Читать книгу Standard methods for the examination of water and sewage - Association of Official Agricultural Chemists - Страница 10

This method requires a rod with a platinum wire 1 mm. in diameter inserted in it about 1 inch from one end of the rod and projecting from it at a right angle at least 25 mm. Near the other end of the rod, at a distance of 1.2 meters from the platinum wire, a small ring shall be placed directly above the wire through which, with his eye directly above the ring, the observer shall look when making the examination.

The rod shall be graduated as follows: The graduation mark of 100 shall be placed on the rod at a distance of 100 mm. from the center of the wire. Other graduations shall be made according to Table 1, which is based on the best obtainable data. The distances recorded in Table 1 are intended to be such that when the water is diluted the turbidity readings will decrease in the same proportion as the percentage of the original water in the mixture. These graduations are those on what is known as the U. S. Geological Survey Turbidity Rod of 1902.^[105]

Table 1.—Graduation of turbidity rod.
Turbidity (parts per million).	Vanishing depth of wire (mm.).
7	1095
8	971
9	873
10	794
11	729
12	674
13	627
14	587
15	551
16	520
17	493
18	468
19	446
20	426
22	391
24	361
26	336
28	314
30	296
35	257
40	228
45	205
50	187
55	171
60	158
65	147
70	138
75	130
80	122
85	116
90	110
95	105
100	100
110	93
120	86
130	81
140	76
150	72
160	68.7
180	62.4
200	57.4
250	49.1
300	43.2
350	38.8
400	35.4
500	30.9
600	27.7
800	23.4
1000	20.9
1500	17.1
2000	14.8
3000	12.1

Procedure.—Lower the rod vertically into the water as far as the wire can be seen and read the level of the surface of the water on the graduated scale. This will indicate the turbidity.

The following precautions shall be taken to insure correct results:

Observations shall be made in the open air, preferably in the middle of the day and not in direct sunlight. The wire shall be kept bright and clean. If for any reason observations cannot be made directly under natural conditions a pail or tank may be filled with water and the observation taken in that, but if this is done care shall be taken that the water is thoroughly stirred before the observation is made, and no vessel shall be used for this purpose unless its diameter is at least twice as great as the depth to which the wire is immersed. Waters which have a turbidity greater than 500 shall be diluted with clear water before the observations are made, but if this is done the degree of dilution shall be reported.