This International Standard specifies four methods for the determination of turbidity of water. Two semiquantitative methods, employed for example in field work, are specified: a) measurement of turbidity using the transparency testing tube (applicable to pure and lightly polluted water); b) measurement of turbidity using the transparency testing disk (especially applicable to surface water). Two quantitative methods, using optical turbidimeters, are specified: c) 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. d) measurement of the attenuation of a radiant flux, more applicable to highly turbid waters (for example waste or polluted waters). Turbidity measured by this method is expressed in formazin attenuation units (FAU); results typically range between 40 FAU and 4000 FAU.