ASTM D6855-03

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: D6855 – 03
Standard Test Method for
Determination of Turbidity Below 5 NTU in Static Mode
This standard is issued under the fixed designation D6855; 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 ISO 7027 (The International Organization for Standardiza-
tion) Water Quality—for the Determination of Turbidity
1.1 This test method covers the static determination of
turbidity in water (see 4.1).
3. Terminology
1.2 This test method is applicable to the measurement of
3.1 Definitions—For definitions of terms used in this
turbidities under 5.0 nephelometric turbidity units (NTU).
method refer to Terminology D1129.
1.3 This test method was tested on municipal drinking
3.2 Definitions:
water, ultra-pure water and low turbidity samples. It is the
3.2.1 calibration turbidity standard—a turbidity standard
user’s responsibility to ensure the validity of this test method
that is traceable and equivalent to the reference turbidity
for waters of untested matrices.
standard to within statistical errors, including commercially
1.4 This standard does not purport to address all of the
prepared 4000 NTU Formazin, stabilized formazin (see 9.2.3),
safety concerns, if any, associated with its use. It is the
and styrenedivinylbenzene (SDVB) (see 9.2.4). These stan-
responsibility of the user of this standard to establish appro-
dards may be used to calibrate the instrument.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. Refer to the
NOTE 1—Calibration standards may be instrument specific.
MSDSs for all chemicals used in this procedure.
3.2.2 calibration verification standards—defined standards
used to verify the accuracy of a calibration in the measurement
2. Referenced Documents
range of interest. These standards may not be used to perform
2.1 ASTM Standards:
calibrations, only calibration verifications. Included standards
D1129 Terminology Relating to Water
are opto-mechanical light scatter devices, gel-like standards, or
D1192 Guide for Equipment for SamplingWater and Steam
any other type of stable liquid standard.
in Closed Conduits
NOTE 2—Calibration verification standards may be instrument specific.
D1193 Specification for Reagent Water
D2777 Practice for Determination of Precision and Bias of
3.2.3 nephelometric turbidity measurement—the measure-
Applicable Test Methods of Committee D19 on Water
ment of light scatter from a sample in a direction that is at 90°
D3370 Practices for Sampling Water from Closed Conduits
with respect to the centerline of the incident light path. Units
D5847 Practice for Writing Quality Control Specifications
are NTU (Nephelometric Turbidity Units); when ISO 7027
for Standard Test Methods for Water Analysis
technology is employed units are in FNU (Formazin Nephelo-
E691 Practice for Conducting an Interlaboratory Study to
metric Units).
Determine the Precision of a Test Method
3.2.4 ratio turbidity measurement—the measurement de-
2.2 Other Referenced Standards:
rived through the use of a nephelometric detector that serves as
USEPA Method 180.1 Methods for Chemical Analysis of
the primary detector and one or more other detectors used to
Water and Wastes, Turbidity
compensate for variation in incident light fluctuation, stray
light, instrument noise, or sample color.
3.2.5 reference turbidity standard—a standard that is syn-
thesized reproducibly from traceable raw materials by a skilled
This test method is under the jurisdiction of ASTM Committee D19 on Water
and is the direct responsibility of Subcommittee D19.07 on Sediments, Geomor-
analyst. All other standards are traced back to this standard.
phology, and Open-Channel Flow.
The reference standard for turbidity is formazin (see 9.2.2).
Current edition approved Jan. 10, 2003. Published April 2003. DOI: 10.1520/
3.2.6 seasoning—the process of conditioning laboratory
D6855-03.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or glassware with the standard to be diluted to a lower value. The
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
process reduces contamination and dilution errors. SeeAppen-
Standards volume information, refer to the standard’s Document Summary page on
dix X2 for the suggested procedure.
the ASTM website.
Withdrawn. The last approved version of this historical standard is referenced
on www.astm.org.
4 5
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D6855 – 03
3.2.7 stray light—all light reaching the detector other than be kept scrupulously clean both inside and outside and dis-
that contributed by the sample. For example: ambient light carded when they become etched or scratched. The sample
leakage, internal reflections and divergent light in optical cells must not be handled where the light strikes them when
systems. positioned in the instrument well.
3.2.8 turbidimeter—an instrument that measures light scat- 6.3.1 Sample cell caps and liners must also be scrupulously
ter using a nephelometric detector. Examples include photo- clean to prevent contamination of the sample.
electric nephelometers and ratio photoelectric nephelometers. 6.4 Ideally, the same indexed sample cell should be used
3.2.9 turbidity—an expression of the optical properties of a first for standardization followed by unknown (sample) deter-
sample that causes light rays to be scattered and absorbed mination. If this is not possible, then sample cells must be
rather than transmitted in straight lines through the sample. matched. Refer to the instrument manual for instructions on
Turbidity of water is caused by the presence of suspended and matching sample cells.
dissolved matter such as clay, silt, finely divided organic
NOTE 3—Indexing of the sample cell to the instrument well is accom-
matter, plankton, other microscopic organisms, organic acids,
plished by placing a mark on the top of the sample cell and a similar mark
and dyes.
on the upper surface of the well so that the sample cell can be placed in
the well in an exact position each time.
NOTE 4—Sample cells can be matched by first filling with dilution
4. Summary of Test Method
water (see 8.2).Allow the sample cell to stand for 5 to 10 min to allow for
4.1 The optical property expressed as turbidity is measured
bubbles to vacate the sample. This is followed by cleaning and polishing
bythescatteringeffectthatsuspendedparticulatematerialhave
the outside of the cell. Cells are then measured on the same turbidimeter
on light; the higher the intensity of scattered light, the higher
and should read no different than 0.01 NTU.
the turbidity. In samples containing particulate material, the
6.5 Condensation of optical elements or sample cells can
manner in which sample interferes with light transmittance is
lead to severe errors in measurement.
relatedtothesize,shapeandcompositionoftheparticlesinthe
water, and also to the wavelength of the incident light.
7. Apparatus
4.2 The method is based upon a comparison of the intensity
7.1 Two types of instruments are available for the nephelo-
of light scattered by the sample with the intensity of light
metric method, the nephelometer and ratio nephelometer (see
scattered by a reference suspension. Turbidity values are
Figs. 1 and 2).
determined by a nephelometer, which measures light scatter
7.2 The resolution of the instruments should permit detec-
from a sample in a direction that is at 90° with respect to the
tion of differences of 0.01 NTU or less in waters having
centerline of the incident light path.
turbidities of less than 5.0 NTU. The instrument must measure
the range from#0.02 to 5.0 NTU. See 12.1 for calibration of
5. Significance and Use
instruments. Calibration verification in the immediate range of
5.1 Turbidity is undesirable in drinking water, plant effluent
interest must be performed using acceptable, defined verifica-
waters, water for food and beverage processing, and for a large
tion standards (see 12.2).
number of other water-dependent manufacturing processes.
Removal is often accomplished by coagulation, settling, and NOTE 5—Consult manufacturer’s instructions for guidance associated
with verification methods and verification devices.
filtration. Measurement of turbidity provides a rapid means of
process control for when, how, and to what extent the water
7.2.1 Consult the manufacturer to ensure that your instru-
must be treated to meet specifications.
ment meets or exceeds the specifications of this method.
5.2 This test method is suitable to turbidity such as that
7.3 Photoelectric Nephelometer:
found in drinking water, process water, and high purity
7.3.1 Thisinstrumentusesalightsourceforilluminatingthe
industrial water.
sample and a single photodetector with a readout device to
indicate the intensity of light scattered at right angle(s) (90°) to
6. Interferences
the centerline of the path of the incident light. The photoelec-
6.1 For this application, bubbles, color and large particles, tric nephelometer should be designed so that minimal stray
although they cause turbidity, may result in interferences in lightreachesthedetectorintheabsenceofturbidityandshould
measured turbidity as determined by this method. Bubbles be free from significant drift after a short warm-up period. The
cause a positive interference and color typically causes a light source shall be a Tungsten lamp operated at a color
negative interference. Dissolved material that imparts a color temperature between 2200 and 3000 K (USEPA Method
to the water may cause errors in pure nephelometric reading- 180.1). Light Emitting Diodes (LEDs) or laser diodes in
s,unless the instrument has special compensating features to defined wavelengths ranging from 400 to 900 nm may also be
reduce these interferences. Certain turbulent motions also used if accurately characterized to be equivalent in perfor-
create unstable reading conditions of nephelometers. mance to tungsten using calibration and calibration verification
6.2 Color is characterized by absorption of specific wave- standards. If LEDs or laser diodes are used, then the LED or
lengths of light. If the wavelengths of incident light are Laser diode should be coupled with a monitor detection device
significantly absorbed, a negative interference will result un- to achieve a constant output . LEDs and laser diodes should be
less the instrument has special compensating features. characterized by a wavelength of between 400 and 900 nm
6.3 Scratches, finger marks, or dirt on the walls of the with a bandwidth of less than 60 nm. (Examples of LEDs
sample cell may give erroneous readings. Sample cells should include: White light with a defined bandwidth and 860 6 30
D6855 – 03
FIG. 1 Photoelectric Nephelometer
FIG. 2 Ratio Photoelectric Nephelometer (Single Beam Design)
nm per ISO 7027.) The total distance traversed by incident sample color. As needed by the design, additional photodetec-
light and scattered light within the sample is not to exceed 10 tors may be used to sense the intensity of light scattered at
cm. The angle of light acceptance to the detector shall be other angles. The signals from these additional photodetectors
centered at 90° to the centerline of the incident light path and may be used to compensate for variations in incident light
shall not exceed 6 10° from the 90° scatter path center line. fluctuation, instrument stray light, instrument noise and/or
The detector must have a spectral response that is sensitive to sample color. The ratio photoelectric nephelometer should be
the spectral output of the incident light used. so designed that minimal stray light reaches the detector(s),
7.3.2 Differences in physical design of photoelectric and should be free from significant drift after a short warm-up
nephelometers will cause slight differences in measured values period.The light source should be a tungsten lamp, operated at
for turbidity even though the same suspension is used for a color temperature between 2200 and 3000 K (USEPA
calibrations. Comparability of measurements made using in- Method 180.1). LEDs and laser diodes in defined wavelengths
struments differing in optical and physical design is not ranging from 400 to 900 nm may also be used. If an LED or a
recommended. To minimize initial differences, the following laser diode is used in the single beam design, then the LED or
design criteria should be observed (see Fig. 1). laser diode should be coupled with a monitor detection device
7.4 Ratio Photoelectric Nephelometer: to achieve a consistent output. The distance traversed by
7.4.1 Ratio Photoelectric Nephelometer—(see Fig. 2 for incident light and scattered light within the sample is not to
single beam design; see Fig. 3 for multiple beam design.) This exceed 10 cm. The angle of light acceptance to the nephelo-
instrument uses the measurement derived through the use of a metric detector(s) should be centered at 90° to the centerline of
nephelometric detector that serves as the primary detector and the incident light path and should not exceed 610° from the
oneormoreotherdetectorsusedtocompensateforvariationin scatter path center line. The detector must have a spectral
incident light fluctuation, stray light, instrument noise, or response that is sensitive to the spectral output of the incident
D6855 – 03
FIG. 3 Ratio Photoelectric Nephelometer (Multiple Beam Design)
light used. The instrument calibration (algorithm) must be 7.6 Sample Chambers:
designed such that the scaleable reading is from the nephelo- 7.6.1 For those units not using sample cells, the sample is
metric detector(s), and other detectors are used to compensate placed directly into the sample chamber. For those units, the
for instrument variation described in 7.3.1. sample chamber must be the following:
7.4.2 Differences in physical design of ratio photoelectric 7.6.1.1 Be kept scrupulously clean. Scratches, fingerprints
nephelometers will cause slight differences in measured values and dirt on the walls of the sample chamber may give
for turbidity even when the same suspension is used for erroneousresults.Seethemanufacturer’srecommendationsf
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