Secchi disc :
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Secchi is opaque circular disc divided into 4 quadrants, alternate quadrants painted black.
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Disc is attached to cord & lowered until it disappears.
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The disk is lowered a few more cm and then is slowly raised again until it reappears. This second reading of depth is also noted.
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The average of these two readings is taken, corresponding depth is called Secchi dept.
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Higher Secchi dept, more clear water is.
Turbidity tube:
- It is an alternative to Secchi disk. The length of water column makes pattern disappear is required Secchi depth.
Note:A Siemens star is a device used to test the resolution of optical instruments, printers and displays. It consists of a pattern of bright"spokes" on a dark background that radiate from a common center and become wider as they get further from it. It is useful tool for comparing two printers’ resolutions.
Advantage:
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Inexpensive
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Easy and reliable
Disadvantage:
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Accuracy depends on eyesight of measure r.
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Quality of water changes with session
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Need to take multiple reading to improve accuracy.