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See Caroline for an example of turbidity tube that uses a Secchi disk. This method will be highly dependent on ambient lighting.
See The Turbidity Tube:Simple and Accurate Measurement of Turbidity in the Field for some previous research in this area.
Waterproof LEDs are availabe at low cost.
There are two operating modes for the turbidity tubes. Either they can be slowly filled until the light (or secchi disk) disappears or the tube is filled and then the light source can be submerged until it disappears. This later technique is simpler and makes it possible to repeat the measurement easily.