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Turbidity Sensor for Water: How It Works, Uses & Selection Guide
Cloudy water is easy to see. But visual appearance alone cannot tell us how much suspended material is affecting the water. That is why turbidity measurement is an important part of water quality monitoring.
Turbidity is commonly monitored in wastewater treatment, surface water, aquaculture, industrial water and environmental monitoring applications.
For occasional testing, a portable turbidity meter may be sufficient. For continuous monitoring, an online turbidity sensor can measure water turbidity and send data to a controller, PLC or monitoring platform.
This guide explains what turbidity means, how optical turbidity sensors work, what NTU and FNU represent, where turbidity sensors are used and how to select the right sensor for your application.
When choosing an online turbidity sensor, consider the expected NTU range, water type, measurement method, materials, installation conditions, fouling risk and communication requirements.
What Is Turbidity in Water?
Turbidity describes the cloudiness or haziness of water caused by suspended and colloidal particles that scatter light.
These particles may include:
In general, water containing more light-scattering suspended material may appear more turbid.
However, turbidity is not a direct count of particles and does not identify their chemical composition.
Is Turbidity the Same as Total Suspended Solids (TSS)?
No. Although turbidity and TSS can be related, they represent different measurements.
Measures an optical property related to light scattering by suspended particles in water. Common reporting units include NTU and FNU, depending on the method.
TSS is generally determined using a laboratory method that measures the mass concentration of retained suspended solids, commonly reported in mg/L.
Particle size, shape, color and optical properties influence light scattering. Therefore, two samples with similar TSS concentrations may produce different turbidity readings.
Turbidity should not automatically be converted into TSS without a validated relationship for the specific water being monitored.
How Does a Turbidity Sensor Work?
Most online water turbidity sensors use an optical measurement principle.
The RK500-07 uses a method based on detecting light scattered by particles in water at approximately 90° to the incident light.
This type of optical measurement is commonly called the nephelometric method.
Because it measures light scattering, the method can detect optical changes caused by suspended particles without requiring an operator to collect every sample manually.
Why Do Turbidity Sensors Use 860 nm Infrared Light?
The RK500-07 specification identifies an 860 nm infrared light source for its turbidity measurement system.
Near-infrared light around 860 nm is commonly associated with ISO 7027-type nephelometric measurement methods.
Compared with some visible-light techniques, this approach can reduce the effect of sample color on the optical reading.
The selected wavelength and optical arrangement are important, but users should still confirm the measurement method required for their application, especially when the results will be used for regulatory or compliance reporting.
NTU vs FNU: What Do Turbidity Units Mean?
Two common units in turbidity measurement are NTU and FNU.
Widely used for reporting turbidity measured by nephelometric instruments.
Commonly associated with ISO 7027-type infrared nephelometric methods.
For the RIKA SENSOR RK500-07, the current V5.0 technical parameter table specifies the selectable measurement ranges in NTU.
Online Turbidity Sensor vs Portable Turbidity Meter
Both instruments can measure turbidity, but they are intended for different monitoring needs.
One important advantage of online measurement is that operators can see how water turbidity changes over time, rather than relying only on isolated readings.
Where Are Turbidity Sensors Used?
Turbidity monitoring is used in a wide range of water-related applications. However, the purpose and required sensor design can differ significantly between them.
1. Wastewater Treatment
Wastewater may contain suspended solids, biological material, flocs and other particles.
Turbidity measurement can help operators observe changes in water clarity during suitable treatment stages, including clarification, filtration or treated-water monitoring.
However, turbidity is not a direct substitute for TSS or other wastewater parameters. The measurement should be selected according to the monitoring objective.
2. Industrial Wastewater
Industrial wastewater conditions differ between facilities. Some processes contain suspended minerals, while others may involve organic contamination, chemicals or corrosive substances.
For these applications, the sensor must be selected according to the expected turbidity range, wetted-material compatibility, fouling conditions, temperature and pressure.
3. River and Surface Water Monitoring
Rivers, lakes, reservoirs and other surface waters may experience turbidity changes associated with rainfall, runoff, erosion, sediment disturbance and biological activity.
Continuous measurement can help environmental monitoring teams observe these changes and develop longer-term records.
For unattended stations, waterproofing, power supply, output interface and maintenance access are also important.
4. Aquaculture
In aquaculture, suspended particles may come from sediment, uneaten feed, organic material or other biological activity.
Turbidity can be used as one part of a wider water-quality monitoring program.
Depending on the system, parameters such as dissolved oxygen, temperature, pH and conductivity may also be relevant.
5. Mariculture and Corrosive Water
Marine environments create different material requirements from freshwater.
The RK500-07 series includes a titanium-alloy Type B configuration intended for applications such as mariculture, strongly corrosive sewage and other complex conditions.
This highlights why material compatibility is an important selection factor alongside turbidity range.
6. Pipeline Turbidity Monitoring
Some industrial systems require turbidity measurement directly in a pipeline rather than in an open tank or basin.
The RK500-07 Type C is designed for pipeline installation using 316L and UPVC materials.
The current product drawing indicates a connection designed for UPVC with an outer diameter of 32 mm.
In pipeline applications, bubbles, reflective surfaces, flow conditions and the position of the optical measurement area should be considered.
7. Clean-in-Place (CIP) Applications
The RK500-07 product specification also lists Clean-in-Place applications.
However, actual compatibility must be confirmed against the cleaning chemical, concentration, temperature, pressure and cleaning procedure.
How to Choose a Turbidity Sensor for Water
Choosing a turbidity sensor is not simply about finding an instrument that can measure NTU.
A sensor suitable for relatively clear water may not be the best choice for highly turbid wastewater, seawater or a fouling-prone process.
What Turbidity Range Should You Choose?
Range selection is one of the most important decisions.
Relatively clear water may require good performance at low turbidity, while wastewater and sediment-rich water may require a wider range.
The RK500-07 offers the following selectable ranges:
A wider range is not automatically better. The selected range should match the actual process conditions and required measurement performance.
Should You Choose a Self-Cleaning Turbidity Sensor?
Optical measurement requires a clean optical window.
In wastewater, aquaculture or other fouling-prone applications, sediment, biological growth or deposits may accumulate on the measurement surface.
The RK500-07 Type D includes an integrated cleaning brush designed to help reduce fouling on the optical measurement area.
How to Install an Online Turbidity Sensor
Good installation is essential for stable optical turbidity measurement.
The installation should provide representative water conditions while minimizing interference from bubbles, deposits, reflective surfaces and other disturbances.
Common Installation Factors That Affect Readings
Special Considerations for Reflective Pipelines
Stainless-steel and other reflective pipe surfaces may influence optical readings when the measurement area is too close to the wall.
The RK500-07 V5.0 installation guidance recommends maintaining at least 58 mm clearance from a reflective surface in the illustrated arrangement.
For reflective pipelines, the document also recommends aligning the line between the two small optical openings with the pipeline flow direction to reduce reflection effects.
How Do You Maintain a Turbidity Sensor?
Because a turbidity sensor uses an optical measurement system, maintaining a clean optical window is important.
The RK500-07 specification recommends keeping the optical surface clean and wiping it gently when necessary.
The actual maintenance frequency depends on water conditions. Heavily contaminated wastewater or biofouling-prone environments may require more frequent inspection than relatively clear water.
RIKA SENSOR RK500-07 Turbidity Sensor: Four Configurations
The RK500-07 series provides four configurations for different water environments, installation conditions and maintenance requirements.
| Type | Specified Application | Main Material | Key Feature |
|---|---|---|---|
| Type A | General freshwater environments | 316L stainless steel | General-purpose water monitoring |
| Type B | Mariculture, strongly corrosive sewage and complex conditions | Titanium alloy | Corrosion-resistant construction |
| Type C | Pipeline installation | 316L + UPVC | Pipeline-oriented installation structure |
| Type D | Applications requiring an integrated cleaning brush | 316L stainless steel | Self-cleaning / reduced-maintenance design |
RK500-07 Technical Specifications
The following specifications are based on the RK500-07 Turbidity Sensor V5.0 document. Values that differ by configuration are identified separately.
| Parameter | Specification |
|---|---|
| Measurement Principle | Optical, 90° scattered-light measurement |
| Light Source | 860 nm infrared |
| Measurement Ranges | 0–10 / 0–100 / 0–1000 / 0–4000 NTU |
| Accuracy | Types A/B/C: ±1% FS; Type D: ±2% FS |
| Resolution | Types A/B/C: 0.01 NTU at ≤10 NTU; 0.1 NTU above 10 NTU. Type D: 0.1 NTU |
| Response Time | 1 s |
| Power Supply | 7–30 VDC |
| Output | 4–20 mA and RS485 simultaneously |
| Ingress Protection | IP68 |
| Operating Temperature | Types A/B/C: 0 to +60°C; Type D: -5 to +60°C |
| Specified Pressure Resistance | 0.8 MPa nominal rating in the specification; confirm permitted working pressure for the selected version before installation |
| Default Cable Length | 5 m |
| Optical Window | Sapphire measurement window |
| Self-Cleaning | Type D includes a cleaning brush |
Can One Turbidity Sensor Measure Both Clean Water and Wastewater?
Not necessarily.
Low-turbidity water and highly turbid wastewater can create very different measurement requirements.
The correct sensor should match the expected turbidity range, water characteristics, installation and required performance.
What Should You Tell a Turbidity Sensor Supplier Before Buying?
Preparing the following project information helps make sensor selection faster and more accurate.
Frequently Asked Questions
What does a turbidity sensor measure?
It measures an optical property associated with light scattering by suspended particles in water. It does not directly identify the particles or their chemical composition.
What does NTU mean?
NTU stands for Nephelometric Turbidity Unit. It is commonly used to report turbidity measurements obtained using nephelometric methods.
Is higher NTU always a sign of unsafe water?
No. Higher turbidity generally indicates more light scattering, but turbidity alone does not determine water safety or identify the type of contamination.
Is turbidity the same as TSS?
No. Turbidity is an optical measurement, while TSS is generally a mass-based measurement. A site-specific relationship is needed before using turbidity to estimate TSS.
Can a turbidity sensor continuously monitor water?
Yes. Online turbidity sensors can be installed for continuous monitoring when the selected configuration and installation conditions are suitable.
Can a turbidity sensor be installed in a pipeline?
Yes, provided the sensor is designed for pipeline installation. The RK500-07 Type C has a pipeline-oriented configuration using 316L and UPVC.
Does a self-cleaning turbidity sensor need maintenance?
Yes. A cleaning brush can reduce fouling and manual cleaning requirements, but inspection and cleaning may still be necessary.
What range should I choose for an online turbidity sensor?
Choose a range that covers the expected water conditions while meeting the required measurement performance. RK500-07 offers selectable 0–10, 0–100, 0–1000 and 0–4000 NTU ranges.
Can an online turbidity sensor connect to a PLC?
Yes, if the interfaces are compatible. RK500-07 provides 4–20 mA and RS485 outputs according to its current technical specification.
Can turbidity measurement identify corrosion in a pipeline?
Not directly. Suspended corrosion products may affect turbidity, but a turbidity reading alone cannot prove corrosion, identify particle composition or determine a corrosion rate.
Final Thoughts: The Best Turbidity Sensor Depends on the Water
When choosing a turbidity sensor, it is easy to compare only measurement range, accuracy and price.
But reliable continuous turbidity measurement depends on more than those specifications.
A sensor that works well in freshwater may not be ideal for seawater, heavily contaminated wastewater or a reflective pipeline.
For online monitoring, the goal is not simply to display an NTU value. It is to obtain a stable, maintainable measurement that is representative of the water being monitored.
Online Turbidity Monitoring with RIKA SENSOR
RIKA SENSOR provides online turbidity measurement solutions for environmental monitoring, aquaculture, wastewater treatment and industrial water applications.
The RK500-07 series includes configurations for general freshwater, corrosive water, pipeline installation and applications where an integrated cleaning brush can help reduce fouling.
With multiple turbidity ranges, IP68 protection and 4–20 mA plus RS485 outputs, the sensors can be integrated into compatible water-quality monitoring and industrial control systems.
Need Help Selecting a Turbidity Sensor for Water?
Tell us your water type, expected NTU range, operating temperature and pressure, installation method, fouling conditions and communication requirements. RIKA SENSOR can help evaluate the appropriate turbidity sensor configuration for your project.
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