ASTM A698/A698M-92(1997)e1
(Test Method)Standard Test Method for Magnetic Shield Efficiency in Attenuating Alternating Magnetic Fields
Standard Test Method for Magnetic Shield Efficiency in Attenuating Alternating Magnetic Fields
SCOPE
1.1 This test method provides 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 means for establishing the desired value of alternating magnetic field strength, of suitable uniformity, inside a standard test coil.
1.3 This test method provides tests for determining the strength of the alternating magnetic field inside the standard coil.
1.4 This test method provides tests for determining the attenuation of the established alternating magnetic field due to the insertion of a magnetic shield around the point of measurement.
1.5 This test method shall be used in conjunction with and shall conform to the requirements of Practice A34.
1.6 The values stated in either customary units (cgs-emu and inch-pound) or SI units are to be regarded separately as standard. Within the text, the SI units are shown in brackets. The values stated in each system are not exact equivalents: therefore, each system shall be used independently of the other. Combining values from the two systems may result in nonconformance with the specification.
1.7 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Standards Content (Sample)
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Designation: A 698/A 698M – 92 (Reapproved 1997) An American National Standard
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method for
Magnetic Shield Efficiency in Attenuating Alternating
Magnetic Fields
This standard is issued under the fixed designation A 698/A 698M; 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 (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Editorially, SI units were enclosed in brackets instead of parentheses.
1. Scope 3. Terminology
1.1 This test method provides means for determining the 3.1 The following symbols and abbreviated definitions ap-
performance quality of a magnetic shield when placed in a ply only to this test method (the official symbols and definitions
magnetic field of alternating polarity. are listed in Terminology A 340).
1.2 This test method provides means for establishing the 3.2 attenuation ratio—the ratio of the measured voltage in
desired value of alternating magnetic field strength, of suitable the pickup coil before and after the insertion of the shield under
uniformity, inside a standard test coil. test, E /E .
1 2
1.3 This test method provides tests for determining the 3.3 A—ac ammeter (usually root mean square (rms) but
strength of the alternating magnetic field inside the standard may be peak reading type).
coil. 3.4 E —voltage in the pickup coil, after establishing the
1.4 This test method provides tests for determining the field of test but before insertion of the shield into the test
attenuation of the established alternating magnetic field due to position.
the insertion of a magnetic shield around the point of measure- 3.5 E —highest voltage observed in the pickup coil, at the
ment. field of test, while rotating the test shield in the test position.
1.5 This test method shall be used in conjunction with and 3.6 H —magnetic field strength, Oe [A/m].
p
shall conform to the requirements of Practice A 34/A 34M. 3.7 I —ac magnetizing current, peak amperes.
p
1.6 The values stated in either customary units (cgs-emu 3.8 I—ac magnetizing current, rms amperes.
and inch-pound) or SI units are to be regarded separately as 3.9 l —length of demagnetizing air coil, cm [m].
c
standard. Within the text, the SI units are shown in brackets. 3.10 N—number of turns per coil section (one half of total
The values stated in each system are not exact equivalents: Helmholtz coil turns).
therefore, each system shall be used independently of the other. 3.11 N —number of turns in demagnetizing coil.
Combining values from the two systems may result in noncon- 3.12 R—mean radius of Helmholtz coil, cm [m].
formance with the specification. 3.13 V—electronic voltmeter (average volts3p/2 2).
=
1.7 This standard does not purport to address all of the
4. Summary of Test Method
safety concerns, if any, associated with its use. It is the
4.1 Frequency—Tests under this test method are normally
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica- conducted at a test frequency of 60 Hz.
bility of regulatory limitations prior to use.
NOTE 1—Other test frequencies are used by mutual agreement. In
general, as test frequencies are increased, it will be necessary to make
2. Referenced Documents
corresponding changes in pickup or test coils and in the size of the power
2.1 ASTM Standards: supply.
A 34/A 34M Practice for Sampling and Procurement Test-
4.2 Magnetic Field Strength—Tests may be conducted at
ing of Magnetic Materials
any desired magnetic field strength. The standard test level for
A 340 Terminology of Symbols and Definitions Relating to
ac magnetic fields is 0.5 or 2.0 peak Oe [39 or 159 peak A/m]
Magnetic Testing
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
This test method is under the jurisdiction of ASTM Committee A-6 on
preamplifiers should also be satisfactory. It may be necessary
Magnetic Properties and is the direct responsibility of Subcommittee A06.01 on Test
to change the rms ammeter and power supplies for use at
Methods.
Current edition approved Jan. 15, 1992. Published June 1992. Originally higher test frequencies. It may also be desirable to reduce the
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published as A 698 – 74. Last previous edition A 698 – 74 (1985) .
number of area turns in the pickup coil at these frequencies.
Annual Book of ASTM Standards, Vol 03.04.
A 698/A 698M
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 repro-
FIG. 2 Diagram of Connections for the Shield Test
ducible method for determination of shielding factors (attenu-
ation ratios) in simple alternating magnetic fields.
larger shields are to be measured, the Helmholtz coil diameter
5.2 Since the sensing or pickup coil is of finite size, the
shall be at least three times the length of the test specimen or
measured shielding factor tends to be the average value for the
four times its diameter, or both. By mutual agreement, small
space enclosed by the coil. Due care is required when inter-
specimens may be measured in smaller Helmholtz coils pro-
preting results when the coil is located near an opening in the
viding the ratio of coil to specimen sizes described above is
shield.
maintained. The framework shall be constructed to allow easy
5.3 This test method is suitable for design, specification
access for insertion of the shield test specimen. The pickup coil
acceptance, service evaluation, quality assurance, and research
should be fastened to a support that maintains its position and
purposes.
orientation at the center of the Helmholtz coil system. The test
6. Apparatus fixture also requires an adjustable specimen support that
permits perpendicular positioning in relation to the axis of the
6.1 Helmholtz Coil:
coil and permits 360° rotation. Also the test fixture shall be
6.1.1 The standard Helmholtz coil shall have an inside
equipped with a centering adjustment which holds the shield
diameter of 48 in. [1.22 m]. It shall consist of two identical coil
specimen in place at the center of the Helmholtz coil system
sections of 48 turns each, wound on separate forms that are
during the test. The axis of the test specimen shall be 90° to the
rigidly fastened together or as two separate 48-turn coils
direction of the field. No magnetic material shall be used in the
wound on a single cylindrical form. The frame shall be
construction of the test apparatus, and the use of electrically
constructed of nonmagnetic materials. A satisfactory wire size
conducting materials should also be avoided wherever pos-
is No. 14 varnish-insulated copper wire wound into concen-
sible.
trated multi-layer coils of approximately square cross section.
6.2 Pickup Coil—The pickup coil shall be wound to have a
The two coils shall be mounted to have a common axis with
value of area turns that is adequate to provide sufficient voltage
the spacing between coil centers equal to the coil radius.
for good instrument scale deflections when the coil is in the
The electrical connections between coil sections shall be such
weakest field to be measured. In general, for broad-range
that they are connected in series with their fields aiding to form
requirements, the pickup coil should have an area turns product
the Helmholtz coil (Figs. 1 and 2).
of approximately 50 000-cm turns. For best reproducibility,
6.1.2 The standard Helmholtz coil will measure shields up
the coil should be covered with a thin nonmagnetic electrically
to 16 in. [0.40 m] in length and 12 in. [0.30 m] in diameter. If
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 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
TABLE 1 Suggested Pickup Coils
Approximate Dimensions
Pickup Wire Number
Mean
Coil Size of
Length,
Diameter,
Number No. Turns
in. [mm]
in. [mm]
1 40 10 000 1.0 [25.4] 1.0 [25.4]
2 40 20 000 1.25 [31.8] 1.25 [31.8]
FIG. 1 Shield Testing Apparatus
A 698/A 698M
recommended for use with various size shields. size relationship specified in 6.2.
6.3 Ammeter—A true rms ammeter, having rated full-scale 7.3 Large magnetic shields of any size may be tested
accuracy of at least 1 %, may be used to set the magnetizing providing the ratio of Helmholtz coil to magnetic shield size is
current in the Helmholtz coil. The peak value of magnetizing maintained in accordance with the requirements of 6.2.
current is calculated from the measured value of rms amperes.
8. Standard Test Values
When desired, the ammeter may be one which reads directly in
peak amperes. 8.1 The standard test conditions shall be an alternating field,
6.4 Electronic Voltmeter—A wide voltage range, average at a frequency of 60 Hz and at a magnetic field strength, H ,of
p
responding type, calibrated p/2 2 3 average (sometimes 0.5 or 2.0 Oe [39.8 or 159 A/m], which is calculated from a
=
called rms calibrated), electronic voltmeter having a rated measured peak value of current or from the assumed peak
full-scale accuracy of at least 1 % shall be used to measure value of the measured rms current using the relationship I 5 I
p
induced voltage in the pickup coil. This meter shall have a 2 . By mutual agreement, tests may be conducted at other
=
minimum impedance of 1 MV to avoid exce
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