The New Atlanta Turbidimeter

Abstract

is a measure of suspended matter which obstructs the passage of light through a medium. This may be due to silt, clay, suspended minerals, organic matter, microorganisms and the like. It is expressed in terms of the turbidity produced by a given weight of silica. The comparison of turbidities is considered from the viewpoint of a unit based upon appearance, rather than quantity. Equal weights of suspended matter do not necessarily constitute the same turbidities, as in the case of silt and sand, which produce a less turbidity than finely divided clay particles. Therefore, the ratio between silica turbidity determined optically, and suspended matter determined gravimetrically, is an important index of the character of suspended matter. This is borne out by the fact that the Turbidity Coefficient equals suspended matter divided by the silica turbidity. The colloidal material that we will refer to from time to time means matter that is so finely divided that it does not settle out rapidly. The Standard Unit in this discussion will refer to that used by the United States Geological Survey, namely: A water which contains one hundred milligrams of silica per liter in such a state of fineness that a bright platinum wire, one millimeter in diameter, can just be seen when the center of the wire is 100 millimeters below the surface of the water and the eye of the observer is 1.2 meters above the wire. The turbidity of such a water is arbitrarily fixed at 100 parts per million. The Platinum Wire and Jackson Candle Turbidimeters give satisfactory results on most raw waters, but neither give entirely accurate results on applied and filtered waters since modern practice demands a water containing a minimum turbidity. Well operated filters having delivered to them a properly coagulated and settled water should remove all of the readily visible turbidity from the applied water. Accepted practice recommends that