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: E703 − 20

Standard Practice for

Electromagnetic (Eddy Current) Sorting of Nonferrous
Metals1
This standard is issued under the fixed designation E703; 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* 2.2 ASNT Documents:3

SNT-TC-1A Recommended Practice for Personnel Qualifi-
1.1 This practice describes a procedure for sorting nonfer-
cation and Certification in Nondestructive Testing
rous metals using the electromagnetic (eddy current) method.
ANSI/ASNT CP-189 Standard for Qualification and Certifi-
The procedure is intended for use with instruments using
cation of Nondestructive Testing Personnel
absolute or comparator-type coils for distinguishing variations
in mass, shape, conductivity, and other variables such as alloy, 2.3 AIA Standard:4
heat treatment, or hardness that may be closely correlated with NAS-410 Qualification and Certification of Nondestructive
the electrical properties of the material. Selection of samples to Testing Personnel
evaluate sorting feasibility and to establish standards is also 3. Terminology
described.
3.1 Standard terminology relating to electromagnetic ex-
1.2 This standard does not purport to address all of the amination may be found in Terminology E1316, Section C:
safety concerns, if any, associated with its use. It is the Electromagnetic Testing.
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter- 4. Summary of Practice
mine the applicability of regulatory limitations prior to use. 4.1 The techniques that are primarily used in electromag-
1.3 This international standard was developed in accor- netic sorting employ the absolute (single-) and comparative
dance with internationally recognized principles on standard- (two-) coil methods using either encircling or probe coils. The
ization established in the Decision on Principles for the decision of whether to use single-coil or two-coil operation is
Development of International Standards, Guides and Recom- usually based on empirical data. In the absolute-coil method
mendations issued by the World Trade Organization Technical (encircling or probe), the equipment is standardized by placing
Barriers to Trade (TBT) Committee. standards of known properties in the test coil. The value of the
examined electrical parameter, which may be correlated with
2. Referenced Documents alloy, heat treatment temper, or hardness, is read on the display
2.1 ASTM Standards:2 of an indicator. In the comparative coil method (encircling or
E105 Practice for Probability Sampling of Materials probe coils), the test specimen in one coil is compared with a
E122 Practice for Calculating Sample Size to Estimate, With reference standard in a second coil to determine whether the
Specified Precision, the Average for a Characteristic of a test specimen is within or outside of the required limits.
Lot or Process 4.1.1 Absolute Coil Method:
E543 Specification for Agencies Performing Nondestructive 4.1.1.1 Encircling Coil—Various reference standards are
Testing inserted consecutively in the test coil, and the controls of the
E1316 Terminology for Nondestructive Examinations instrument are adjusted to obtain an appropriate response.
Typically, three samples would be used representing the upper,
lower, and mid-range for which standardization is required.
1
The examination is then conducted by inserting the specimens
This practice is under the jurisdiction of ASTM Committee E07 on Nonde-
structive Testing and is the direct responsibility of Subcommittee E07.07 on
to be sorted into the test coil, and observing the instrument
Electromagnetic Method. response.
Current edition approved June 1, 2020. Published July 2020. Originally approved
in 1979. Last previous edition approved in 2014 as E703 – 14. DOI: 10.1520/
3
E0703-20. Available from American Society for Nondestructive Testing (ASNT), P.O. Box
2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or 28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http://www.asnt.org.
4
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Available from Aerospace Industries Association of America, Inc. (AIA), 1000
Standards volume information, refer to the standard’s Document Summary page on Wilson Blvd., Suite 1700, Arlington, VA 22209-3928, http://www.aia-aerospace.org.
the ASTM website.

*A Summary of Changes section appears at the end of this standard

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States

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 E703 − 20
4.1.1.2 Probe Coil—The probe coil is placed consecutively 5.5 Successful sorting of nonferrous material depends
on the reference standards and the controls of the instrument mainly on the variables present in the sample and the proper
are adjusted for appropriate response (see 4.1.1.1). The exami- selection of frequency and fill factor.
nation is then conducted by placing the probe on the specimens 5.6 The accuracy of a sort will be affected greatly by the
to be sorted and observing the instrument response. coupling between the test coil field and the examined part
4.1.2 Comparative Coil Method: during the measuring period.
4.1.2.1 Encircling Coil—Reference standards representing
the minimum or maximum limits, or both, of acceptance or 6. Basis of Application
sorting category are inserted in the reference and test coil. The 6.1 Personnel Qualification—If specified in the contractual
instrument controls are adjusted for appropriate responses. The agreement, personnel performing examinations to this practice
examination is then conducted by inserting specimens to be shall be qualified in accordance with a nationally recognized
sorted in the test coil, leaving the known reference in the nondestructive testing (NDT) personnel qualification practice
reference coil, and observing the instrument response. or standard, such as ANSI/ASNT-CP-189, SNT-TC-1A, NAS-
4.1.2.2 Probe Coil—Both probe coils are placed on the 410, ISO 9712, or a similar document and certified by the
reference standard representing the minimum or maximum employer or certifying agency, as applicable. The practice or
limits, or both, of acceptance or sorting category. The instru- standard used and its applicable revision shall be identified in
ment controls are adjusted for appropriate responses. The the contractual agreement between the using parties.
examination is then conducted by placing the test probe on the
specimens to be sorted (the other probe is left on the reference 6.2 Qualification of Nondestructive Testing Agencies—If
standard) and observing the instrument response. specified in the contractual agreement, NDT agencies shall be
qualified and evaluated as described in Specification E543. The
4.2 The range of instrument response must be so adjusted in applicable edition of Specification E543 shall be specified in
the initial step that the anticipated deviations will be within the the contractual agreement.
range of readout.
6.3 Acceptance Criteria—Since acceptance criteria are not
4.3 Both absolute and comparative methods using encir- specified in this practice, they shall be specified in the
cling coil(s) require comparing the specimens to be examined contractual agreement.
with the reference standards. Two or more samples represent-
ing the limits of acceptance may be required. In the absolute 7. Interferences
method, the electrical reference signal from the instrument is 7.1 The influence of the following variables must be con-
adjusted with the reference standard in the coil. In the sidered for proper interpretation of the results:
comparative method, any electromagnetic condition, that is not 7.1.1 The correlation shall be established so that electrical
common to the test specimen and the reference standard, will properties of various groups do not overlap and are well
produce an imbalance in the system. The comparative method defined in the standardization procedure used. For any given
is usually more stable since it suppresses most of the interfer- temper and alloy combination, there is an acceptable range of
ences. values. These ranges must be considered, not just a single
4.4 The examination process may consist of manual inser- value.
tion of one specimen after another into the test coil or an 7.1.2 The examination frequency must be selected to pro-
automated feeding and classifying mechanism may be em- vide a well-defined separation of variables.
ployed. In automated setups, it is sometimes necessary to 7.1.3 The temperature of the reference standard and test
establish empirically the time required for the test specimen to specimen shall be controlled within limits that will permit a
remain in the test coil while the reading is being taken, well-defined range of conductivity or permeability, or both, for
especially if low frequencies are employed. which the correlation of the group or groups is valid. Cooling
of the reference standard when high field strengths are used or
5. Significance and Use allowing test specimens to cool or heat to an established
5.1 Absolute and comparative methods provide a measure ambient range, or both, may be required.
for sorting large quantities of nonferrous parts or stock with 7.1.4 The geometry, mass, and thickness of the reference
regard to composition or condition, or both. standard and test specimen shall be controlled within limits that
5.2 The comparative or two-coil method is used when will permit sorting.
high-sensitivity examination is required. The advantage of this 7.1.5 Magnetic permeability variations can interfere when
method is that it almost completely suppresses interferences. sorting paramagnetic materials.
7.1.6 Signal response can result from a change in relative
5.3 The ability to accomplish these types of separations motion between the test specimen and the test coil, such as the
satisfactorily is dependent upon the relation of the electric length of time the specimen is in a test coil (see 4.4).
characteristics of the nonferrous parts to their physical condi- 7.1.7 Conductivity has an unambiguous relationship to
tion. hardness for certain alloys. However, when alloys are mixed,
5.4 These methods may be used for high-speed sorting in a identical conductivity does not necessarily indicate the same
fully automated setup where the speed of examination may hardness.
approach many specimens per second depending on their size 7.1.8 Care must also be exercised in using conductivity to
and shape. sort overheated parts quenched at a high temperature as the

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 E703 − 20
conductivity reading for acceptable parts may repeat at a large the samples may also be conducted on a measurements (or
increase in temperature. variables) basis; that is, determining actual readings on the
7.1.9 Lift-off can result in a change in the test system output units in the sample. The majority of acceptance sampling is
with probe coils. This effect is a change in the magnetic carried out on a sampling by attributes basis and the usual
coupling between the test specimen and probe coil. Care must acceptance sampling table is designed for accept/reject.
be exercised to prevent this effect from interfering with
9.3 Process control sampling may be conducted on material
examination results; either mechanical or electronic compen-
during the course of production to prevent large quantities of
sation must be used.
defective parts being found in the acceptance examinations.
7.1.10 For certain heat-treatable (aluminum) alloys, conduc-
Many parts and materials are subjected to several successive
tivity values can also repeat themselves during the aging cycle
machining or processing operations before they become fin-
at a constant temperature. Thus, for such alloys, conductivity is
ished units. Parts can be most effectively controlled during
not unique as a monitor of temper, etc. In this case, you must
production by examining small samples of these parts at
have an understanding of the temper expected of each alloy to
frequent regularly scheduled intervals. The object of this
determine if this test is possible.
process check is to provide a continuous picture of the quality
8. Apparatus of parts being produced. This helps prevent production of
defective parts by stopping and correcting the problem as soon
8.1 Electronic Apparatus—The electronic apparatus shall be
as it begins to appear in the manufacturing process and thereby
capable of energizing the test coils with alternating currents of
keeping the process in control. Sampling may be by attributes
suitable frequencies and power levels and shall be capable of
or by variables and process control sampling tables are used.
sensing changes in the electromagnetic response of the coils.
The measurements (variables) control chart is by far the most
Equipment may include any suitable signal-processing devices
effective process control technique.
(phase discriminator, filter circuits, etc.) and the output may be
displayed by meter, oscilloscope, recorder, signaling devices, 9.4 Statistical sampling tables have four definite features:
or any suitable combination required for the particular appli- (1) specification of sampling data—that is, the size of the
cation. samples to be selected, the conditions under which the samples
8.2 Test coils may be of the encircling or probe-coil type are to be selected, and the conditions under which the lot will
and shall be capable of inducing an electromagnetic field in the be accepted or rejected; (2) protection afforded—that is, the
test specimen and reference standard, and sensing changes in element of risk that the sampling schedules in a given table will
the electric or magnetic characteristics of the test specimen. reject good lots or accept bad ones; (3) disposal procedure—
8.2.1 When selecting the test coil, the objective should be to that is, a set of rules that state what is to be done with lots after
obtain a coil fill factor as large as possible. This means that the sampling has been completed; and (4) cost required—that is,
inside of the test coil should be filled by the test specimen as average inspection cost required to accept or reject a lot.
much as possible. This is of primary importance for examina-
tions requiring high sensitivity. 10. Reference Standards
8.2.2 For complicated test specimen shapes, a correspond- 10.1 Two reference standards of the precise size and con-
ing insert shall be provided to ensure that each test specimen figuration of the product to be examined are usually used to set
can be placed in the same position within the test coil. These up for sorting by the absolute coil method (see 11.2). Three
inserts, as well as any other accessories, should consist of reference standards of the precise size and configuration are
non-ferromagnetic, electrically nonconductive material. usually used for sorting by the comparative coil (see 11.3)
8.3 Mechanical Handling Apparatus— A mechanical device method.
for feeding and sorting the test specimens may be used to 10.2 Three reference standards are usually required for a
automate a particular application. three-way mix (see 11.4).
9. Sampling 10.3 The reference standard should be selected to represent
9.1 Sampling (see Practices E105 and E122) is a method to the extremes of acceptable and unacceptable groups or a range
obtain assurance that materials are of satisfactory quality. of hardness or conductivity to assure no overlap in the sort.
Instead of 100 % inspection, a portion of the material is 10.4 Other arrangements can be used and are acceptable but
examined to show evidence of the quality of the whole. There are not described in this procedure.
are two important needs for this approach: first, the final
inspection or examination is made to assure that products 10.5 Reference standards must be stored in such a way as to
delivered are in conformance with specification requirements; minimize exposure to conditions that can affect their temper
second, to control parts and assemblies while they are being and conductivity.
processed. Statistical acceptance sampling tables and statistical
process-control sampling tables have been developed to meet 11. Standardization
these needs. 11.1 The electromagnetic sorting method is primarily one of
9.2 Acceptance sampling may be conducted on an accept/ comparison between specimens. Empirical data and physical
reject (or attributes) basis; that is, determining whether or not examinations on samples representing properties to be sepa-
the units of the sample meet the specification. Examination of rated determine the validity of the sort. The standardization

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 E703 − 20
procedure shall be based on the properties of the sample 11.5 A similar procedure to that used with encircling coils is
requiring separation. The sort may require more than one used with probe coils (11.2 – 11.4). Instead of placing reference
operation. standards in the coil(s), position the probe(s) in a suitable
location on the reference standard(s).
11.2 When using the absolute coil method (encircling),
insert the reference standard to a fixed position in the coil and 12. Procedure
adjust the test instrument to get an on-scale meter or oscillo- 12.1 Connect the required test coil to the instrument. Place
scope reading, or both. Replace the acceptable reference insert(s) or other positioning fixture in the coil(s) if required.
standard with a known unacceptable reference standard in the
same exact position and adjust the sensitivity of the instrument 12.2 Switch on the instrument and allow it to warm up for
to maximize the indicated difference reading without exceed- at least the length of time recommended by the manufacturer.
ing 90 % of the available scale range. 12.3 Make all necessary setup and control adjustments in
accordance with the manufacturer’s recommendation. Adjust
11.3 When using the comparative coil method (encircling),
frequency, field strength, sensitivity, and other necessary con-
select a reference piece (usually one that falls within the
trols to values determined for the electromagnetic sort.
acceptable limits of the specimens being examined), place it in
the reference coil and set this coil and piece in a location so that 12.4 Standardize the sorting system in accordance with 11.2
it will not be accidentally disturbed during the sorting opera- when using the absolute coil method, 11.3 when using the
tion. For this method, when used with a two-way mix, choose comparative coil method, or 11.5 when using probe coils.
two reference standards, one of which represents the accept- Standardize at the start of the examination run and at least once
able and the other the unacceptable group. Place the acceptable every hour of continuous operation or whenever improper
reference standard at a fixed position in the test coil coinciding functioning of the system is suspected (see 12.7).
with the position of the reference piece in the reference coil and 12.5 For manual operation, insert the test specimens manu-
balance the instrument. Replace this acceptable reference ally in the test coil.
standard with one representing the unacceptable group and 12.5.1 Read the results on an indicator.
adjust the test instrument’s phase, sensitivity, and coil current, 12.5.2 Manually remove the specimens from the test coil.
to maximize the indicator reading without exceeding 90 % of 12.6 For automatic sort, transmit the test specimens con-
the available scale range. Reinsert the acceptable reference tinuously through the test coil.
standard and alternately readjust the instrument controls to 12.6.1 In passing through the coil, each test specimen is
retain a null value for the acceptable reference standard and analyzed by the test instrument.
maximum indication for the unacceptable reference standard. 12.6.2 A signal, corresponding to the conductivity of the
11.4 For a three-way sort, it is best to have three reference respective test specimen, is sent to a sorting gate where the
standards, two of which represent the high and low limits of specimens are automatically sorted into preselected groups.
acceptability for one group or one each of the two unacceptable 12.7 Verify the standardization of the instrument at the end
groups. The third reference standard represents the acceptable of examining each lot. If the standardization is found to have
lot of material. changed since the last check so that it affects the sort,
11.4.1 A typical case of high and low limits of acceptability restandardize the system and reexamine all of the material
reference standards is in measurements where reference stan- examined since the last check.
dards representing maximum and minimum acceptable hard-
13. Interpretation of Results
ness are required. In this instance, insert the third reference
standard representing the acceptable lot into the test coil and 13.1 The results of most nondestructive evaluation proce-
adjust the instrument for a null or zero reading. Then adjust the dures are based on the comparison of an unknown with a
controls to maximize the indications without exceeding 690 % reference standard. Unless the significant interrelationships of
of the available scale range from the null for each of the other material or product properties are understood and measurable
two reference standards (maximum and minimum). Alternate for both standard and unknown samples, erroneous examina-
readjustment of the controls may be necessary to retain the null tion results may be obtained.
reading, as well as the maximum and minimum limits for 13.2 Electromagnetic sorting is best used for repetitive
acceptance. examinations on material that has the same shape, composition,
11.4.2 For a three-way sort when three dissimilar grades of and metallurgical structure; that is, it is not generally used for
material become mixed, place the third reference standard examinations on grossly different materials. Electromagnetic
(acceptable group) into the test coil and null. Then successively sorting is generally not useful if there is limited knowledge of
insert into the test coil the two reference standards representing the properties of the unknown or material to be examined.
the other two grades and adjust the instrument’s controls to 13.3 Interpretation of data depends upon the degree to
maximize the indications without exceeding 690 % of the which the materials to be examined compare with the reference
available scale range from the null for each of the other two materials. It is necessary to have all variables, except that one
reference standards. Alternate readjustment of the controls may selected as a basis for sorting, under control if the measured
be necessary to retain the null reading as well as the indication variation is to be properly interpreted. Results can often be
for the other two reference standards. interpreted or explained by a processing change, such as

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 E703 − 20
temperature, composition, and inclusions, when the measured other location, plus such other items as may be agreed upon by
property is known to be a function of the processing procedure. the using parties. The following information should be re-
corded:
13.4 When products with different shapes, alloys, or con-
14.1.1 Description of apparatus:
ductivities are to be sorted, only a qualitative interpretation of
14.1.1.1 Type of equipment.
results can be made. The materials can be said to be different,
14.1.1.2 Model number.
but the reason for the difference may not be understood. 14.1.1.3 Serial number.
13.5 When the spread in value of the measured variable is 14.1.2 Output device (if used):
sufficient, electromagnetic sorting can be 100 % effective. 14.1.2.1 Type.
However, there may be cases where a single examination will 14.1.2.2 Model number.
not show a clear separation. Often a second examination or 14.1.2.3 Serial number.
procedure can be used to further define the separation of 14.1.3 Coil:
materials. For example, a different examination frequency may 14.1.3.1 Size.
show the effect of a second variable. 14.1.3.2 Type.
14.1.4 Other interconnecting apparatus.
13.6 Shape and surface variations can mask the examination 14.1.5 Reference standards.
results. If surface hardness is desired as the basis for sorting, all 14.1.6 Examination frequency.
material should have composition and surface roughness under 14.1.7 Description of materials:
sufficient control so that the effects of variations in hardness 14.1.7.1 Geometry.
can be separated. 14.1.7.2 Chemistry.
13.7 Measurement bias depends upon factors that include 14.1.7.3 Heat treatment.
equipment, techniques, temperature control of parts and refer- 14.1.8 Method of standardization.
ence standard, geometry, types of materials, and operator. 14.1.9 Scanning speed.
Variations in these factors can affect the bias of the sort. 14.1.10 Temperature of the reference standard.
14.1.11 Temperature of the test specimen.
14. Report 14.1.12 Examination method.

14.1 The written report of an electromagnetic sort should 15. Keywords

contain any information about the examination setup that will 15.1 encircling coils; electromagnetic sorting; nonferrous
be necessary to duplicate the examination at the same or some metals; probe coils; sorting

SUMMARY OF CHANGES

Committee E07 has identified the location of selected changes to this standard since the last issue (E703 – 14)
that may impact the use of this standard.

(1) Made updates to 7.1.1 and 7.1.10. (2) Added 10.5.

ASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentioned
in this standard. Users of this standard are expressly advised that determination of the validity of any such patent rights, and the risk
of infringement of such rights, are entirely their own responsibility.

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