| 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"
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| Abstract from DBPedia | The Secchi disk (or Secchi disc), as created in 1865 by Angelo Secchi, is a plain white, circular disk 30 cm (12 in) in diameter used to measure water transparency or turbidity in bodies of water. The disc is mounted on a pole or line, and lowered slowly down in the water. The depth at which the disk is no longer visible is taken as a measure of the transparency of the water. This measure is known as the Secchi depth and is related to water turbidity. Since its invention, the disk has also been used in a modified, smaller 20 cm (8 in) diameter, black and white design to measure freshwater transparency. Similar disks, with a black-and-yellow pattern, are used as fiducial markers on vehicles in crash tests, crash-test dummies, and other kinetic experiments.透明度板（とうめいどばん）とは、水の透明度や濁度を調べるために利用される、円形の板である。大きく分けて2種類存在し、直径30センチの白い円盤のタイプと、直径20センチの白い円盤を円の中心を通る直交した2本の直線で4等分して隣り合わない任意の2つの区画を黒く塗ったタイプがある。いずれのタイプも円盤に棒か紐を取り付けて、水中へとゆっくりと沈めてゆき、円盤が肉眼で何メートルの深さまで見えるかを確認するという方法で用いられる。イタリア人のアンジェロ・セッキーが考案したことから、セッキー板やセッキ板（イタリア語：Disco di Secchi）とも呼ばれ、また円形であることからセッキー円盤やセッキ円盤とも呼ばれる。 (Source: http://dbpedia.org/resource/Secchi_disk) |
