Turbidity Measurement by Nephelometry

How does nephelometry work?

Suspended particles in water scatter incident light in all directions. In nephelometric turbidity measurement, a light beam is sent into the sample, and scattered light is sensed by a detector placed at a 90° angle to the incident light. The scattered light intensity increases with particle number and size and is converted to a turbidity value.

For low-level turbidity measurements, the light source, optical arrangement and bubble separation system are specially designed so that very small signal changes can be detected. The sample is fed to the measurement cell at a steady flow.

What nephelometry measures and where it is used

Turbidity is the main indicator of water's visual clarity and filtration efficiency. Typical applications:

• Monitoring filter outlets and each filter unit at drinking water treatment plants
• Process water lines
• Wastewater recovery and advanced treatment outlets
• Water quality control in the food and beverage industry

Where nephelometry works well and where it struggles

An online nephelometer makes it possible to notice small changes in filter performance early and to optimize backwash timing. Continuous measurement eliminates the waiting time of laboratory analyses.

Limitations: air bubbles in the sample can cause falsely high readings, so a bubble separator should be used and sample pressure kept balanced. When measurement cell windows become contaminated, the reading drifts; periodic cleaning and calibration with primary standards are required. Because measurement relies only on particles scattering light, it gives no information about the type of particle.

Choosing an instrument for nephelometry

When selecting, the measurement range, sample pressure and flow, display configuration and output and communication options are determined. For low-level measurements at filter outlets, a bubble separator and regular optical cleaning are important for reliable results. During commissioning, Tetra configures the sample line, bubble removal arrangement and alarm outputs according to site operating conditions.

Tetra and turbidity analyzers

Tetra supplies, installs, commissions, calibrates and services turbidity analyzers in Türkiye. The product range includes the Thermo Orion AquaClear low-level turbidity analyzer, which offers integrated, remote or displayless options and current output, relay and Ethernet options. The AV38 local interface transmitter can be used with this system.