ASTM A698/A698M-20

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Designation: A698/A698M − 15 A698/A698M − 20
Standard Test Method for
Magnetic Shield Efficiency in Attenuating Alternating
Magnetic Fields
This standard is issued under the fixed designation A698/A698M; 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
1.1 This test method covers the means for determining the performance quality of a magnetic shield when placed in a magnetic
field of alternating polarity.
1.2 This test method provides a means of evaluating and grading magnetic shielding materials to determine their suitability for
use in the production of magnetic shields.
1.3 This test method shall be used in conjunction with and shall conform to the requirements of Practice A34/A34M.
1.4 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each
system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the
two systems may result in non-conformance with the standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.6 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.
2. Referenced Documents
2.1 ASTM Standards:
A34/A34M Practice for Sampling and Procurement Testing of Magnetic Materials
A340 Terminology of Symbols and Definitions Relating to Magnetic Testing
3. Terminology
3.1 The following symbols and abbreviated definitions apply only to this test method (the official symbols and definitions are
listed in Terminology A340):
3.2 Definitions of Terms Specific to This Standard:
3.2.1 attenuation ratio—the ratio of the measured voltage in the pickup coil before and after the insertion of the shield under
test, E /E .
1 2
3.3 Abbreviations:
3.3.1 A—ac ammeter (usually root mean square (rms) but may be peak reading type).
3.3.2 E —voltage in the pickup coil, after establishing the field of test but before insertion of the shield into the test position.
3.3.3 E —highest voltage observed in the pickup coil, at the field of test, while rotating the test shield in the test position.
3.3.4 H —magnetic field strength, Oe [A/m].
p
3.3.5 I —ac magnetizing current, peak amperes.
p
3.3.6 I—ac magnetizing current, rms amperes.
This test method is under the jurisdiction of ASTM Committee A06 on Magnetic Properties and is the direct responsibility of Subcommittee A06.01 on Test Methods.
Current edition approved April 1, 2015June 1, 2020. Published May 2015July 2020. Originally approved in 1974. Last previous edition approved in 20072015 as
A698/A698MA698/A698M – 15.–07. DOI: 10.1520/A0698_A0698M-15.10.1520/A0698_A0698M-20.
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 Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
A698/A698M − 20
3.3.7 l —length of demagnetizing air coil, cm [m].
c
3.3.8 N—number of turns per coil section (one half of total Helmholtz coil turns).
3.3.9 N —number of turns in demagnetizing coil.
3.3.10 R—mean radius of Helmholtz coil, cm [m].
3.3.11 V—electronic voltmeter (average volts 3π/2 =2).
4. Summary of Test Method
4.1 Frequency—Tests under this test method are normally conducted at a test frequency of 60 Hz.
NOTE 1—Other test frequencies are used by mutual agreement. In general, as test frequencies are increased, it will be necessary to make corresponding
changes in pickup or test coils and in the size of the power supply.
4.2 Magnetic Field Strength—Tests may be conducted at any desired magnetic field strength. The standard test level for ac
magnetic fields is 0.5 or 2.0 peak Oe [39.8 or 159 peak A/m] and in the presence of the earth’s magnetic field (see 9.2).
4.3 Instrumentation—The instruments described are for use at 60 Hz. At higher frequencies, electronic voltmeters and
preamplifiers should also be satisfactory. It may be necessary to change the rms ammeter and power supplies for use at higher test
frequencies. It may also be desirable to reduce the number of area turns in the pickup coil at these frequencies.
4.4 Field Attenuation—The usable range of attenuation ratios is determined by the strength of the starting field and the
sensitivity of the available instrumentation. Measurements shall not be made when signal voltages cannot be clearly distinguished
from amplifier noise or other transient voltages.
5. Significance and Use
5.1 This test method provides an easy, accurate, and reproducible method for determination of shielding factors (attenuation
ratios) in simple alternating magnetic fields.
5.2 Since the sensing or pickup coil is of finite size, the measured shielding factor tends to be the average value for the space
enclosed by the coil. Due care is required when interpreting results when the coil is located near an opening in the shield.
5.3 This test method is suitable for design, specification acceptance, service evaluation, quality assurance, and research purposes
on magnetic shields.
5.4 Provided geometrically identical shields are compared, this test method is also suitable for evaluation and grading of
magnetic shielding materials.
6. Apparatus
6.1 Helmholtz Coil:
6.1.1 The standard Helmholtz coil shall have an inside diameter of 48 in. [1.22 m]. It shall consist of two identical coil sections
of 48 turns each, wound on separate forms that are rigidly fastened together or as two separate 48-turn coils wound on a single
cylindrical form. The frame shall be constructed of nonmagnetic materials. A satisfactory wire size is No. 14 varnish-insulated
copper wire wound into concentrated multi-layer coils of approximately square cross section. The two coils shall be mounted to
have a common axis with the spacing between coil centers equal to the coil radius. The electrical connections between coil
sections shall be such that they are connected in series with their fields aiding to form the Helmholtz coil (Figs. 1 and 2).
6.1.2 The standard Helmholtz coil will measure shields up to 16 in. [0.40 m] in length and 12 in. [0.30 m] in diameter. If larger
shields are to be measured, the Helmholtz coil diameter shall be at least three times the length of the test specimen or four times
its diameter, or both. By mutual agreement, small specimens may be measured in smaller Helmholtz coils providing the ratio of
coil to specimen sizes described above is maintained. The framework shall be constructed to allow easy access for insertion of the
shield test specimen. The pickup coil should be fastened to a support that maintains its position and orientation at the center of
the Helmholtz coil system. The test fixture also requires an adjustable specimen support that permits perpendicular positioning in
relation to the axis of the coil and permits 360° rotation. Also the test fixture shall be equipped with a centering adjustment which
holds the shield specimen in place at the center of the Helmholtz coil system during the test. The axis of the test specimen shall
be 90° to the direction of the field. No magnetic material shall be used in the construction of the test apparatus, and the use of
electrically conducting materials should also be avoided wherever possible.
6.2 Pickup Coil—The pickup coil shall be wound to have a value of area turns that is adequate to provide sufficient voltage for
good instrument scale deflections when the coil is in the weakest field to be measured. In general, for broad-range requirements,
the pickup coil should have an area turns product of approximately 50 000-cm turns. For best reproducibility, the coil should be
covered with a thin nonmagnetic electrically conductive foil shield (copper, aluminum, and so forth) which should be connected
to the system common point or to ground. Care must be taken to make sure the edges of this shield material are insulated in such
a manner as to prevent a shorted turn effect around the flux linking the coil area. Coil leads to the instrument shall be enclosed
in a grounded electrostatic shield which may also be connected to the foil of the coil shield. The pickup coil should be positioned
A698/A698M − 20
FIG. 1 Shield Testing Apparatus
FIG. 2 Diagram of Connections for the Shield Test
at the center of the Helmholtz coil, equidistant from the two coil sections, and oriented so its axis coincides with that of the
Helmholtz coil. Either or both the diameter and length of the pickup coil shall be equal to or less than one half of the diameter
of the shield test specimen and should be less than one tenth of the Helmholtz coil radius. The pickup coils listed in Table 1 are
recommended for use with various size shields.
6.3 Ammeter—A true rms ammeter, having rated full-scale accuracy of at least 1 %, may be used to set the magnetizing current
in the Helmholtz coil. The peak value of magnetizing current is calculated from the measured value of rms amperes. When desired,
the ammeter may be one which reads directly in peak amperes.
6.4 Electronic Voltmeter—A wide voltage range, average responding type, calibrated π/2 =23 average (sometimes called rms
calibrated), electronic voltmeter having a rated full-scale accuracy of at least 1 % shall be used to measure induced voltage in the
pickup coil. This meter shall have a minimum impedance of 1 MΩ to avoid excessive loading effects on the pickup coil. It shall
be accurately calibrated at the frequency of test.
6.5 Preamplifier—When specimens having high attenuation are tested or when pickup coils with relatively small values of area
turns are used, the electronic voltmeter may be unable to resolve the low value of pickup voltage obtained from inside the shield.
In such cases, a linear high-impedance preamplifier having an extremely low noise level voltage in its output is placed between
the pickup coil and the electronic voltmeter to extend its low voltage range. This preamplifier should have an input impedance of
at least 1 MΩ at the test frequency. The preamplifier should be connected into the circuit for voltage measurement, both with and
without the shield in test position. A calibrated attenuator may be used if necessary.
TABLE 1 Suggested Pickup Coils
Approximate Dime
...