ASTM D7726-11(2023)
(Guide)Standard Guide for The Use of Various Turbidimeter Technologies for Measurement of Turbidity in Water
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...
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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 service@astm.org. 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
...
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