ASTM D7937-15(2023)
(Test Method)Standard Test Method for In-situ Determination of Turbidity Above 1 Turbidity Unit (TU) in Surface Water
Standard Test Method for In-situ Determination of Turbidity Above 1 Turbidity Unit (TU) in Surface Water
SIGNIFICANCE AND USE
5.1 Turbidity is monitored to help control processes, monitor the health and biology of aquatic environments and to determine the impact of environmental events such as storms, floods, runoff, etc. Turbidity is undesirable in drinking water, plant-effluent waters, water for food and beverage production, and for a large number of other water-dependent manufacturing processes. Turbidity is often reduced by coagulation, sedimentation and water filtration. The measurement of turbidity may indicate the presence of particle-bound contaminants and is vital for monitoring the completion of a particle-waste settling process. Significant uses of turbidity measurements include:
5.1.1 Compliance with permits, water-quality guidelines, and regulations;
5.1.2 Determination of transport and fate of particles and associated contaminants in aquatic systems;
5.1.3 Conservation, protection and restoration of surface waters;
5.1.4 Measure performance of water and land-use management;
5.1.5 Monitor waterside construction, mining, and dredging operations;
5.1.6 Characterization of wastewater and energy-production effluents;
5.1.7 Tracking water-well completion including development and use; and
5.1.8 As a surrogate for other constituents in water including sediment and sediment-associated constituents.
5.2 The calibration range of a turbidimeter shall exceed the expected range of TU values for an application but shall not exceed the measurement range specified by the manufacturer.
5.3 Designs described in this standard detect and respond to a combination of relative absorption, intensity of light scattering, and transmittance. However, they do not measure these absolute physical units as defined in 3.2.15 and 3.2.19.
5.4 Several different turbidimeter designs may be used for this test method and one design may be better suited for a specific type of sample or monitoring application than another. The selection flowchart in Annex A1 provides guidance for th...
SCOPE
1.1 This test method covers the in-situ field measurements of turbidity in surface water. The measurement range is greater than 1 TU and the lesser of 10 000 TU or the maximum measurable TU value specified by the turbidimeter manufacturer.
1.1.1 Precision data was conducted on both real world and surrogate turbidity samples up to about 1000 TU. Many of the technologies listed in this test method are capable of measuring above that provided in the precision section (see Section 16).
1.2 “In-situ measurement” refers in this test method to applications where the turbidimeter sensor is placed directly in the surface water in the field and does not require transport of a sample to or from the sensor. Surface water refers to springs, lakes, reservoirs, settling ponds, streams and rivers, estuaries, and the ocean.
1.3 Many of the turbidity units and instrument designs covered in this test method are numerically equivalent in calibration when a common calibration standard is applied across those designs listed in Table 1. Measurement of a common calibration standard of a defined value will also produce equivalent results across these technologies. This test method prescribes the assignment of a determined turbidity values to the technology used to determine those values. Numerical equivalence to turbidity standards is observed between different technologies but is not expected across a common sample. Improved traceability beyond the scope of this test method may be practiced and would include the listing of the make and model number of the instrument used to determine the turbidity values.
1.4 In this test method, calibration standards are often defined in NTU values, but the other assigned turbidity units, such as those in Table 1 are equivalent. For example, a 1 NTU formazin standard is also a 1 FNU, a 1 FAU, a 1 BU, and so forth.
1.5 This test method was tested on different natural waters and with standards th...
General Information
Relations
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: D7937 − 15 (Reapproved 2023)
Standard Test Method for
In-situ Determination of Turbidity Above 1 Turbidity Unit
(TU) in Surface Water
This standard is issued under the fixed designation D7937; 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 such as those in Table 1 are equivalent. For example, a 1 NTU
formazin standard is also a 1 FNU, a 1 FAU, a 1 BU, and so
1.1 This test method covers the in-situ field measurements
forth.
of turbidity in surface water. The measurement range is greater
than 1 TU and the lesser of 10 000 TU or the maximum 1.5 This test method was tested on different natural waters
measurable TU value specified by the turbidimeter manufac- and with standards that served as surrogates for samples. It is
turer. recommended to validate the method response for waters of
1.1.1 Precision data was conducted on both real world and untested matrices.
surrogate turbidity samples up to about 1000 TU. Many of the
1.6 This standard does not purport to address all of the
technologies listed in this test method are capable of measuring
safety concerns, if any, associated with its use. It is the
above that provided in the precision section (see Section 16).
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
1.2 “In-situ measurement” refers in this test method to
mine the applicability of regulatory limitations prior to use.
applications where the turbidimeter sensor is placed directly in
1.7 This international standard was developed in accor-
the surface water in the field and does not require transport of
dance with internationally recognized principles on standard-
a sample to or from the sensor. Surface water refers to springs,
ization established in the Decision on Principles for the
lakes, reservoirs, settling ponds, streams and rivers, estuaries,
Development of International Standards, Guides and Recom-
and the ocean.
mendations issued by the World Trade Organization Technical
1.3 Many of the turbidity units and instrument designs
Barriers to Trade (TBT) Committee.
covered in this test method are numerically equivalent in
calibration when a common calibration standard is applied
2. Referenced Documents
across those designs listed in Table 1. Measurement of a
2.1 ASTM Standards:
common calibration standard of a defined value will also
D1129 Terminology Relating to Water
produce equivalent results across these technologies. This test
D1193 Specification for Reagent Water
method prescribes the assignment of a determined turbidity
D2777 Practice for Determination of Precision and Bias of
values to the technology used to determine those values.
Applicable Test Methods of Committee D19 on Water
Numerical equivalence to turbidity standards is observed
D3864 Guide for On-Line Monitoring Systems for Water
between different technologies but is not expected across a
Analysis
common sample. Improved traceability beyond the scope of
D4411 Guide for Sampling Fluvial Sediment in Motion
this test method may be practiced and would include the listing
D7315 Test Method for Determination of Turbidity Above 1
of the make and model number of the instrument used to
Turbidity Unit (TU) in Static Mode
determine the turbidity values.
E177 Practice for Use of the Terms Precision and Bias in
1.4 In this test method, calibration standards are often
ASTM Test Methods
defined in NTU values, but the other assigned turbidity units,
E691 Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method
This test method is under the jurisdiction of ASTM Committee D19 on Water
and is the direct responsibility of Subcommittee D19.07 on Sediments,
Geomorphology, and Open-Channel Flow. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved July 1, 2023. Published July 2023. Originally approved contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
in 2015. Last previous edition approved in 2015 as D7937 – 15. DOI: 10.1520/ Standards volume information, refer to the standard’s Document Summary page on
D7937-15R23. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7937 − 15 (2023)
TABLE 1 Summary of Known in-situ Instrument Designs, Applications, Ranges, and Reporting Units
Design and Reporting Unit Prominent Application Key Design Features Typical Instrument Range Suggested Application Ranges
Nephelometric Non-Ratio (NTU) White light turbidimeters Detector centered at 90° rela- 0.0–40 0.0–40 Regulatory
Comply with EPA 180.1 for low tive to the incident light beam.
level turbidity monitoring. Uses a white light spectral
source.
Ratio White Light Turbidimeters Complies with U.S. EPA regula- Used a white light spectral 0–10 000 0–40 Regulatory
(NTRU) tions and EPA 2130B. Can be source. Primary detector cen- 0–10 000 other
used for both low and high level tered at 90°. Other detectors
measurement. located at other angles. An in-
strument algorithm uses a com-
bination of detector readings to
generate the turbidity reading.
Nephelometric, Near-IR Complies with ISO 7027. The Detector centered at 90° rela- 0–1 000 0–40 Regulatory (non-US)
Turbidimeters, Non- Ratiometric wavelength is less susceptible tive to the incident light beam. 0–1 000 other
(FNU) to color interferences. Appli- Uses a near-IR (780-900 nm)
cable for samples with color monochromatic light source.
and good for low level monitor-
ing.
Nephelometric Near-IR Complies with ISO 7027. Appli- Uses a near-IR monochromatic 0–10 000 0–40 Regulatory
Turbidimeters, Ratio Metric cable for samples with high lev- light source (780–900 nm). Pri- 0–10 000 other
(FNRU) els of color and for monitoring mary detector centered at 90°.
to high turbidity levels. Other detectors located at other
angles. An instrument algorithm
uses a combination of detector
readings to generate the turbid-
ity reading.
Formazin Back Scatter (FBU) Not applicable for regulatory Uses a near-IR monochromatic 100–10 000+ 100–10 000
purposes. Best applied to high light source in the 780–900 nm
turbidity samples. Backscatter range. Detector geometry is 30
is common probe technology ± 15° relative to the incident
and is best applied in higher light beam.
turbidity samples.
Backscatter Unit (BU) Not applicable for regulatory Uses a white light spectral 10–10 000+ 100–10 000+
purposes. Best applied for source (400–680 nm range).
samples with high level turbidid- Detector geometry is 30 ± 15°
ity. relative to the incident light
beam.
Formazin Attenuation Unit May be applicable for some Detector is geometrically cen- 20–1 000 20–1 000 Regulatory
(FAU) regulatory purposes. This is tered at 180° relative to incident
commonly applied with spectro- beam (attenuation) Wavelength
photometers. Best applied for is 780–900 nm.
samples with high level turbidid-
ity.
Light Attenuation Unit (AU) Not applicable for some regula- Detector is geometrically cen- 20–1 000 20–1 000
tory purposes. This is com- tered at 180° relative to incident
monly applied with spectropho- beam (attenuation). Wavelength
tometers. is 400–680 nm.
Nephelometric Turbidity Multi- Is applicable to EPA regulatory Detectors are geometrically 0.02–4000 0–40 Regulatory
beam Unit (FNMU) method GLI Method 2. Appli- centered at 90° and 180°. An 0–4 000 other
cable to drinking water and instrument algorithm uses a
wastewater monitoring applica- combination of detector
tions. readings, which may differ for
turbidities varying magnitude.
Forward Scatter Ratio Unit The technology encompasses a The technology is sensitive to 1-800 FSRU Forward Scatter Ratio Unit
(FSRU) single, light source and two de- turbidities as low as 1 TU. The The measurement of ambient (FSRU)
tectors. Light sources can vary ratio technology helps to com- waters such as streams, lakes,
from single wavelength to poly- pensate for color interference rivers.
chromatic sources. The detec- and fouling.
tion angle for the forward scat-
ter detector is between 0 and
90° relative to the centerline of
the incident light beam.
Forward Scatter Unit (FSU) The technology encompasses a The technology is sensitive to 1-1000 FSU Forward Scatter Unit (FSU)
single, light source and one de- turbidities as low as 1 TU. The The measurement of ambient
tector between 0 and 90° rela- ratio technology helps to com- waters such as streams, lakes,
tive to the centerline of the inci- pensate for color interference rivers and process waters.
dent light beam. and fouling.
D7937 − 15 (2023)
2.2 Other Referenced Standards: 3.2.6 calibration-verification standards, n—defined stan-
EPA 180.1 Determination of Turbidity by Nephelometry dards used to verify the instrument performance in the mea-
EPA 2130B Analytical Method For Turbidity Measurement surement range of interest.
ISO 7027 (International Organization for Standardization) 3.2.6.1 Discussion—Calibration-verification standards may
Water Quality for the Determination of Turbidity not be used to adjust instrument calibration, but only to check
GLI Method 2 Turbidity that the instrument measurements are in the expected range.
Examples of calibration-verification standards are opto-
3. Terminology
mechanical light-scatter devices, gel-like standards, or any
other type of stable liquid standard. Calibration-verification
3.1 Definitions—For definitions of terms used in this test
standards may be instrument-design specific.
method, refer to Terminology D1129.
3.2.7 color, n—the hue (red, yellow, blue, etc.) of a water
3.2 Definitions of Terms Specific to This Standard—Unless
sample produced by the combination of: the selective absorp-
otherwise noted, the term ‘light’ means visible light or near-
tion of visible light, the spectral reflectivity, and the degree of
infrared (NIR) radiation or both.
darkness or blackness of suspended matter.
3.2.1 ambient light, n—light or optical path or both that does
3.2.7.1 Discussion—The combination above is defined by
not originate from the light source of a turbidimeter.
the Munsell (1) color-classification scheme.
3.2.2 attenuation, n—the amount of incident light that is
3.2.8 detector, n—a solid-state device that converts light
scattered and absorbed before reaching a detector, which is
into electrical current or voltage.
geometrically centered at 180° relative to the centerline of the
incident light beam.
3.2.9 detector angle, n—the angle between the axis of the
3.2.2.1 Discussion—Attenuation is inversely proportional to detector acceptance cone and the axis of the source light or
transmitted signal.
NIR beam.
3.2.9.1 Discussion—The detector angle equals 180° – θ (θ is
Attenuated Turbidity = Absorbed Light + Scattered Light
the scattering angle).
3.2.2.2 Discussion—The application of attenuation in this
3.2.10 narrow-band source, n—a light source with a full
test method is as a distinct means of measuring turbidity. When
bandwidth (at half of the source’s maximum intensity)
measured in the FAU or AU mode, the turbidity value is a
(FWHM) located at wavelengths less than 5 nm.
combination of scattered (attenuated) light plus absorbed light.
The scattered light is affected by particle size and is a positive
3.2.11 operating spectrum, n—the wavelength-by-
response. The absorption due to color is a negative response. wavelength products of source intensity, filter transmittance,
The sum of these two responses results in the turbidity value in
and detector sensitivity.
the appropriate unit. 3.2.11.1 Discussion—The operating spectrum determines
the relative contributions of wavelengths in the light-to-current
3.2.3 automatic power control (APC), n—the regulation of
conversions made by a turbidimeter.
light power from a source such that illumination of the sample
remains constant with time and temperature. 3.2.12 ratio turbidity measurement, n—the measurement
derived through the use of a primary detector and one or more
3.2.4 broadband, white-light source, n—a visible-light
other detectors to compensate for variation in incident-light
source that has a full bandwidth at half of the source’s
intensity, stray light, sample color, window transmittance, and
maximum intensity (FWHM) located at wavelengths greater
dissolved NIR-absorbing matter.
than 200 nm.
3.2.4.1 Discussion—Tungsten-filament lamps (TFLs) and 3.2.13 reference turbidity standard, n—a standard that is
white LEDs are examples of broadband sources. synthesized reproducibly from traceable raw materials by a
skilled analyst.
3.2.5 calibration turbidity standard, n—a turbidity standard
3.2.13.1 Discussion—All other standards are traced back to
that is traceable and equivalent to the reference turbidity
this standard. The reference standard for turbidity is formazin.
standard to within defined accuracy; commercially prepared
4000 NTU Formazin, stabilized formazin, and styrenedivinyl- 3.2.14 sample volume, n—the water-sample volume
benzene (SDVB) are calibration turbidity standards. wherein light from a turbidimeter source interacts with sus-
3.2.5.1 Discussion—These standards may be used to cali- pended particles and is subsequently detected.
brate the instrument. All meters should read equivalent values
3.2.15 scattering (also referred to as scatter), n—light
for formazin standards. SDVB-standard readings are instru-
interaction that alters the direction of light transport through a
ment specific and should not be used on meters that do not have
sample without changing the wavelength.
defined values specified for that instrument. Calibration stan-
3.2.15.1 Discussion—The light interaction can be with sus-
dards that exceed 10 000 turbidity units are commercially
pended particles, water molecules, and variations in the sam-
available.
ple’s refractive index.
3.2.16 scattering angle (θ), n—the angle between a source
light or NIR beam, and the scattered beam.
Available from United States Environmental Protection Agency (EPA), William
Jefferson Clinton Bldg., 1200 Pennsylvania Ave., NW, Washington, DC 20004,
http://www.epa.gov.
4 5
Available from American National Standards Institute (ANSI), 25 W. 43rd St., The boldface numbers in parentheses refer to the list of references at the end of
4th Floor, New York, NY 10036, http://www.ansi.org. this standard.
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