Turbidity Measurement: Online Turbidimeters
Low-level turbidity measurement at filter outlets and in drinking water: nephelometric online turbidity analyzers, digital sensor and local interface options.
Measurement methods: Nephelometry / Turbidity
What is turbidity and why is it measured?
Turbidity is the loss of visual clarity caused by very small particles suspended in water scattering light. Particles such as clay, silt, organic matter, microorganisms and precipitates cause turbidity. In drinking water treatment, turbidity is the most sensitive indicator of filter performance: a rise at the filter outlet is the first sign of filter bed loss and of particle breakthrough that would reduce disinfection effectiveness. In industry, it is monitored for pre-membrane water quality, process water and wastewater discharge quality.
Where is turbidity measured?
- Drinking water treatment plants: At the raw water inlet, sedimentation outlet and each filter outlet.
- Membrane and reverse osmosis pretreatment: In feedwater quality control.
- Process and wastewater lines: In discharge and recovery control.
How is turbidity measured?
Nephelometry measures the scattering of light by particles in the sample. Light from a source is sent into the sample; a detector positioned at a set angle to the beam (usually 90°) senses the scattered light. As the amount of particles increases, scattered light rises. Low-level turbidimeters are designed to catch small changes in clean filtered water, and they need a flow cell design that reduces the effect of air bubbles. In digital sensor systems, the measured value is shown on a local interface and transferred to the control system through a current output, relay or Modbus.
Choosing a turbidimeter
- Measurement range: low-level filter outlet or high-turbidity process water
- Air bubble management and flow cell design
- Integrated, remote or displayless configuration
- Periodic cleaning and calibration plan
Turbidity measurement with Tetra
Tetra supplies the Thermo Orion AquaClear turbidity analyzer and AV38 local interface transmitter in Türkiye. It provides installation, supply of consumables, commissioning, calibration and service.
Frequently asked questions
What is the difference between the turbidity units NTU and FNU?
NTU is the unit used for measurements with a white light (tungsten lamp) source and a 90° detector (e.g. EPA 180.1). FNU is used for measurements made according to ISO 7027 with an infrared (860 nm) light source and a 90° detector. Although both units are based on the same formazin standard, the different light sources can give different results on the same sample; the method used should be stated in reporting.
How is a turbidimeter calibrated?
Primary calibration is performed with formazin or stabilized formazin standards. Between calibrations, instrument stability is checked with secondary (verification) standards provided by the manufacturer. Calibration frequency is determined by the manufacturer's recommendation and the criticality of the application; verification should be more frequent for low-level filter outlet measurements.
What causes falsely high readings in low-level turbidity measurement?
The most common cause is air bubbles in the sample; bubbles scatter light and are detected as particles. Contamination or condensation on the measurement cell window, stray light and deposits in the cell also create marked deviation at low values. A bubble-removing flow cell, regular cleaning and keeping the sample line pressurized and short reduce these errors.
Which turbidimeter should be selected for colored samples?
In instruments with a white light source, sample color absorbs some of the light and can make turbidity appear lower than it is. Instruments with an infrared light source (ISO 7027) are less affected by color interference, so they are more suitable for colored waters or those containing organic matter. When selecting, also check which method the applicable legislation or specification requires.
Products measuring this parameter
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