. Thus, these particles can be organic or mineral matter, colloidal particles, bacteria, algae, etc.

Turbidity can be expressed in NTU, in FNU (=1.5*NTU) or in FAU when it is a water sample.

The turbidity of a water sample is generally between 0 and 400 NTU and between 40 and 4000 FAU.

Clément Duprat, Work-study engineer, Calnesis

What is the point of measuring turbidity?

Particles suspended in a fluid interact with light by reflection, scattering and even absorption. Their presence therefore has an impact on the penetration of visible light into the fluid, and indirectly influences other parameters (color, temperature, etc.) and certain processes (photosynthesis for example). It is therefore an essential quality control parameter, particularly from a health point of view, on a wide variety of samples.

Turbidity is also related to the number of suspended particles, as well as their size and shape. It is therefore an easier alternative to measure continuously in a process than the concentration of suspended solids (SS) itself. It can also be used to control the synthesis and growth of particles of interest in a process.

Turbidity measurement is essential in the quality control of a water sample. Turbid water, ie highly charged with suspended particles, is already an indicator of the existence of problems.

How to measure this parameter?

The Calnesis laboratory performs turbidity measurements using two photometric methods:

• Nephelometry, adapted to the study of weakly turbid (or quasi-clear) solutions. This method consists of measuring the drop in light intensity due to the DIFFUSION of light rays by the particles.
• Turbidimetry, adapted to the study of highly turbid solutions. This method consists of measuring the drop in light intensity caused by the ABSORPTION of light rays by the particles.

The turbidity measurement does not characterize the species of particles present in the analyzed sample, but only the quantity of these particles.