This part of ISO 7027 specifies two quantitative methods using optical turbidimeters or nephelometers for the determination of turbidity of water. a) measurement of diffuse radiation, applicable to water of low turbidity (for example drinking water); turbidity measured by this method is expressed in formazin nephelometric units (FNU); results typically range between 0 FNU and 40 FNU. Depending on the instrument design, it may also be applicable to waters of higher turbidity. b) measurement of the attenuation of a radiant flux, more applicable to highly turbid waters (for example waste waters or other cloudy waters); turbidity measured by this method is expressed in formazin attenuation units (NTU); results typically range between 40 NTU and 4 000 NTU. Formazine attenuation units are also expressed as FAU (equivalent to NTU). NOTE The semi quantitative methods using transparency testing tubes and disks will be transferred to a second part of ISO 7027.