ISO 7027-1:2016 specifies two quantitative methods using optical turbidimeters or nephelometers for the determination of turbidity of water:
a) nephelometry, procedure for measurement of diffuse radiation, applicable to water of low turbidity (for example drinking water);
b) turbidimetry, procedure for measurement of the attenuation of a radiant flux, more applicable to highly turbid waters (for example waste waters or other cloudy waters).
Turbidities measured according to the first method are presented as nephelometric turbidity units (NTU). The results typically range between <0,05 NTU and 400 NTU. Depending on the instrument design, it can also be applicable to waters of higher turbidity. There is numerical equivalence of the units NTU and formazin nephelometric unit (FNU).
Turbidity measured by the second method is expressed in formazin attenuation units (FAU), results typically range between 40 FAU and 4 000 FAU.