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It is allowable to substitute for the candle an electric light. Calibrate the apparatus to correspond with the United States Geological Survey scale. The figures in Table 2 on page ssss1 are believed to be approximately correct for the candle turbidimeter but should be checked by the experimenter. It is allowable to calibrate the tube of the instrument with waters of known turbidity prepared by making a series of dilutions of the silica standard with distilled water. From the figures obtained in calibrating plot a curve from which the turbidity of a sample may be read when the depth of water in the tube has been obtained.
Table 2.—Graduation of candle turbidimeter. Depth of liquid (cm.). Turbidity (parts per million of silica). 2.3 1000 2.6 900 2.9 800 3.2 700 3.5 650 3.8 600 4.1 550 4.5 500 4.9 450 5.5 400 5.6 390 5.8 380 5.9 370 6.1 360 6.3 350 6.4 340 6.6 330 6.8 320 7.0 310 7.3 300 7.5 290 7.8 280 8.1 270 8.4 260 8.7 250 9.1 240 9.5 230 9.9 220 10.3 210 10.9 200 11.4 190 12.0 180 12.7 170 13.5 160 14.4 150 15.4 140 16.6 130 18.0 120 19.6 110 21.5 100The results of turbidity observations shall be expressed in whole numbers which correspond to parts per million of silica and recorded as follows: