How turbidity measurement works using an optical 90° scattered light probe

Water turbidity is a parameter that indicates the amount of suspended solid particles (sludge, colloids, bacteria, oxides, organic matter) that make the water cloudy. The higher the turbidity, the lower the water clarity and the higher the concentration of suspended solids. The S461 ST turbidity sensor uses the 90° scattered light optical principle (nephelometry), which is today one of the most reliable technologies for measuring turbidity in process water, wells, surface water and wastewater.

Measurement principle: 90° scattered light

Inside the probe there are:

A LED light source that emits a stable light beam, A photodetector positioned at 90° to the light beam

When the water is clear, the light passes through it with very little deviation.

When suspended particles are present, they intercept the light and scatter it in all directions. A portion of this scattered light reaches the detector located at 90°. The more particles are present, the stronger the scattered light signal becomes. This signal is converted into a turbidity value expressed in NTU (Nephelometric Turbidity Units). 

Why the 90° geometry is so reliable, The 90° detector position allows the sensor to:

Exclude the direct transmitted light, minimize the influence of water color, measure only the light actually scattered by suspended particles, this makes the measurement stable and accurate even in dark, colored or heavily polluted water. 

Importance of proper installation geometry

For accurate measurement, the optical window of the probe must face a free and unobstructed measurement volume. 

As shown in the illustration:

At least 5 cm (2 in) of free space must be maintained in front of the optical cone

At least 10 cm (4 in) distance must be kept from walls, tank bottoms or pipe surfaces

This prevents:

Optical reflections, False high turbidity readings, Signal instability, Signal conversion and process control

The probe converts the scattered light intensity into a 4–20 mA output signal, which can be connected to: 

Controllers

PLCs, Data loggers, SCADA systems, This allows turbidity to be:

Continuously monitored

Used to control filters, backwash cycles or chemical dosing. Logged for process optimization and regulatory compliance.

Technical conclusion

The 90° optical turbidity measurement principle used by the S461 ST probe provides reliable and repeatable monitoring of water quality, even in demanding applications such as wastewater, well water and industrial process water.

Correct mechanical installation and proper optical geometry are essential to turn the sensor into a truly effective process control instrument.

👉 In water treatment systems, turbidity is not just a number: it is one of the earliest indicators of contamination, process instability and filtration problems.

More info: @info