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Turbidity is the most important parameter for determining drinking water quality

This article is about the most important measurement parameter turbidity and the measurement methods in different application areas. Our turbidity experts have developed first-class measuring instruments Measuring devices such as the Turb 430 T and the Turb 750 T that enable you, as a customer, to solve your water quality determination challenges easily and quickly.

Fishing in the mud - reliable and plausible

Hardly any other measurement method is so diverse, has such a great limitation of the measurement method, arouses so many misunderstandings as turbidity. As an important measurement parameter and indicator, turbidity is very helpful for the assessment of water quality and safety in applications, products, production processes and especially in the determination of drinking water! And so turbidity is one of the most exciting parameters of water analysis at all. Learn more about turbidity measurement in drinking water here in this application report!

What is turbidity?

clean water and drinking water measurement

Turbidity is the optical impression created when there are freely moving particles in a solution or in gases that have a different refractive index than their surroundings, i.e. the solutions or gas mixtures. And this already explains why turbidity in aqueous solution is not as clear and easy to measure as, for example, electrochemical parameters: The subjectivity of the perception, particles that are non-uniform in shape, size and color, the mobility and thus the different orientation to the observer have to be captured. The transfer into an optical measuring system with a suitable measuring angle, detector and standard, which can be used as a basis for calibration, had to reflect this mobility and snapshots of constantly moving parts - no easy task!

But with the latest optics and electronics, excellent measurement results with high reproducibility and plausibility can be achieved today, such as with the WTW® measurement by Intelligent Reproducibility and Plausibility Checks (IRPC), in which the fastest multiple measurements are performed and evaluated for the measured value output.

In addition to pure measurement, it has been possible to supplement or replace the formazin standards used to date with high drift susceptibility, especially in the lower range, and a tolerance of at least 10% by modern polymer standards with 1% solder-to-lot production accuracy. Polymer standards are optimized for the respective optics.The measuring methods used are partly standardized or industry-specific with regard to the equipment. In drinking water, a nephelometric measurement is performed with 90° detector arrangement. Depending on the country standard DIN ISO or US EPA, however, the two different light sources infrared and white light with the output units FNU (Formazine Nephelometric Unit) or NTU (Nephelometric Turbidity Unit) are already used for this: However, white light may be inferior or unsuitable for other application areas such as the juice industry with colored solutions due to additional absorption influences during measurement.

This is also where the user comes into play for an accurate measurement result

Every user should always keep in mind what he wants to achieve with the measurement, which of the possible light sources and optical systems are best suited and which results can realistically be expected with the method.

Our turbidity experts around the world have developed measuring instrumentssuch as the Turb 430 T and the Turb 750 T that allow you, the customer, to solve your water quality determination challenges easily and quickly. 

On the one hand, this depends very much on the application

  • If one wants to make a quality check of a product with high turbidity, a result corridor of more than 10% is usually meaningful enough.
  • In drinking water, on the other hand, the focus is on particles as an indicator of the purity of water for consumption without risk from bacteria and viruses. For this reason, a limit value of 1 NTU is specified here by standard.
  • For a high measured value quality, there is a requirement for precise operation, especially in the lower range, with the exclusion of external interfering factors such as inaccurate calibration due to drifted calibration standards, glass inhomogeneities and scratches of the cuvette, sample temperature, etc. In the meantime, even smaller limits for drinking water are sometimes under discussion, but these do not actually seem reasonable in view of the process limits.

To select the appropriate device for the task in terms of optics and light source

1. An IR-Instrument should be used for colored solutions, since in the infrared range the absorption influences of the coloring do not play a role. This is also reflected in industry recommendations such as fruit juice processing.

2. Particle sizes in the low measuring range can be a selection criterion for the light source.
For turbidities above 1100 NTU/FNU, instruments with ratiometry are required to evaluate different scattering angles, but due to manufacturer-specific, non-standardized optical arrangements, they usually do not output comparable values with other instruments, which is irrelevant for a closed system to evaluate production and product quality.

Free turbidity measurement guide from our experts

Turb 750 IR

For optimizing the working method, choosing the right instrument, standards and the pitfalls in measurement, you will find news and interesting facts about turbidity with tips and tricks for a successful measurement for download.

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