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 │ Vanishing depth of wire (mm.).
(parts per million). │
│
───────────────────────────────────┼───────────────────────────────────
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.