Used to measure water clarity in more turbid estuarine or freshwater settings, this secchi disc is the 200mm alternating white and black design. Made of 6mm white acrylic with perminant black vinyl stickers, and features a 500g disc weight attached to the underside.
 
The device is held together by a 12cm eye ring bolt with two self locking nuts to prevent it from coming unattached. The bolt and nut assembly are marine grade steel to prevent corrosion.