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Turbidity can be measured by the cloudiness in liquids.

Turbidity, a measure of cloudiness in liquids, has been recognized as a simple and basic indicator of water quality.


What is Turbidity

Turbidity, a measure of cloudiness in liquids, has been recognized as a simple and basic indicator of water quality. It has been used for monitoring drinking water, including that produced by filtration for decades. Turbidity measurement involves the use of a light beam, with defined characteristics, to determine the semi-quantitative presence of particulate material present in the water or other fluid sample. The light beam is referred to as the incident light beam. Material present in the water causes the incident light beam to scatter and this scattered light is detected and quantified relative to a traceable calibration standard. The higher the quantity of the particulate material contained in a sample, the greater the scattering of the incident light beam and the higher the resulting turbidity.

Any particle within a sample that passes through a defined incident light source (often an incandescent lamp, light emitting diode (LED) or laser diode), can contribute to the overall turbidity in the sample. The goal of filtration is to eliminate particles from any given sample. When filtration systems are performing properly and monitored with a turbidimeter, turbidity of the effluent will be characterized by a low and stable measurement. Some turbidimeters become less effective on super-clean waters, where particle sizes and particle count levels are very low. For those turbidimeters that lack sensitivity at these low levels, turbidity changes that result from a filter breach can be so small that it becomes indistinguishable from the turbidity baseline noise of the instrument.

This baseline noise has several sources including the inherent instrument noise (electronic noise), instrument stray light, sample noise, and noise in the light source itself. These interferences are additive and they become the primary source of false positive turbidity responses and can adversely impact the instrument detection limit.


Laser-based Tech

Within the past decade, newer laser-based techniques in turbidity analysis have emerged and have proved to be a more sensitive method for monitoring filter performance. These laser-based technologies can identify filtration integrity problems earlier and with better detection levels. Laser turbidimeters better address the need for low-level turbidity analysis in cleaner water samples as they possess enhanced optical designs that yield greater sensitivity and baseline stability.

The laser turbidimeter utilizes a highly collimated (light whose rays are parallel), laser-based light source that is primarily monochromatic. The characteristics of this light source allow the light energy to be concentrated and focused into a very small volume within the sample chamber in each instrument. This combination provides an incident beam with a high power density, which is efficiently scattered by particles within a sample. The detector is also of greater sensitivity and provides a greater response to scattered light. Preferably, the peak of the detector response spectrum should completely overlap with the spectrum emitted by the incident light source to generate maximum optical sensitivity. This combination of detector sensitivity, collimated light source, and the high power density of the incident light provides a very high signal-to-noise ratio for the laser turbidimeter. This signal-to-noise ratio enhances the sensitivity to detect very small changes in turbidity that can be distinguished from a measurement baseline. In other words, a high signal-to-noise ratio is an indication of a sensitive turbidimeter.

Laser turbidimeters and other instruments that provide high signal-to-noise ratios will yield extremely stable measurement baseline levels in comparison to traditional turbidimeters. Stable baselines allow for the detection of very fine changes in the turbidity within a sample that would otherwise be indistinguishable with conventional turbidimeters. Further, this baseline can be characterized in terms of stability and then serve as an additional analysis parameter. This parameter would complement the directional trending of the turbidity measurement value itself.

The advent of laser turbidimeters has improved the detection of filtration integrity deficiencies. These instruments possess highly improved optical qualities to produce a very stable process measurement system. This enhanced stability provides additional information that can be deciphered from the laser turbidity measurement. It is important to understand that turbidity is a method-based measurement and only turbidity measurements derived from the same methodology should be compared quantitatively using accuracy specifications. The difference in absolute turbidity values may indicate an offset between the two methods, which also may vary depending on calibration. This condition must always be considered when comparison between turbidity measurements is conducted. Also, the value of calibration and calibration verification in turbidity measurements cannot be overestimated. The quality of calibration is dependent upon quality of the standards, which play crucial role in establishing and verifying the quality of turbidity measurement.

Laser turbidimeters make low-level turbidity analysis possible.

Turbidity Standards

The subject of standards in turbidimetric measurement is complicated partly by the variety of types of standards in common use and acceptable for reporting purposes by organizations such as the USEPA and Standard Methods, and partly by the terminology or definition applied to them. In the 19th Edition of Standard Methods for the Examination of Water and Wastewater, clarification was made in defining primary versus secondary standards. Standard Methods define a primary standard as one that is prepared by the user from traceable raw materials, using precise methodologies and under controlled environmental conditions. In turbidity, Formazin is the only recognized true primary standard and all other standards are traced back to Formazin. Further, instrument algorithms and specifications for turbidimeters should be designed around this primary standard.

Standard Methods now define secondary standards as those standards a manufacturer (or an independent testing organization) has certified to give instrument calibration results equivalent (within certain limits) to results obtained when an instrument is calibrated with user-prepared Formazin standards (primary standards). Various standards that are suitable for calibration are available, including commercial stock suspensions of 4,000 NTU Formazin, stabilized Formazin suspensions (StablCal™ Stabilized Formazin Standards, which is also referred to as StablCal Standards, StablCal Solutions, or StablCal), and commercial suspensions of microspheres of styrene divinylbenzene copolymer.

At the time of this writing, calibration verification items supplied by instrument manufacturers, such as sealed sample cells filled with a latex suspension or with metal oxide particles in polymer gel, are used to check a calibration and are not used in performing instrument calibrations. If there is a discrepancy on the accuracy of a standard or an instrument, the primary standards (i. e., user-prepared Formazin) are to be used to govern the validity of the issue.

Primary standards are also used for measuring and determining the value of all other standards. Under the USEPA definition, secondary standards are used to verify the calibration of a turbidimeter. However, secondary standards should not to be used for calibrating instruments. Examples of these standards include the metal oxide gels, latex, and any non-aqueous standards that are defined to monitor calibrations on a day-to-day basis. This usage depends on the design of the standard. On the other hand, Formazin, StablCal standards, and Amco AEPA-1 Alternative Standards are designed to calibrate the instruments.

Various standards exists for turbidity measurement.

StablCal Stabilized Formazin Standards

A new turbidity standard has been developed for use in calibrating or verifying the performance of any turbidimeter. StablCal Stabilized Formazin Standards contain the same light scattering polymer as traditional Formazin primary turbidity standards. By using a different matrix, the polymer in StablCal standards will not deteriorate over time as is the case with low turbidity Formazin standards. Due to this enhanced stability, StablCal standards of any concentration ranging up to 4,000 NTU can be manufactured and packaged in ready to-use formats. Thus, time is saved and direct exposure to the standard is minimized.

The StablCal Stabilized Formazin Standards have been shown to be both stable and read comparable to traditional, freshly prepared Formazin standards. Standards in the range of 0.30 to 4,000 NTU have been demonstrated to remain within 5 percent of their original preparation values for a minimum of two years. From a comparability standpoint, StablCal Standards can be interchangeably used as calibration standards on any turbidimeter with only very minimal differences in the instrument response. The stabilization of Formazin has resulted in the development of the StablCal standards. These standards serve as the solutions to those problems that were associated with traditional Formazin standards. This stabilization allows for these standards to be packaged in structures that greatly reduce any kind of potential exposure to the user from the standard. Further, when comparing StablCal to traditional Formazin standards of equal concentration, studies have shown that residual hydrazine sulfate in StablCal is reduced by two to three orders of magnitude. The stabilization of Formazin in the StablCal standards provides the user with ready-to-use standards, and the large quantity of time required to prepare low turbidity traditional Formazin standards is now eliminated. Users can take these stabilized standards and use them in the field, and at the same time be confident that the standards are accurate and repeatable in these non-laboratory settings.

StablCal is a used for calibrating your TU5 Turbidimeter.

Ultra-High Turbidity Measurement

Ultra-high turbidity measurements are generally turbidity measurements where nephelometric light scatter can no longer be used to assess particle concentration in samples. In a sample with a measurement path length of 1-inch, nephelometric light-scatter signals begin to decrease at turbidities exceeding 2000 NTU. At this point, an increase in turbidity will result in a decrease in nephelometric signal.

In addition, color can be a major interference in ultra-high turbidity measurements. Because of the influence of sample color, the application of strict nephelometric turbidity has been limited, particularly in industrial processes that involve beverages, food products, cell cultures, and dispersed oil in water.

However, other measurements can be used to determine the turbidity of such samples. Three of these are transmitted, forward scatter, and back-scatter methods. Transmitted and forward-scatter signals are inversely proportional to increased turbidity and give good response to 4,000 NTU. Above 4,000 NTU (when using the standard 1-inch path), transmitted and forward-scatter signals are so low that instrument noise becomes a major interfering factor. On the other hand, back-scatter signals will increase proportionally with increases in turbidity. Back-scatter measurements have been determined to be highly effective at determining turbidity specifically in the range of 1,000 to 10,000 NTU (and higher). Below 1,000 NTU, back-scatter signal levels are very low, and instrument noise begins to interfere with the measurements. With a combination of detectors, turbidity can now be measured from ultra-low to very high levels.

This type of measurement is known as Ratio turbidimetry. The ratio turbidimeter’s optical configuration is the key to several performance characteristics. Among them are good stability, linearity, sensitivity, low stray light and color rejection. In a Ratio instrument, a large transmitted-light detector measures the light that passes through the sample. A neutral density filter attenuates the light incident on this detector and the combination is canted at 45 degrees to the incident light, so that reflections from the surface of the filter and detector do not enter the sample cell area. A forward-scatter detector measures the light scattered at 30 degrees from the transmitted direction. A detector at 90 degrees nominal to the forward direction measures light scattered from the sample normal to the incident beam. And a fourth, back-scatter detector measures the light scattered at 138 degrees nominal from the transmitted direction. This detector “sees” light scattered by very turbid samples when the other detectors no longer produce a linear signal. The signals from each of these detectors are then mathematically combined to calculate the turbidity of a sample.

The use of ultra-high turbidity measurement has many applications. It is used in the monitoring of fat content in milk, paint resin constituents such as titanium dioxide, liquor solutions in pulp and paper processing mills, and ore slurries in milling operations.

Ultra-high turbidity measurements are used for turbidities exceeding 2000 NTU.

Turbidity vs. Suspended Solids

Ultra-high turbidity measurements are generally used as a mechanism for monitoring process control either directly or as a surrogate to the lengthy gravimetric analysis for total suspended solids (TSS). A correlation needs to be established between the turbidity and TSS of the sample. If such a correlation exists, then a turbidimeter can be used to monitor TSS changes in a sample, resulting in a prompt analysis. The user must first determine the relationship of turbidity to varying conditions in the process stream. In determining this relationship, dilutions of the sample are made and the turbidity and TSS of each dilution is measured. A plot of turbidity (y-axis) versus each corresponding dilution (x-axis) is then made. The slope of the best fit line will indicate the nature of this relationship. The response time to a change in the TSS of a process can be reduced from hours to seconds with the use of a turbidimeter.

When can turbidity be measured to detect changes in total suspended solids (TSS).


Turbidity Applications

What is turbidity and why is it important to measure?

Turbidity 101: What It Is, And Why It Is So Important

Turbidity, a measure of the cloudiness or haziness of a fluid, was originally intended as a qualitative measure of the aesthetics of drinking water. Today’s turbidity designs and methods have been regimented in an attempt to bring quantitative consistency to the measurement for both aesthetic and pathogenic qualities of drinking water.


Where does turbidity fit in EPA drinking water compliance?

Turbidity’s Role In EPA Compliance

Turbidity measurement is both a nebulous, oft-misunderstood concept and the master link in a chain of events affecting U.S. EPA drinking water compliance. It can influence, or be influenced by, almost every other link in a water treatment process. Here is a quick overview of turbidity’s relationship to drinking water compliance standards and some tips for keeping a water treatment process in balance.


What are the applications of turbidity measurement?

Drinking Water Turbidity Monitoring: 7 Key Considerations

Turbidity, as a measure of cloudiness or haze in water, has many useful applications for industrial processes, pharmaceutical manufacturing, environmental monitoring, and utility applications. Unlike general commercial applications, however, the use of turbidity readings in municipal drinking water treatment comes with unique demands and considerations related to regulatory compliance.



Which measurement options are right for you?

Every facility and every operation is different. Depending on your specific needs, there may be several options to consider. Are you looking for a portable solution? Do you need greater coordination between process and lab measurements? Or perhaps your operation needs streamlined maintenance, faster testing, or better accuracy in data logging and transfer? Whatever your needs, Hach is ready to help with information, technology, and support.

Explore the key factors in the different types of turbidity instruments below.


The filter gallery of a drinking water plant. The particulates the cause turbidity are filtered out here, and turbidity testing on both influent and effluent will indicate the quality improvement of water. Hach process turbidimeters and benchtop lab turbidimeters are engineered to eliminate the mis-match between lab and process measurements of the same water samples.?

Sometimes it is essential to monitor turbidity continuously. Continuous monitoring of drinking water effluent confirms compliance and also provides immediate notification of process upsets. Continuous monitoring of wastewater treatment systems provides real-time process optimization data. Whenever immediate turbidity results are required, either for compliance or process control, a process turbidity analyzer is the best solution.

In addition to delivering continuous analyses, process turbidity analyzers are simple, accurate, and low maintenance. Process analyzers eliminate grab sampling and analyst errors. Proper sampling, sample preparation, and cell handling are critical for accurate laboratory turbidity measurements where sample settling, cell orientation, and glass cell imperfections can have significant effects. Properly maintained and calibrated process turbidity analyzers avoid all of these potential issues. Maintenance and calibration of Hach process turbidity analyzers are simple procedures facilitated by automated cleaning routines and Hach StablCal turbidity standards.

Process turbidity analyzers may be used in conjunction with laboratory measurements for verification or calibration. Recent advances in turbidimetric technology in Hach TU5 instruments ensure that process/laboratory verification measurements are closer than ever. Process turbidity analyzers may also be correlated to gravimetric total suspended solids (TSS) measurements for creation of a turbidity/TSS correlation, or to total iron colorimetric measurements for creation of a turbidity/iron corrosion correlation.

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Technician using a turbidity instrument. The versatile standard for grab sample turbidity measurements, Hach lab turbidimeters are engineered to eliminate the mis-match between process turbidity measurements and lab measurements of the same sample. Or, quickly measure multiple samples from multiple sources.

Benchtop turbidimeters are the versatile standard for measuring grab sample turbidity. Whether it’s periodic analyses of raw or settled water, calibration of process turbidity analyzers, or compliance monitoring, a benchtop turbidimeter is the best solution for measuring multiple sample from multiple sources.

Hach benchtop turbidimeters are compliant with EPA requirements for drinking water and wastewater reporting. Extremely sensitive nephelometric measurements are ideal for drinking water combined effluent monitoring. Advanced ratio nephelometric measurements are ideal for high turbidity samples or those with large particles or color. Calibration of Hach benchtop turbidity analyzers is a simple procedure facilitated by Hach StablCal turbidity standards.

Benchtop turbidity analyzers may be used in conjunction with process analyzer measurements for verification or calibration of the process analyzer. Recent advances in turbidimetric technology in Hach TU5 instruments ensure that process/laboratory verification measurements are closer than ever.

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Technician using a turbidity instrument in the field. Portable turbidimeters need to be rugged and reliable. The Hach 2100Q portable sampler gives rapid results of streambed turbidity, surface water turbidity, construction runoff turbidity, and other applications in the field where you need reliable turbidity measurement and analysis.

A handheld portable turbidity meter is ideal for taking measurements in remote locations. A quick examination of surface water, storm water, construction site runoff, or even a spot-check in the drinking water distribution system can provide turbidity data necessary to demonstrate discharge compliance, indicate an upset condition, or direct stream dispositioning. Whenever and wherever a rapid field turbidity result is needed, a portable turbidity meter is the best solution.

Hach portable turbidity meters are simple, accurate, rugged, and easy to take anywhere. Field turbidity measurements eliminate the need to collect samples from multiple distant locations for analysis back in the laboratory. Sample data are logged for each measurement and easily transferred to a computer for analysis so data are never lost. Calibration of Hach portable turbidity meters is a simple procedure facilitated by Hach StablCal turbidity standards. The 2100Q portable turbidity meter is compliant with EPA 180.1 design criteria.

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Explore the TU5 Series turbidimeters

See what makes the TU5 Series the most precise and advanced turbidimeters on the market, with a patented optics system, easy-to-use interface, and faster calibration, cleaning and response times to changes in turbidity.

Options and Features

The Hach family of turbidimeters leverage reliable technology to provide fast, accurate turbidity measurement and analysis across a wide range of applications.

Hach offers a suite of TU5 Turbidity analyzers, equipped with the patented, breakthrough 360° x 90° degree laser technology, allowing for fast and accurate readings in a number of hydrological settings. With designed-in calibration between process and lab, and secure data access and data logging, you'll be confident in your operation.

Compare our different products used for process, lab and portable applications by browsing the options below.

The Hach family of TU5 Turbidimeters






Service & Support


Online Support

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Hach Training Center provides relevant, hands-on training to your team, giving them the experience they need to master various theories and techniques—and produce results you can trust for quality assurance, environmental safety, and regulatory compliance. Hach experts offer a large course catalog of workshop training, personalized training, and digital learning designed to increase proficiency and confidence for plant operators, instrument and field technicians, laboratory personnel, and plant managers and superintendents.


Hach ServicePlus ® Programs have been developed to help solve your maintenance and support problems. Whether it’s a lack of resources or skills, an instrument that is down, compliance concerns or the need for a predictable budget, we have programs to fit the unique challenges you face in your organization.