#WishIknew – How does nephelometric turbidity measurement work?


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I wish I knew how nephelometric turbidity measurement works!

We have a number of ways we can measure turbidity thus there are also different nephelometric turbidity units but later more about that. The old Jackson Candle method, for example, had a person basically eyeballing it. Now we have more advanced techniques, such as radiometrics that use multiple light detectors to improve accuracy.

Most of these methods have two things in common, a light source and a  light detector. The nephelometric turbidity measurement method, widely used in industries such as water and wastewater, has its detector set at a 90-degree angle from the light source. Why 90 degrees? Because it’s the angle considered most sensitive to light scattered by suspended particles, regardless of size.

Courtesy of Fondriest Environmental

To measure a sample’s turbidity, the light source sends a beam through the sample. When this beam hits particles, the detector senses the scattered light. The more light the detector senses, the higher the turbidity. But these measurements are accurate only in the range between 0 and 40 nephelometric turbidity units (NTU). Within this range, turbidity and light scatter have a linear relationship, which disappears above 40 nephelometric turbidity units.

The U.S. Environmental Protection Agency (EPA) Method 180.1 uses nephelometric turbidity units. And the European ISO 7027 uses the Formazin Nephelometric Units (FNU) standard. Both use nephelometric devices to measure a liquid’s turbidity. These methods differ mainly in light source. Method 180.1 uses tungsten lamps with color temperatures between 2000 and 3000 kelvin (K). ISO 1027 uses a light source within the range of 830-890 nanometers, which means near-infrared light.

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