Precise Turbidity Readings by Electrical Methods

Abstract

The machine is so designed as to read all turbidities of the raw influent water into the plant. One is able then with only a glance at the pointer to tell the exact condition of the raw water. The machine depends upon its action by the variation of light on a photo-electric cell. This cell is so made that the resistance will vary in direct proportion as the square of the power of light thrown on it. The variant light is made by passing light through a column of water. As the turbidity changes the light on the cell will vary accordingly. The machine consists of only four parts, an ordinary electric light, water tube, photo-electric cell and an extremely sensitive galvanometer. Both ends of the water tube are sealed with a convex lens, both focusing in the same direction, that is, the lower one focuses into the water tube and the upper one on to the plate of the photo-electric cell. Water is made to circulate in at the bottom of the tube, through the tube and out at the top, keeping it entirely full, due to the rise of the effluent. At the bottom of the tube, under the lower lens is placed the electric light. At the top is placed the photo-electric cell in a light-tight compartment, and so arranged that the light from the tube will focus on the plate of the cell. It will be seen then with this arrangement that, as the water gets slightly turbid, the intensity of the light on the cell will vary. This varies the resistance in the cell, and this variation is recorded directly on the galvanometer, which scale is calibrated to be read in turbidities. Some idea of just how sensitive the cell is can be appreciated when it is found that with a change of 0.16 foot candle of light there is a change of 60 micro-amperes, or a deflection of fifteen on the scale reading. These cells are made so that there is very little lag in response and the resistance is constantly the same with the same variation in light.