What is an FNU?

An FNU is a Formazin Nephelometric Unit. It is a measurement of turbidity commonly used in Europe and is similar to a Nephelometric Turbidity Unit (NTU). The difference is based on the wavelength of light used to make the measurement.

Turbidity is a measurement of the light-absorbing and light-scattering properties of a liquid. A wide variety of methods to measure those properties are available.

Devices that measure turbidity in FNUs use a light-emitting diode (LED) with a wavelength of 860±60 nanometers as a light source, and measure the scattered light at an angle of 90±2.5 degrees to the incident light beam (ISO 7027).
Devices that measure turbidity in NTUs use a tungsten lamp with a color temperature of 2,200 to 3,000 K, and measure the scattered light at an angle of 90±30 degrees to the incident light beam (EPA method 180.1).

In simplified terms, then, NTUs are measured with a white light, while FNUs are measured with an infrared light. Both measure the scattered light at roughly 90 degrees to the incident light beam. Most submersible, multiparameter turbidimeters use an infrared light source.

Due to the fact that suspended particles scatter light of different wavelengths with varying efficiency, FNU data often are not directly comparable to NTU data. The U.S. Geological Survey began making this distinction in the various methods of measuring turbidity on October 1, 2004. For more information, please see Chapter 6.7 of the USGS National Field Manual, available online at http://water.usgs.gov/owq/FieldManual/Chapter6/6.7_contents.html.

Accessibility FOIA Privacy Policies and Notices

U.S. Department of the Interior | U.S. Geological Survey
URL: http://or.water.usgs.gov/grapher/fnu.html
Page Contact Information: USGS Office of Water Quality
Page Last Modified: Thursday - Apr 21, 2005 at 13:22:07 EDT