Turbidity Measurement Principle

Secchi disc :

  • Secchi is opaque circular disc divided into 4 quadrants, alternate quadrants painted black.

  • Disc is attached to cord & lowered until it disappears.

  • 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.

  • The average of these two readings is taken, corresponding depth is called Secchi dept.

  • 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.


  • Inexpensive

  • Easy and reliable


  • Accuracy depends on eyesight of measure r.

  • Quality of water changes with session

  • Need to take multiple reading to improve accuracy.