Infrared light is directed at a cuvette containing the
sample water. This light is scattered in all directions off
the particles in the water. A detector, consisting of a
photodiode, is placed at a 90° angle to the light source.
The amount of light being scattered directly into the
detector is measured in volts and translated into turbidity
units. This style of turbidity sensor is called a
nephelometer. A standard is used to calibrate the
Turbidity Sensor in units of NTU, Nephelometric
Turbidity Units. Other units such as JTU (Jackson
Turbidity Units), and FTU (Formazin Turbidity Units),
have values similar to NTU, but are not exactly the same.