Several forms of turbidimeter or diaphanometer[73] have been suggested
for use. The simplest and most satisfactory form is the candle
turbidimeter.[116] This consists of a graduated glass tube with a flat
polished bottom, enclosed in a metal case. This is supported over an
English standard candle and so arranged that one may look vertically
down through the tube at the flame of the candle. The observation is
made by pouring the sample of water into the tube until the image of the
flame of the candle just disappears from view. Care shall be taken not
to allow soot or moisture to accumulate on the lower side of the glass
bottom of the tube so as to interfere with the accuracy of the
observations. The graduations on the tube correspond to turbidities
produced in distilled water by certain numbers of parts per million of
silica standard. In order to insure uniform results it is necessary to
have the distance between the top rim of the candle and the bottom of
the tube constant, and this distance shall be 7.6 cm. or 3 inches. The
observations shall be made in a darkened room or with a black cloth over
the head.
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 8 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 │ Turbidity
(cm.). │ (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│ 100
───────────────────────────────────┴───────────────────────────────────
The results of turbidity observations shall be expressed in whole
numbers which correspond to parts per million of silica and recorded as
follows:
Turbidity between │ 1│and│ 50│recorded to nearest│unit
" " │ 51│ " │ 100│ " " " │ 5
" " │ 101│ " │ 500│ " " " │ 10
" " │ 501│ " │ 1000│ " " " │ 50
" " │1001│ " │greater│ " " " │ 100