Photon/Optical Detectors
type: Concept

SECCHI DISKS
type: Concept
change note: 2021-08-05 17:06:46.0 [sritz] insert AltLabel (id: null category: primary text: SECCHI DISKS language code: en);
definition: The Secchi (rhymes with etch-e) disk originated with Fr. Pietro Angelo Secchi, an astrophysicist, who was requested to measure transparency in the Mediterranean Sea by Commander Cialdi, head of the Papal Navy. Secchi was the scientific advisor to the Pope. Secchi used some white disks to measure the clarity of water in the Mediterranean in April of l865. Various sizes of disks have been used since that time, but the most frequently used disk is an 8 inch diameter metal disk painted in alternate black and white quadrants. The Secchi disk is used to measure how deep a person can see into the water. It is lowered into a body of water until the observer loses sight of it. The disk is then raised until it reappears. The depth of the water where the disk vanishes and reappears is the Secchi disk reading. Even though the Secchi disc measurement of water clarity is an approximate evaluation of the transparency of water, it is used primarily for its simplicity. A more accurate measurement of underwater irradiance can be made by the use of photometer. A Secchi Disk measures water clarity. Water clarity may be affected by three different factors -- algae, sediment and / or water color. One of the major reasons why Secchi disc measurements decrease from spring to fall, is due to the increase of plankton-- both phytoplankton and zooplankton. Since zooplankton graze on phytoplankton, (algae i.e.), the Secchi disc readings may increase during the summer by the reduction of algae in the water. Algae blooms may occur when the amount of available nutrients increases faster than the macrophytes (plants rooted in the bottom of a lake)can absorb them. This summary has been adapted from the MSLA page on "THE SECCHI DISK - WHAT IS IT?" Additional information on Secchi Disk available at "http://www.pollardwater.com/emarket/pages/L0921002disks.asp"
in scheme: instruments