This 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