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How Is Turbidity Measured?
Apr 22, 2017

The most common measurement for turbidity in the United States are the Nephelometric Turbidity Units (NTU).

There are several ways you can check turbidity in water, the most direct being a measure of attenuation, or reduction in strength, of a light source as it passes through a water sample. An older system was called the Jackson Candle method, with units expressed as JTU or Jackson Turbidity Units. It used a candle flame viewed through a clear column filled with water. The length of water that the candle could be seen through related to the turbidity in the water sample. With the advent of electronic meter technology this method is no longer used.

The particles suspended in the water will scatter a light beam focused on them. The scattered light is then measured at various angles from the incident light path. This is now accepted as a more precise measure of turbidity. To measure turbidity this way use a nephelometer, such as the LaMotte 2020we. Nephele is the Greek word for "cloud"; metric means “measure."Nephelometric, therefore, means "measuring cloudiness." Most nephelometers measure the scattered light at 90°. If more light is able to reach the detector it means there are many small particles scattering the source beam, less light reaching the detector means fewer particles. Nephelometric Turbidity Units (NTU) are the units of measurement used by a nephelometer meeting EPA design criteria. The amount of light scattered is influenced by many aspects of the particles like color, shape, and reflectivity. Because of this, and the fact heavier particles may settle quickly and may not contribute to the turbidity reading, the relationship between turbidity and total suspended solids (TSS) can change depending on the location that the test sample was collected.

Measuring turbidity in environmental applications, such as the oceans, rivers and lakes, a Secchi disk can be used. This is a black and white disk that is lowered into the water until it can no longer be seen.  At that depth (called Secchi depth) the correlating number is recorded as a measure of the clarity in the water. The advantage in using this device in open waters is the ability to measure turbidity at various depths where multiple turbidity layers are present. This device is also easy to use and relatively inexpensive.