A specialized and simplified laboratory form of the fixed-angle scattering meter is the nephelometric turbidimeter. The word 'turbidity' is used in a general sense to indicate the extent to which a liquid lacks clarity, i.e. scatters light as perceived by the human eye. In the most common type of instrument, a beam of light is directed along the axis of a cylindrical glass cell containing the liquid sample under study. Light scattered from the beam within a rather broad angle centred on 90 ° is measured by a photomultiplier located at one side of the cell (Fig. 4.7). The 'turbidity' (Tn) of the sample in nephelometric turbidity units (NTU) is measured relative to that of an artificial standard with reproducible light-scattering properties. The standard can be a suspension of latex particles, or of the polymer formazin, made up in a prescribed manner. Turbidimeters, as at present constituted, do not attempt to provide a direct estimate of any fundamental scattering property of the water and the nephelometric turbidity units are essentially arbitrary in nature. Nevertheless, the turbidity measured in this way should be directly related to the average volume scattering function over an angular range centred on 90 and so for waters of given optical type (e.g. waters with moderate to high turbidity due to inorganic particles) should bear an approximately linear relation to the scattering coefficient. Since turbidimetric measurements are so easily made, and since there already exist a large amount of turbidimetric data on inland water bodies, comparative studies on a
variety of natural waters to determine the empirical relation between nephelometric turbidity and scattering coefficient would be valuable. Some existing indirect measurements (see below) suggest that for turbid waters, by a convenient coincidence, b/Tn is about 1m~lNTU~l. Turbi-dimeters are, however, not well suited for characterizing waters with very low scattering values, such as the clear oceanic types.
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