Читать книгу Standard methods for the examination of water and sewage - Association of Official Agricultural Chemists - Страница 11

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 (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	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