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What Is a Turbidity Sensor? How to Choose?

turbidity sensor

Before learning the turbidity sensor, we need to understand what is turbidity. Turbidity is a measure of water quality that reflects the amount of suspended particles in a water sample by observing the amount of light scattered through it. The more pronounced the light scattering, the higher the turbidity of the water. Typically, the factors that affect water turbidity are the content of suspended solids and tiny particles. These tiny particles can be composed of sediment, clay, organic and inorganic materials, dissolved organic compounds, and algae. Water with high turbidity often requires purification processes before it can be used in industrial and domestic applications. This is because a decrease in turbidity often implies a reduction in harmful substances, bacteria, and viruses in the water.

Why need to measure water turbidity?

Turbidity in Drinking Water

The higher the turbidity in drinking water, the greater the risk of gastrointestinal diseases in humans. This is because contaminants such as viruses and bacteria can attach themselves to suspended solids. These solids can interfere with disinfection processes. Drinking water is typically treated surface water or groundwater because the particles and microorganisms naturally present in surface water and groundwater are not suitable for direct human consumption. Currently, drinking water regulations stipulate that turbidity should not exceed 1 NTU (Nephelometric Turbidity Unit).

water treatment

According to the World Health Organization, when turbidity exceeds 1 NTU, higher disinfectant dosages or contact times are required to ensure adequate treatment. Additionally, monitoring turbidity in drinking water can help operators assess the proper functioning of water treatment equipment. When measured turbidity in drinking water does not meet the requirements, it prompts an investigation into whether the water treatment system’s pipes are damaged or if there are issues with the pump system.

Turbidity in Industrial Water

Monitoring turbidity in industrial water is also essential because high turbidity levels can damage equipment and reduce its lifespan. For instance, water with high turbidity can increase frictional resistance in equipment such as water pumps and pipelines, leading to higher energy consumption. Impurities in the water can cause abrasion on equipment, especially in terms of transmission machinery and agitators. Prolonged abrasion can significantly decrease the equipment’s operational lifespan.

High turbidity in industrial water often indicates the presence of a significant amount of algae, bacteria, and other microorganisms. These organisms can grow on the inner walls of pipelines, causing blockages. Practical experience has shown that in industrial cooling water systems with side-stream filtration, supplementary water turbidity can be controlled to within 5 NTU.

Turbidity in Rivers and Reservoirs

Excessive turbidity in water can reduce the amount of light penetrating deep into bodies of water such as rivers, lakes, and reservoirs. This reduction in light can inhibit the growth of certain aquatic plants and have negative effects on species like fish and shellfish that rely on algae as a food source. High turbidity can also hinder the ability of fish to absorb dissolved oxygen. This phenomenon has been observed and documented throughout the Chesapeake Bay in the Mid-Atlantic region of the United States.

This video from @NEIWPCC

What is a turbidity sensor?

A turbidity sensor is a specialized instrument used to measure the concentration of suspended particles in a solution. When light passes through water, it is refracted and reflected, and when light is reflected, its angle and intensity change. These changes can be used to detect the turbidity of water and calculate the degree of cloudiness or turbidity of the water.

How does a turbidity sensor work?

The working principle of a turbidity sensor involves measuring the intensity of the interaction between light and suspended particles in a liquid sample. The interaction between light and suspended particles affects in two main ways: scattering and absorption.

light changing through water

Scattering: Particles in the liquid scatter incident light in different directions. The degree and pattern of scattering depend on factors such as the size, shape, and concentration of the particles. Some of the scattered light can be directed towards a detector.

Absorption: Some particles in the liquid may absorb certain wavelengths of light. The extent of light absorption depends on the nature of the particles and their optical properties. This absorption reduces the intensity of the light beam as it passes through the sample.

Turbidity unit

When measuring turbidity, you will usually see the units NTU, JTU or FTU. The conversion relationship between them is:

UnitJTU (degrees)FTU (NTU)SiO2(mg/l)
JTU (degrees)1192.5
FTU (NTU)0.05310.13
  • JTU is the unit of turbidity measured with diatomite or kaolin as the turbidity standard solution and measured by the Jackson candle turbidimeter.
  • NTU is the unit of turbidity measured with methyl wax as the turbidity standard solution and measured with a scattered light nephelometer.
  • FTU is the unit of turbidity measured by a transmitted light nephelometer.

Three types of turbidity sensor

1. Absorption Turbidity Sensors

An absorption turbidity sensor is a device used to measure the turbidity of a liquid by evaluating the extent to which the liquid sample absorbs light. Its light source typically consists of an LED (Light Emitting Diode) that emits light at a specific wavelength or within a certain wavelength range. The emitted light passes through the liquid sample being tested. When the light encounters suspended particles or molecules in the liquid, some of the light is absorbed by these particles. The remaining transmitted light is measured by a photodetector. The sensor then converts the detected light intensity into a turbidity measurement value. This is usually done by comparing the intensity of transmitted light with the intensity of incident light (the light before it passes through the sample).

Turbidity measurement results are typically expressed in units such as NTU (Nephelometric Turbidity Units) or FTU (Formazin Turbidity Units), depending on the calibration and standards used by the sensor.

Absorption turbidity sensors are widely used in applications such as water treatment, environmental monitoring, industrial processes, and laboratory environments to assess the clarity and quality of liquid samples.

2. Nephelometric Turbidity Sensors

A nephelometric turbidity sensor is a device used to measure the turbidity or cloudiness of a liquid sample by evaluating the light scattering caused by suspended particles in the liquid.

The light source of a turbidity sensor is typically an LED (Light Emitting Diode), and the emitted light is directed into the liquid sample being tested. As the light passes through the sample, it encounters suspended particles or molecules. These particles scatter the incident light in various directions. The sensor includes photodetectors or photodiodes placed at specific angles to detect the scattered light. It measures the intensity of the scattered light at a specific angle relative to the incident light. The amount of scattered light is directly related to the concentration and size of suspended particles in the liquid.

Turbidity Sensor

Turbidity measurement results are usually expressed in units such as NTU (Nephelometric Turbidity Units) or FNU (Formazin Nephelometric Units), depending on the calibration and standards used by the sensor.

Nephelometric turbidity sensors are used to measure the turbidity of drinking water, pharmaceuticals, or other types of low-turbidity water samples.

3. Suspended Solid Turbidity Sensors

A Total solid/suspended solid turbidity sensor is a device used to measure the concentration of total suspended solids in a liquid sample. Total suspended solids refer to the mass of solid particles suspended in a liquid, typically measured in milligrams per liter (mg/L) or parts per million (ppm).


An LED emits light at a specific wavelength into a liquid sample containing suspended solids. As the light passes through the sample, it interacts with the suspended particles. Detectors are used to measure the intensity of scattered light. The degree of scattering is proportional to the concentration and size of suspended solid particles in the liquid. The sensor processes the detected scattered light to calculate the concentration of total suspended solids in the sample.

TSS measurement results are typically displayed on a screen or provided as an electrical signal.

This turbidity sensor is primarily used in environments with higher turbidity levels, such as water and wastewater treatment, agriculture, industrial production, and other applications where the concentration of suspended solids in the liquid needs to be determined.

How to choose the best turbidity sensor?

Selecting the best turbidity sensor for your particular application depends on several factors, including the nature of the sample you are testing, the accuracy you need, environmental conditions, and your budget. Here are some key considerations to help you choose the right turbidity sensor:

Measure Range

Consider the range of turbidity values you need to measure. Ensure that the sensor’s measurement range covers the expected turbidity levels in your sample. Some sensors are designed for low turbidity levels, while others are suitable for high turbidity levels.

Solution Turbidity Comparison


Accuracy is crucial, especially for applications that require precise turbidity measurements. Look for a sensor with high accuracy. Renke RS-ZD-N01-* turbidity sensor has a built-in temperature compensation function, which avoids the influence of solution temperature changes on the measurement results. The monitoring results are more accurate.


Check whether the sensor requires calibration and what calibration standards are needed. Calibration ensures measurement accuracy. Some sensors may require calibration using known turbidity standard solutions.


Consider the nature of your sample. For instance, if your sample contains large particles, choose a sensor that can handle these particles without clogging or interference.

Environmental Conditions

Think about the environmental conditions in which you will use the sensor. Some sensors are designed for harsh environments and can withstand temperature fluctuations, humidity, and other challenging conditions.

Response Time

Sensor response time is essential, especially when you need real-time monitoring. Faster response times are crucial for process control applications.


Sensors that are easy to clean and maintain can save you time and effort. Self-cleaning turbidity sensors can significantly reduce maintenance costs.


Turbidity sensors come in a range of prices, so consider your budget constraints. While cost-effective options exist, be cautious not to sacrifice quality for a lower price if accuracy and reliability are critical.

By carefully evaluating these factors, you can select a turbidity sensor that best suits your specific measurement requirements and application conditions.

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