Turbidity is an indicator of water quality, and accurate measurement is important for protecting water resources. One of the tools we can use for this is called the Nephelometric Turbidity Unit (NTU) sensor.

NTU sensors have two main parts: a light source and a light sensor. The light source and light sensor are placed at a 90-degree angle to each other like so:

When you use a NTU sensor to test a water sample, the following occurs:

  1. The NTU sensor shines a light directly through the water sample.
  2. The beam of light hits any particles suspended in the water sample.
  3. The particles scatter the light in many different directions.
  4. The light sensor measures how much light has been scattered to the side.
  5. A reading is produced.

When the water is clear (low turbidity), most of the light will pass straight through the sample, missing the light sensor. This will give a low NTU reading.

A clear sample is read by an NTU sensor. A light source shines through the water sample, and most of the light passes through to the opposite side.

As the water becomes cloudier (more turbid), more light will be scattered towards the light sensor. This will give a higher NTU reading.

A cloudy sample is read by an NTU sensor. A light source shines through the water sample, and the light is scattered towards the light sensor.

NTU sensors are very useful. People at water treatment plants use them to check if our drinking water is clean. Scientists use them to check the water in rivers and lakes. Even people at fish farms may use them to make sure the water is healthy for the fish.