Turbidity is the measure of the clarity of water. This haziness or cloudiness of water is caused by a large number of usually very small suspended particles.
In drinking water high turbidity can increase the risk of gastrointestinal diseases because viruses and bacteria can attach to the particles in turbid water. High turbidity will reduce the effectiveness of disinfectants like chlorine so turbidity should be lowered as much as possible before disinfecting.
Turbidity tubes and Secchi disks rely on users' eyesight therefore results will be affected by changes in sunlight conditions, choppy water, time of day, and human error. They also cannot be accurately used in very low turbidity samples (<5 NTU)
Photometers & other turbidity meter are electronic devices that measure the absorbance of wavelengths of light in liquids. They are ideal for collecting accurate results, especially in small samples, but can be more expensive than other measurement methods and require power to operate.
User bias (inaccurate/inconsistent)