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The results of color determinations shall be expressed in whole numbers and recorded as follows:

Color between 1 and 50 recorded to nearest unit 〃 〃 51 〃 100 〃 〃 〃 5 〃 〃 101 〃 250 〃 〃 〃 10 〃 〃 251 〃 500 〃 〃 〃 20.

As the platinum-cobalt standard method is not well adapted for field work, the color of the water to be tested may be compared with that of glass disks held at the end of metallic tubes through which they are viewed by looking toward a white surface. The glass disks are individually calibrated to correspond with colors on the platinum scale. Experience has shown that the glass disks used by the U. S. Geological Survey give results in substantial agreement with those obtained by the platinum determinations, and their use is recognized as a standard procedure.

Inasmuch as the Nessler scale^[62] and the natural water scale^[22][49] which agrees with it except for colors less than 20, have been largely used in the past, the old results may be converted^[117] into terms of the platinum standard by means of the ratios in Table 3, but they must not be considered as universally applicable as the variable sensitiveness of the Nessler solution introduces an uncertain factor.