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The oceanographic secchi disk is, by definition, white on one side, black on the other. Lower it into shallow water until the white can no longer be seen and record its depth. Reverse the disk and lower it again, recording the depth for the black side. White side can be used with Forel-Ule Color Comparator Kit.
|58-A25||Oceanographic Secchi Disks, 200mm, Acrylic|