ASTM D7726-11(2023)

Standard Guide for The Use of Various Turbidimeter Technologies for Measurement of Turbidity in Water

SIGNIFICANCE AND USE
5.1 Turbidity is a measure of scattered light that results from the interaction between a beam of light and particulate material in a liquid sample. Particulate material is typically undesirable in water from a health perspective and its removal is often required when the water is intended for consumption. Thus, turbidity has been used as a key indicator for water quality to assess the health and quality of environmental water sources. Higher turbidity values are typically associated with poorer water quality.  
5.1.1 Turbidity is also used in environmental monitoring to assess the health and stability of water-based ecosystems such as in lakes, rivers and streams. In general, the lower the turbidity, the healthier the ecosystem.  
5.2 Turbidity measurement is a qualitative parameter for water but its traceability to a primary light scatter standard allows the measurement to be applied as a quantitative measurement. When used as a quantative measurement, turbidity is typically reported generically in turbidity units (TUs).  
5.2.1 Turbidity measurements are based on the instruments’ calibration with primary standard reference materials. These reference standards are traceable to formazin concentrate (normally at a value of 4000 TU). The reference concentrate is linearly diluted to provide calibration standard values. Alternative standard reference materials, such as SDVB co-polymer or stabilized formazin, are manufactured to match the formazin polymer dilutions and provide highly consistent and stable values for which to calibrate turbidity sensors.
5.2.1.1 When used for regulatory compliance reporting, specific turbidity calibration standards may be required. The user of this guide should check with regulatory entities regarding specifics of allowable calibration standard materials.  
5.2.2 The traceability to calibrations from different technologies (and other calibration standards) to primary formazin standards provides for a basis for defined turbidity uni...
SCOPE
1.1 This guide covers the best practices for use of various turbidimeter designs for measurement of turbidity in waters including: drinking water, wastewater, industrial waters, and for regulatory and environmental monitoring. This guide covers both continuous and static measurements.  
1.1.1 In principle there are three basic applications for on-line measurement set ups. The first is the bypass or slipstream technique; a portion of sample is transported from the process or sample stream and to the turbidimeter for analysis. It is then either transported back to the sample stream or to waste. The second is the in-line measurement; the sensor is submerged directly into the sample or process stream, which is typically contained in a pipe. The third is in-situ where the sensor is directly inserted into the sample stream. The in-situ principle is intended for the monitoring of water during any step within a processing train, including immediately before or after the process itself.  
1.1.2 Static covers both benchtop and portable designs for the measurement of water samples that are captured into a cell and then measured.  
1.2 Depending on the monitoring goals and desired data requirements, certain technologies will deliver more desirable results for a given application. This guide will help the user align a technology to a given application with respect to best practices for data collection.  
1.3 Some designs are applicable for either a lower or upper measurement range. This guide will help provide guidance to the best-suited technologies based given range of turbidity.  
1.4 Modern electronic turbidimeters are comprised of many parts that can cause them to produce different results on samples. The wavelength of incident light used, detector type, detector angle, number of detectors (and angles), and optical pathlength are all design criteria that may be different among instruments. When these sensors are all calibra...

General Information

• Status: Published
• Publication Date: 31-Oct-2023
• ICS: 17.180.30 - Optical measuring instruments
• Technical Committee: D19 - Water
• Drafting Committee: D19.07 - Sediments, Geomorphology, and Open-Channel Flow

Relations

• Replaces: ASTM D7726-11(2016)e1 - Standard Guide for The Use of Various Turbidimeter Technologies for Measurement of Turbidity in Water
• Effective Date: 01-Nov-2023
• Refers: ASTM D7315-17(2023) - Standard Test Method for Determination of Turbidity Above 1 Turbidity Unit (TU) in Static Mode
• Effective Date: 01-Nov-2023
• Refers: ASTM D6855-17(2023) - Standard Test Method for Determination of Turbidity Below 5 NTU in Static Mode
• Effective Date: 01-Nov-2023
• Refers: ASTM D1129-13(2020)e2 - Standard Terminology Relating to Water
• Effective Date: 01-May-2020
• Refers: ASTM D1129-13(2020)e1 - Standard Terminology Relating to Water
• Effective Date: 01-May-2020
• Refers: ASTM D6855-17 - Standard Test Method for Determination of Turbidity Below 5 NTU in Static Mode
• Effective Date: 01-Nov-2017
• Refers: ASTM D7315-17 - Standard Test Method for Determination of Turbidity Above 1 Turbidity Unit (TU) in Static Mode
• Effective Date: 01-Jul-2017
• Referred By: ASTM D5978/D5978M-16 - Standard Guide for Maintenance and Rehabilitation of Groundwater Monitoring Wells
• Effective Date: 01-Nov-2023
• Referred By: ASTM D5131-23 - Standard Guide for Record Keeping for Electrodialysis/Electrodialysis Reversal Systems
• Effective Date: 01-Nov-2023
• Referred By: ASTM D5091-23 - Standard Guide for Water Analysis for Electrodialysis/Electrodialysis Reversal Applications
• Effective Date: 01-Nov-2023
• Referred By: ASTM D8002-15e1 - Standard Test Method for Modified Fouling Index (MFI-0.45) of Water (Withdrawn 2024)
• Effective Date: 01-Nov-2023

Overview

ASTM D7726-11(2023): Standard Guide for the Use of Various Turbidimeter Technologies for Measurement of Turbidity in Water is a comprehensive guide developed by ASTM International to help professionals select, implement, and interpret turbidimeter measurements across a wide range of water types. Turbidity, a measure of light scattering by particles in water, is a critical indicator for assessing water quality in contexts such as drinking water production, wastewater treatment, industrial applications, and environmental monitoring. This guide provides best practices for using modern turbidimeter technologies, considering their design, calibration methods, and application environments.

Key Topics

• Significance of Turbidity Measurement

  • Turbidity is a key metric for water quality, reflecting the amount of suspended particulate material.
  • Lower turbidity generally correlates with healthier water-based ecosystems and improved suitability for human consumption.

• Turbidimeter Technologies Covered

  • The document discusses the use of various turbidimeter designs, including both continuous (on-line) and static (benchtop or portable) models.
  • Key measurement setups: bypass/slipstream, in-line, and in-situ techniques.
  • Technologies covered include nephelometric, ratio-metric, near-IR, backscatter, surface scatter, attenuation-based, and multi-beam turbidimeters.

• Traceability and Calibration

  • Emphasizes the importance of calibrating instruments with primary standards, typically formazin-based, to ensure accurate and comparable turbidity readings.
  • Discusses the equivalence of turbidity units (TU, NTU, FNU, etc.) when calibrated against primary standards.

• Measurement Range and Interferences

  • Provides guidance on selecting technologies based on desired measurement range and expected sample characteristics.
  • Addresses common sources of error, including color, particle size distribution, bubbles, temperature, and instrument-related factors.

• Reporting and Metadata

  • Encourages proper documentation of measurement context, technology used, and calibration details to support data comparability and quality assurance, especially for regulatory reporting.

Applications

• Drinking Water Monitoring

  • Ensuring water is free of potentially harmful particulates and meets regulatory standards for turbidity.
  • Selecting highly sensitive nephelometric and ratio-metric technologies for compliance and quality control.

• Wastewater and Industrial Water

  • Monitoring treatment efficacy and the removal of suspended solids.
  • Utilizing high-range or robust in-line and in-situ turbidimeters to withstand challenging sample matrices.

• Environmental Monitoring

  • Assessing ecosystem health in lakes, rivers, and streams, where turbidity affects light penetration and biological processes.
  • Using portable or continuous turbidimeters for field measurements across diverse settings.

• Regulatory Compliance

  • Aligning technology selection and calibration approaches with local and international standards to support compliance with environmental and public health regulations.

Related Standards

Professionals utilizing ASTM D7726-11(2023) may also reference these closely related standards:

• ASTM D1129: Terminology Relating to Water
• ASTM D3977: Test Methods for Determining Sediment Concentration in Water Samples
• ASTM D6855: Test Method for Determination of Turbidity Below 5 NTU in Static Mode
• ASTM D7315: Test Method for Determination of Turbidity Above 1 Turbidity Unit (TU) in Static Mode
• ISO 7027: Water Quality - Determination of Turbidity

These standards provide additional test methods, terminology, and procedures that complement the guidance offered by ASTM D7726-11(2023).

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By applying ASTM D7726-11(2023), water quality professionals can optimize turbidity measurement practices, ensure data reliability across technology types, and better comply with regulatory and operational requirements. This standard is essential for those seeking to advance best practices in water quality monitoring and process control.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D7726 − 11 (Reapproved 2023)
Standard Guide for
The Use of Various Turbidimeter Technologies for
Measurement of Turbidity in Water
This standard is issued under the fixed designation D7726; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope pathlength are all design criteria that may be different among
instruments. When these sensors are all calibrated with the
1.1 This guide covers the best practices for use of various
sample turbidity standards, they will all read the standards the
turbidimeter designs for measurement of turbidity in waters
same. However, samples comprise of completely different
including: drinking water, wastewater, industrial waters, and
matrices and may measure quite differently among these
for regulatory and environmental monitoring. This guide cov-
different technologies.
ers both continuous and static measurements.
1.4.1 This guide does not provide calibration information
1.1.1 In principle there are three basic applications for
but rather will defer the user to the appropriate ASTM turbidity
on-line measurement set ups. The first is the bypass or
method and its calibration protocols. When calibrated on
slipstream technique; a portion of sample is transported from
traceable primary turbidity standards, the assigned turbidity
the process or sample stream and to the turbidimeter for
units such as those used in Table 1 are equivalent. For example,
analysis. It is then either transported back to the sample stream
a 1 NTU formazin standard is also equivalent in measurement
or to waste. The second is the in-line measurement; the sensor
magnitude to a 1 FNU, a 1 FAU, and a 1 BU standard and so
is submerged directly into the sample or process stream, which
forth.
is typically contained in a pipe. The third is in-situ where the
1.4.2 Improved traceability beyond the scope of this guide
sensor is directly inserted into the sample stream. The in-situ
may be practiced and would include the listing of the make and
principle is intended for the monitoring of water during any
model number of the instrument used to determine the turbidity
step within a processing train, including immediately before or
values.
after the process itself.
1.5 This guide does not purport to cover all available
1.1.2 Static covers both benchtop and portable designs for
technologies for high-level turbidity measurement.
the measurement of water samples that are captured into a cell
and then measured.
1.6 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this
1.2 Depending on the monitoring goals and desired data
standard.
requirements, certain technologies will deliver more desirable
results for a given application. This guide will help the user
1.7 This standard does not purport to address all of the
align a technology to a given application with respect to best
safety concerns, if any, associated with its use. It is the
practices for data collection.
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
1.3 Some designs are applicable for either a lower or upper
mine the applicability of regulatory limitations prior to use.
measurement range. This guide will help provide guidance to
1.8 This guide does not purport to address all of the safety
the best-suited technologies based given range of turbidity.
concerns, if any, associated with its use. It is the responsibility
1.4 Modern electronic turbidimeters are comprised of many
of the user of this standard to establish appropriate safety and
parts that can cause them to produce different results on
health practices and determine the applicability of regulatory
samples. The wavelength of incident light used, detector type,
limitations prior to use. Refer to the MSDSs for all chemicals
detector angle, number of detectors (and angles), and optical
used in this procedure.
1.9 This international standard was developed in accor-
dance with internationally recognized principles on standard-
This guide is under the jurisdiction of ASTM Committee D19 on Water and is
the direct responsibility of Subcommittee D19.07 on Sediments, Geomorphology,
ization established in the Decision on Principles for the
and Open-Channel Flow.
Development of International Standards, Guides and Recom-
Current edition approved Nov. 1, 2023. Published December 2023. Originally
ɛ1 mendations issued by the World Trade Organization Technical
approved in 2011. Last previous edition approved in 2016 as D7726 – 11 (2016) .
DOI: 10.1520/D7726-11R23. Barriers to Trade (TBT) Committee.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7726 − 11 (2023)
2. Referenced Documents 3.2.4.1 Discussion—Measurement intervals range from sec-
2 onds to months, depending on monitoring goals of a given site.
2.1 ASTM Standards:
D1129 Terminology Relating to Water 3.2.5 design, n—a more detailed technology description that
will encompass all of the elements making up a technology,
D3977 Test Methods for Determining Sediment Concentra-
tion in Water Samples plus any inherent criteria used to generate a specific turbidity
value.
D6698 Test Method for On-Line Measurement of Turbidity
Below 5 NTU in Water (Withdrawn 2023) 3.2.5.1 Discussion—The design will typically translate into
D6855 Test Method for Determination of Turbidity Below 5 a specific make or model of an instrument.
NTU in Static Mode
3.2.6 detection angle, n—the angle formed with its apex at
D7315 Test Method for Determination of Turbidity Above 1
the center of the analysis volume of the sample, and such that
Turbidity Unit (TU) in Static Mode
one vector coincides with the centerline of the incident light
2.2 Other References:
source’s emitted radiation and the second vector projects to the
USGS National Field Manual for the Collection of Water
center of the primary detector’s view.
Quality Data
3.2.6.1 Discussion—This angle is used for the differentia-
Wagner’s Field Manual Guidelines and Standard Procedures
tion of turbidity-measurement technologies that are used in this
for Continuous Water-Quality Monitors: Station
guide.
Operation, Record Computation, and Data Reporting
3.2.6.2 attenuation-detection angle, n—the angle that is
formed between the incident light source and the primary
3. Terminology
detector, and that is at exactly 0 degrees.
3.1 Definitions:
(1) Discussion—This is typically a transmission measure-
3.1.1 For definitions of terms used in this standard, refer to
ment.
Terminology D1129.
3.2.6.3 backscatter-detection angle, n—the angle that is
3.2 Definitions of Terms Specific to This Standard:
formed between the incident light source and the primary
3.2.1 calibration drift, n—the error that is the result of drift
detector, and that is greater than 90 degrees and up to 180
in the sensor reading from the last time the sensor was
degrees.
calibrated and is determined by the difference between
3.2.6.4 nephelometric-detection angle, n—the angle that is
cleaned-sensor readings in calibration standards and the true,
formed between the incident light source and the detector, and
temperature-compensated value of the calibration standards.
that is at 90 degrees.
3.2.6.5 forward-scatter-detection angle, n—the angle that is
3.2.2 calibration turbidity standard, n—a turbidity standard
formed between the incident light source and the primary
that is traceable and equivalent to the reference turbidity
detector, and that is greater than 0 degrees but less than 90
standard to within statistical errors; calibration turbidity stan-
degrees.
dards include commercially prepared 4000 NTU Formazin,
(1) Discussion—Most designs will have an angle between
stabilized formazin, and styrenedivinylbenzene (SDVB).
135 degrees and 180 degrees.
3.2.2.1 Discussion—These standards may be used to cali-
3.2.6.6 surface-scatter detection, n—a turbidity measure-
brate the instrument.
ment that is determined through the detection of light scatter
3.2.3 calibration-verification standards, n—defined stan-
caused by particles within a defined volume beneath the
dards used to verify the accuracy of a calibration in the
surface of a sample.
measurement range of interest.
(1) Discussion—Both the light source and detector are
3.2.3.1 Discussion—These standards may not be used to
positioned above the surface of the sample. The angle formed
perform calibrations, only calibration verifications. Included
between the centerline of the light source and detector is
verification standards are opto-mechanical light-scatter
typically at 90 degrees. Particles at the surface and in a volume
devices, gel-like standards, or any other type of stable-liquid
below the surface of the sample contribute to the turbidity
standard.
reading.
3.2.4 continuous, adj—the type of automated measurement
3.2.7 fouling, v—the measurement error that can result from
at a defined-time interval, where no human interaction is
a variety of sources and is determined by the difference
required to collect and log measurements.
between sensor measurements in the environment before and
after the sensors are cleaned.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
3.2.8 in-situ nephelometer, n—a turbidimeter that deter-
Standards volume information, refer to the standard’s Document Summary page on
mines the turbidity of a sample using a sensor that is placed
the ASTM website.
3 directly in the sample.
The last approved version of this historical standard is referenced on
www.astm.org.
3.2.8.1 Discussion—This turbidimeter does not require
Available from United States Geological Survey (USGS), USGS Headquarters,
transport of the sample to or from the sensor.
12201 Sunrise Valley Drive, Reston, VA 20192, http://www.usgs.gov/FieldManual/
Chapters6/6.7.htm.
3.2.9 metadata, n—the ancillary descriptive information
Wagner, R. J., et al, Guidelines and Standard Procedures for Continuous
that describes instrument, sample, and ambient conditions
Water-Quality Monitors: Station Operation, Record Computation, and Data
under which data were collected.
Reporting, USGS Enterprise Publishing Network, 2005, available from: http://
pubs.usgs.gov/tm/2006/tm1D3. 3.2.9.1 Discussion—Metadata provide information about
D7726 − 11 (2023)
data sets. An example is the useful background information 3.2.19 turbidimeter, n—an instrument that measures light
regarding the sampling site, instrument setup, and calibration scatter caused by particulates within a sample and converts the
and verification results for a given set of turbidity data measurement to a turbidity value.
(especially when data are critically reviewed or compared 3.2.19.1 Discussion—The detected light is quantitatively
against another data set). converted to a numeric value that is traced to a light-scatter
standard. See Test Method D7315.
3.2.10 nephelometric-turbidity measurement, n—the mea-
surement of light scatter from a sample in a direction that is at 3.2.20 turbidity, n—an expression of the optical properties
90° with respect to the centerline of the incident-light path. of a sample that causes light rays to be scattered and absorbed
rather than transmitted in straight lines through the sample.
3.2.10.1 Discussion—Units are NTU (Nephelometric Tur-
bidity Units). When ISO 7027 technology is employed units 3.2.20.1 Discussion—Turbidity of water is caused by the
presence of matter such as clay, silt, finely divided organic
are FNU (Formazin Nephelometric Units).
matter, plankton, other microscopic organisms, organic acids,
3.2.11 pathlength, n—The greatest distance that the sum of
and dyes.
the incident light and scattered light can travel within a sample
volume (cell or view volume).
4. Summary of Practice
3.2.11.1 Discussion—The pathlength is typically measured
4.1 This guide is to assist the user in meeting and under-
along the centerline of the incident-light beam plus the
standing the following criteria with respect to turbidity mea-
scattered light. The pathlength includes only the distance the
surements:
light and scattered light travel within the sample itself.
4.1.1 The selection of the appropriate technology for mea-
3.2.12 ratio-turbidity measurement, n—the measurement
surement of a given sample with implied characteristics.
derived through the use of a nephelometric detector that serves
4.1.2 Help in the selection of a measurement technology
as the primary detector, and one or more other detectors used
that will help meet the scope of requirements (goals) for use of
to compensate for variation in incidentlight fluctuation, stray
the data.
light, instrument noise, or sample color.
4.1.3 Assist in the selection of a technology that is best
3.2.13 reference-turbidity standard, n—a standard that is
suited to withstand the expected environmental and sample
synthesized reproducibly from traceable raw materials by the
deviations over the course of data collection. Examples of
user.
deviations would be expected measurement range and interfer-
3.2.13.1 Discussion—All other standards are traced back to
ences.
this standard. The reference standard for turbidity is formazin.
4.1.4 Understand both the general strengths and limitations
for a given type (design) of technology in relation to overcom-
3.2.14 seasoning, v—the process of conditioning labware
ing known interferences in turbidity measurement.
with the standard that will be diluted to a lower value to reduce
4.1.5 Provide general procedures that can be used to deter-
contamination and dilution errors.
mine whether a given technology is suitable for use in a given
3.2.15 slipstream, n—an on-line technique for analysis of a
sample or a given application.
sample as it flows through a measurement chamber of an
4.1.6 Understand the need for the user to include critical
instrument.
metadata related to turbidity measurement.
3.2.15.1 Discussion—The sample is transported from the
4.1.7 This guide will help the user select the appropriate
source into the instrument (for example, a turbidimeter),
technology for regulatory purposes.
analyzed, and then transported to drain or back to the process
stream. The term is synonymous with the terms “on-line
5. Significance and Use
instrument” or “continuous monitoring instrument.”
5.1 Turbidity is a measure of scattered light that results from
3.2.16 sonde, n—a monitoring instrument that contains two
the interaction between a beam of light and particulate material
or more measurement sensors that share common power,
in a liquid sample. Particulate material is typically undesirable
transmitting, and data logging.
in water from a health perspective and its removal is often
3.2.16.1 Discussion—A sonde usually has one end that
required when the water is intended for consumption. Thus,
contains the measurement sensors, which are in close proxim-
turbidity has been used as a key indicator for water quality to
ity to each other and together are submerged in a sample.
assess the health and quality of environmental water sources.
Higher turbidity values are typically associated with poorer
3.2.17 stray light, n—all light reaching the detector other
than that contributed by the sample. water quality.
5.1.1 Turbidity is also used in environmental monitoring to
3.2.18 technology, n—a general classification of a turbidi-
assess the health and stability of water-based ecosystems such
meter design that incorporates the type and wavelength of the
as in lakes, rivers and streams. In general, the lower the
incident-light source, detection angles, and the number of
turbidity, the healthier the ecosystem.
detectors used to generate a turbidity measurement and its
defined reporting unit. 5.2 Turbidity measurement is a qualitative parameter for
3.2.18.1 Discussion—In ASTM turbidity test methods, the water but its traceability to a primary light scatter standard
technology is based on type and number of light sources, and allows the measurement to be applied as a quantitative mea-
their respective wavelength, detector angle(s), and number of surement. When used as a quantative measurement, turbidity is
detectors used in the technology to generate the turbidity value. typically reported generically in turbidity units (TUs).
D7726 − 11 (2023)
5.2.1 Turbidity measurements are based on the instruments’ and refer to Table 1), instruments within a group should not be
calibration with primary standard reference materials. These considered to be identical nor it is proposed that sample values
reference standards are traceable to formazin concentrate obtained will be alike. Instruments within each technology may
(normally at a value of 4000 TU). The reference concentrate is still have other design differences whereby samples give
linearly diluted to provide calibration standard values. Alter- different results. For example, pathlength differences between
native standard reference materials, such as SDVB co-polymer two instruments with the same reporting units can impact
or stabilized formazin, are manufactured to match the formazin measurements and the relative difference in results.
polymer dilutions and provide highly consistent and stable
5.4 Discussion of Table 1:
values for which to calibrate turbidity sensors.
5.4.1 Table 1 provides a summary of technologies and their
5.2.1.1 When used for regulatory compliance reporting,
respective reporting units that are in the different ASTM test
specific turbidity calibration standards may be required. The
methods. The reporting unit is a two to four letter-code that has
user of this guide should check with regulatory entities
been assigned to a unique type of technology. The reporting
regarding specifics of allowable calibration standard materials.
unit follows every reported turbidity measurement and serves
5.2.2 The traceability to calibrations from different tech-
as metadata to the respective measurement.
nologies (and other calibration standards) to primary formazin
5.4.2 The key design features are based on three criteria: (1)
standards provides for a basis for defined turbidity units. This
type of light source used, (2) primary detector angle with
provides equivalence in the magnitude of the turbidity unit
respect to the incident light beam, and (3) number of detectors
between the different measurement technologies when they are
used.
all calibrated on standards that are traced to primary formazin.
5.4.2.1 If the measurement unit begins with an “F” then the
This means that a TU is equivalent in its magnitude to a
light source is a near-IR wavelength. Most designs will
nephelometric turbidity unit (NTU), and all other units as
encompass a light source that is in the 860 6 60 nm range. The
described in this guide. See Table 1.
strength of this wavelength is that most natural colors do not
5.2.3 Turbidity is not an inherent property of the sample,
absorb at this level, which reduces or eliminates color inter-
such as temperature, but in part is dependent on the technology
ference. Two things that interfere at the near-IR are carbon
used to derive the value. Even though the magnitude of
black and copper sulfate. Second, the incident light beams are
turbidity units are equivalent and are based on turbidity
easily collimated, which extends the overall operational range.
standards, the units do not maintain this equivalence when
Third, the output of the light source can be regulated to provide
measurement of samples is practiced. Turbidity standards are
a stable output over time. The weakness is that longer
generally free of interferences and samples are not. Depending
wavelengths are less sensitive to smaller particles with respect
on the type of technology employed for measurement, the
to response at very low turbidities.
magnitude of the different interferences on a given sample can
5.4.2.2 If the measurement unit either begins with an “N” or
differ significantly with respect to the different measurement
is a two-letter unit (for example, BU, AU), the incident light
technologies. The user of a turbidity technology should expect
source will be in the 400–680-nm range. The strength of this
to observe a lack of measurement equivalence across different
wavelength range is increased sensitivity to smaller particles
turbidity measurement designs when common samples are
when compared to longer wavelengths (such as those in the
analyzed. See Section 6 on interferences.
near infared (IR) range). The weakness of this wavelength
5.2.4 Depending on the application, some instruments are
range is that color that absorbs at the same wavelengths, as
calibrated on a sample that has been characterized (or defined)
those that are emitted by light source will cause a negative
by some independent means. The calibration may include one
interference. Second, if the source is an incandescent light
or more samples that have been characterized with respect to
source, additional optics is required to maintain collimation
the application of its use. See Test Methods D3977.
and stability over time. The light source will typically need to
5.3 Turbidity is not a quantative measure of any chemical or periodic replacement over the life of an instrument.
physical property of water. Different expected interactions
5.4.2.3 If the measurement unit includes an “R” it is a
between a given measurement technology and a given sample
nephelometric method that utilizes a 90-degree detector plus
with a unique combination of interferences can significantly
one other detector. This is referred to as a ratio metric
impact the final turbidity result. As stated in 5.3, depending on technique and helps to compensate for color interference,
the technology used, the result will differ. It is imperative to
regardless of the wavelength of the incident light source. The
provide a linkage of metadata that is reflective of the design technique also helps to linearize the response to turbidity at
type (that is, technology) used to generate the turbidity values.
higher levels and can provide an extended measurement range.
In all ASTM standards, the measurement units are reflective of The technique can also help to stabilize measurement outputs.
the design criteria and the information is presented in Table 1.
The technique is the most flexible across different applications
5.3.1 The actual reporting units, signified by a two to because of the combination of sensitivity to low turbidity
four-letter code, are based upon distinguishing design criteria ranges and the ability can measure very high turbidity levels.
for each of the common measurement technologies. The intent
5.4.2.4 If the measurement unit has a “B” it indicates a
of attaching the measurement unit to the determined turbidity
backscatter technique. These techniques typically have a wide
value is to indicate the type of technology used.
range, but are not sensitive at low turbidities. They are also
5.3.2 Even though various instrument designs may be more susceptible to color and particulate absorbance interfer-
grouped by technology type (that is, FNU, NTU, FBU, etc., ences.
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5.4.2.5 If the measurement unit has an “A” it indicates an (b) SSU—The “SS” portion of the unit indicates a surface
attenuation or absorbance measurement. The measurement is a scatter technique is being used. The technique positions both
the light source and detector that are in the same horizontal
combination of light that is attenuated and absorbed, in
plane above the sample. Light that is scattered by particles at or
combination. Color is a significant interference, except for
very near the surface and detected at an angle that is at 90
applications that require color to be considered part of the
degrees to the centerline of the incident light beam. The system
overall turbidity measurement. The method is very sensitive to
has a high detection range, but low sensitivity. It is also
wavelength and thus, the reporting unit should also include the
susceptible to color interferences, but to a lesser degree than
wavelength of the incident light beam.
techniques that pass light completely through the sample. The
5.4.2.6 If the measurement unit contains an “M” it indicates
technique is valuable for applications where it is desirable for
a technology in which at least two incident light beams and two
the sample not to touch the optics of the instrument.
detectors are employed. The method also encompasses a ratio
5.4.3 The table provides information regarding to the most
technique. These designs are very similar to ratio techniques as
prominent applications and discusses interference concerns.
are the advantages and limitations.
This information is based on technologies that are in the field
5.4.2.7 Other Units:
at the time this guide was written, but does not constitute
(a) mNTU—The technology indicates a monochromatic
endorsement to any given manufacture of a given technology.
incident light source in visible wavelength range and a neph-
In some cases, a design can be successfully used outside of the
elometric technique. The technology design allows for an
stated applications in Table 1. The user should perform testing
improved limit of detection over conventional light sources. Its
to ensure the technology meets limit of detection, sensitivity,
primary use for low turbidity measurements, such the moni- and range requirements that insure representative data can be
toring for membrane breaches and ultra-purification processes. acquired.
TABLE 1 Summary of Known Commercialized Technologies, Key Design Features, Prominent Sample Applications, Ranges, and
Reporting Units for Turbidity Measurements
Turbidity Reporting
Unit Used in Turbidity Reporting Unit Prominent Application and Suggested Application and
ASTM Test Used in ASTM Test Methods Major Interference Concerns Operating Range Ranges
Methods
Nephelometric The detector is centered at 90º relative to White light turbidimeters. These designs Regulatory for drinking water. The optimal
Non-Ratio (NTU) the incident light beam. The incident light comply with EPA 180.1 for low level turbidity operating range is 0.0 to 40 units if the sample
source is a tungsten filament lamp that is monitoring. Color is a major has no color. Best comparability will be at
operated at a color temperature between negativeinterference and optical variations turbidities below 5 TU.
2200 and 3000 K. cannot be compensated with this technique.
Ratio White This technology applies the same light Complies with the USEPA Interim Enhanced Regulatory for drinking water and wastewater
LightTurbidimeters source as the EPA 180.1 design but uses Surface Water Treatment Rule regulations and (0–40 units). The technology can potentially
(NTRU) several detectors in the measurement. A Standard Methods 2130B. Can be used for measure up to 10 000 units.
primary detector centered at 90° relative to both low and high level measurement. Color
the incident beam plus other detectors interference (negative) is reduced and lamp
located at other angles. An instrument variations are compensated for with this
algorithm uses a combination of detector technique.
readings to generate the turbidity reading.
Nephelometric, This technology uses a light source in the The instrument design is compliant with ISO Regulatory compliance in Europe for drinking
Near- IR near IR range (860 ± 60 nm). The detector 7027. The wavelength is less susceptible to water and wastewater (0–40 units). The
Turbidimeters, is centered at 90º relative to the incident color interferences. The light source is very technology can measure up to 1000 units or
Nonratiometric light beam. stable over time because its output can be more, depending on pathlength.
(FNU) highly controlled. This technique is applicable
for samples with color and for low level
monitoring. Only highly samples that absorb
light above 800 nm can result in negative
interference.
Nephelometric This technology applies the same light Complies with ISO 7027. This technique is Regulatory compliance monitoring in Europe for
Near-IR source that is required by ISO 7027. The applicable for samples with high levels of color drinking water and wastewater (0–40 units). The
Turbidimeters, design uses several detectors in the and for monitoring to high turbidity levels. technology can potentially measure up to 10000
Ratio Metric measurement. A primary detector is Samples that absorb light above 800 nm will units.
(FNRU) centered at 90° relative to the incident result in some negative interference.
beam and other detectors are located at
other angles. An instrument algorithm uses
the combination of detector readings to
generate the turbidity value.
Surface Scatter The technology uses the same white light Turbidity is determined through light scatter Sample flows through the instrument. This is a
Turbidimeters source as in EPA 180.1. The detector from a defined volume of sample beneath the good watershed monitoring instrument and can
(SSU) centered at 90º relative to the incident light surface of a sample. Negative color measure from 0.5 to 10,000 units.
beam. Both the detector an incident light interferences are reduced when compared to
source is mounted in a fixed position in the the non-ratio nephelometric method.
same plane that is immediately above the
sample.
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TABLE 1 Continued
Turbidity Reporting
Unit Used in Turbidity Reporting Unit Prominent Application and Suggested Application and
ASTM Test Used in ASTM Test Methods Major Interference Concerns Operating Range Ranges
Methods
Formazin Back This design applies a near-IR This technology is not applicable for most This technology is best suited for in-situ
Scatter (FBU) monochromatic light source in the (860 ± regulatory purposes. It is best applied to measurement, in which a probe is placed in a
60 nm) nm range as the incident light samples with high turbidity and is commonly sample for continuous monitoring purposes. It is
source. The scattered light detector is used in trending applications. Absorbance and best applied to turbidities in the range of 100–10
typically positioned at 30 ± 15° relative to color above 800 nm will result in negative 000+ unit range.
the incident light beam. However, some interference
designs may have a detection angle that is
approximately 0° relative to the incident
light beam.
Backscatter Unit The design applies a white light spectral This technology is not applicable for most This technology is best suited for in-situ
(BU) source (400–680 nm range). The detector regulatory purposes. It is best applied to measurements in which sample color is part of the
geometry is between 90 and 180° relative samples with high turbidity. The measurement turbidity measurement. It is best applied to
to the incident light beam. will be susceptible to any visible color and turbidities in the 100–10 000+ unit range.
particle absorbance that will result in a
negative interference.
Formazin The incident light beam is at a wavelength The design may be applicable for some This measurement is part of the ISO 7027
Attenuation Unit of (860 ± 60 nm) nm. The detector is regulatory purposes. The measurement is regulation. The optimal turbidity range is between
(FAU) geometrically centered at 0° relative to the commonly performed with spectrophotometers. 20 and 1000 units.
incident light beam. This is typically an It is best suited for samples with high-level
attenuation measurement. turbidity. Particle absorption is a prominent
interference.
Light Attenuation The wavelength of the incident light is in This design is not applicable for some This is best applied to samples in which color is
Unit (AU) the 400–680 nm range. The light scatter regulatory applications. This is commonly part of the turbidity measurement. The best
detector is geometrically centered at 0º performed with spectrophotometers. Color and application is to samples in the turbidity range of
relative to incident beam. This is an absorption are prominent interferences if their 20 to 1000 units.
transmission measurement. respective absorptive spectrum is the same as
the output spectrum of the incident light.
Nephelometric The technology consists of two light This technology is compliant to the EPA Regulatory monitoring at low turbidity levels in the
Turbidity sources and two detectors. The light Regulatory Method GLI Method 2 and ISO 0.02 to 40 unit range. The technology can
Multibeam Unit sources comply with ISO 7027. The 7027. It is applicable to regulatory monitoring measure up to 4000 units.
(FNMU) detectors are geometrically centered at 90° for drinking water, wastewater, and industrial
relative to each incident light beam. The monitoring applications. The technology is very
instrument measures in two phases in stable. This technology will be immune to color
which the detectors are either at 90 or 180º absorbance below 800 nm. Above 800 nm,
relative to the incident light beam, color and particle absorbance interferences
depending on the phase. An instrument will be reduced.
algorithm uses a combination of detector
readings to calculate the reported value.
Laser Turbidity The technology consists of an incident laser This technique complies with the EPA- Regulatory monitoring of drinking water effluent
Units (mNTU) light source at 660 nm and a detector that approved Hach Method 10133. The application and membrane systems. The range is about 7 to
is a high-sensitivity PMT design. The is for the monitoring of filter performance and 5000 mNTU. 1 NTU = 1000 mNTU.
detector is centered at 90º relative to the breakthrough. Color interference can occur it
incident light beam. absorbs the same wavelength of light that is
emitted by the incident light source. However,
color is typically significant in filtered samples.
Forward Scatter The technology encompasses a single light The technology is sensitive to turbidities as The measurement of ambient waters such as
Ratio Unit (FSRU) source and two detectors. Light sources low as 1 TU. The ratio technology helps to streams, lakes, rivers. The range is typically from
can vary from single wavelength to compensate for color interference and fouling. about 1–800 FSRU, depending on the
polychromatic sources. The detection angle manufacturer.
for the forward scatter detector is between
0 and 90º relative to the centerline of the
incident light beam. More commonly, the
forward scatter detection angle will be
between 15 and 45º. The second detector
is at exactly 0º.
5.4.4 Range of Measurement—Table 1 provides guidance on becomes detected. It encompasses both the incident light
the estimated range of use for the different measurement distance and the receive angles for the scattered light detectors.
technologies. A key design criterion is the pathlength of
The longer the pathlength, the lower the measurement ranges,
measurement. This is the actual distance that light travels
through a sample to generate the scatter that ultimately
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TABLE 2 Typical Interferences Associated with Turbidity Measurement
Typical Interferences that Originate from the Sample and their Impact on the Turbidity Measurement
Absorbing Particles (Colored) Negative bias (reported measurement is lower than actual turbidity)
Color in the Matrix Negative bias if the incident light wavelengths overlap the absorptive spectra of a sample
Bubbles Positive or negative bias (reported measurement is higher than the actual turbidity) and can impact measurement accuracy at all
turbidity levels. Depending on the location of bubbles with respect to the optical elements and the technology being employed, the
interferences can be either.
Particle Size Distribution Particle size distribution can be considered a interference but is typically considered an inherent part of the sample. The particle-size
distribution in a sample, and operating spectrum will affect the relative sensitivity of turbidimeters. The intensity of light scattered from
a water sample depends, among other factors, on the ratio of particle diameter to light wavelength. Since the operating wavelength of
a turbidimeter is fixed, particle size is the controlling variable.
Particle Density Negative bias (reported measurement is lower than the actual turbidity)
Temperature Sample temperatures can cause fogging of optical windows or of the sample cell. Depending on which component in a given design
becomes fogged, the resultant interference can be either positive or negative. Some systems have features that can compensate for
fogging, which include the use of desiccants, wipers, or anti-fogging materials. Such features must be demonstrated to perform as
expected as they can otherwise result in measurement error.Changes in sample temperature can also impact the inherent turbidity of
the sample. A sample that increases in temperature after collection can impact particulate solubility or increase microbiological activity,
thereby impacting the turbidity of the sample.
Instrument Based Interferences and their Impact on Turbidity Measurement
Optical Variation Degradation of instrument optical components can have both positive and negative impacts on measurement, but bias is usually
negative.
Sample Cell Variations Either positive or negative bias. This can be minimized through the use of matching and indexing techniques and the application of
silicone oils to reduce reflections due to scratches. The impact of this interference is most severe at turbidity values below 0.1
turbidity units.
Particle Settling Positive or negative bias can result due to the rapid settling of particles and depending on the length of time to perform a
measurement. This is typically associated with grab sample, and laboratory/portable benchtop measurements.
Stray Light Positive bias (reported measurement is slightly higher than the actual turbidity). Stray light has the most significant impact at turbidity
levels below 0.1 turbidity units.
Temperature The stability of certain instrument components can change with temperature. Most modern instruments have internal compensation
features for changes in ambient temperature over a defined range. Check with instrument manufacturers to determine the
temperature for the operational range their respective technology.
Contamination Positive bias (reported measurement is higher than actual turbidity). This is caused by dust contamination on optical surfaces that
cannot be easily cleaned. This is most prominent on laboratory and portable turbidimeters.
but the better the sensitivity. Shorter pathlengths may provide 6.3 Color is sometimes considered an interference, and
a greater range, but a poorer sensitivity and a poorer the limit other times it is not. It is dependent upon the application. For
of detection. example, when performing compliance monitoring for drink-
ing water, color is considered an interference and certain
6. Interferences in Turbidity
measurement techniques will help to reduce its effects. An
application where color is not considered interference would be
6.1 The measurement of turbidity is subject to a combina-
tion of different interferences. Some interferences are inherent the monitoring of a natural water to determine the effectiveness
on the underwater vision for aquatic predators and prey. In this
with the sample itself and others are instrument-based. Table 2
summarizes these interferences. application color is considered to be part of turbidity because
the application relates the effectiveness of underwater vision
6.2 Turbidity interferences will either cause positive or
for the aquatic species. The majority of applications however,
negative bias. Negative bias results in a measurement being
color is considered an interference and typically causes false
below the true turbidity and is typically associated with
negative bias.
measurements greater than 1 TU and can become more
significant as the turbidity of a sample increases. Positive bias 6.4 Summary—The minimization of interferences will im-
interferences are typically associated with extremely low prove measurement reliability. Several different turbidity mea-
turbidity measurements, where stray light becomes a factor in surement methods (that is, instrument designs or technologies)
measurement. Stray light and particulate contamination on have evolved to address one or a combination of interferences
optical surfaces can cause positive interferences and is most and to meet specific monitoring criteria for a particular
prevalent at levels below 0.1 TU. These levels are representa- application. For example, some designs are intended to maxi-
tive in applications involving in highly pure waters. mize sensitivity to turbidity on the cleanest of waters. Other
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TABLE 3 Prework Prior to Technology Selection
Measurement Considerations Questions to Consider
The purpose for the turbidity Process Control? Regulatory Compliance or Assessment, or both? Natural event monitoring?
measurement
The physical location of the Outdoors versus indoors? Remote versus a local site?
instrumentation
Frequency of site attendance Does the monitoring require a specific level of attentiveness? What is the necessary level of frequency?
The measurement sensor application Is the measurement in the environment, in situ, in-line, or in process (on-line)?
and location
Additional desired measurement What other parameters are required? For example, pH, DO, temperature, flow, etc. Will a sonde be used?
parameters
Sample disposal How will the sample be disposed? Sanitary drain? Run the sample back to the source? Sample treatment prior to
disposal? Secondary processing?
Special considerations What are those considerations that should be addressed to insure accurate and reliable measurement?
Ambient light considerations Will ambient light interfere? Will sunlight/moonlight impact the site? Will reflections cause interference? What is the
direction of the interference (positive or negative)?
Biological activity Will my sample be prone to biological activity? What will the impacts of the biology in the sample be on the sensor? Will a
sensor with compensating features (for example, a wiper) be needed?
Temperature What are the expected sample and ambient temperature changes? Will the technology meet the ambient and sample
temperature ranges?
Regulatory compliance Is the measurement subject to regulatory reporting? And if so, to what regulatory reporting method?
designs minimize the effects of an interference such as color. portion in view of the optical detectors for a given technology,
Many designs have features such as bubble traps or bubble which is measured to generate a result. For a static
rejection software to minimize bubble interference. Other measurement, is that portion of a sample stream or process that
methods have been developed to function in a specific type of is collected for measurement.
application or over a discreet turbidity range. Depending on the
7.2 Empirical Sample Assessment—Whenever possible, the
characteristics within a sample and the measurement technol-
sample should be assessed to help determine the best technol-
ogy that was applied, the various components of the turbidity
ogy fit for monitoring a sample. Empirical characterization
measurement and the inherent interferences within the sample
helps to identify interferences that may be reduced or elimi-
can impact the reported value. Different technologies often
nated. Table 4 provides a general list of questions that focus on
produce different turbidity values on the same sample.
potential interferences from a sample. Table 4 also provides
6.5 The combination of a sample’s respective general guidance on the type of technology that may or may
characteristics, its inherent interferences, and these interactions not be suitable for a given interference.
with a given measurement technology can have a significant
impact on resultant turbidity values that are generated. A 8. Equipment Technologies
sample may contain an interference tha
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