Secchi turbidity disk.
https://diqn32j8nouaz.cloudfront.net...y_fullsize.jpg
https://diqn32j8nouaz.cloudfront.net...1_fullsize.jpg
Previously:
https://www.homemadetools.net/forum/...271#post184969
https://www.homemadetools.net/forum/...973#post198836
https://www.homemadetools.net/forum/...073#post149447
https://www.homemadetools.net/forum/...692#post212831
https://www.homemadetools.net/forum/...393#post135340
