1、Designation: A 698/A 698M 07Standard Test Method forMagnetic Shield Efficiency in Attenuating AlternatingMagnetic Fields1This standard is issued under the fixed designation A 698/A 698M; the number immediately following the designation indicates the yearof original adoption or, in the case of revisi
2、on, 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.1. Scope1.1 This test method covers the means for determining theperformance quality of a magnetic shield when pl
3、aced in amagnetic field of alternating polarity.1.2 This test method provides a means of evaluating andgrading magnetic shielding materials to determine their suit-ability for use in the production of magnetic shields.1.3 This test method shall be used in conjunction with andshall conform to the req
4、uirements of Practice A 34/A 34M.1.4 The values stated in either SI units or inch-pound unitsare to be regarded separately as standard. The values stated ineach system may not be exact equivalents; therefore, eachsystem shall be used independently of the other. Combiningvalues from the two systems m
5、ay result in non-conformancewith the standard.1.5 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regul
6、atory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2A 34/A 34M Practice for Sampling and Procurement Test-ing of Magnetic MaterialsA 340 Terminology of Symbols and Definitions Relating toMagnetic Testing3. Terminology3.1 The following symbols and abbreviated definitions ap-ply
7、only to this test method (the official symbols and definitionsare listed in Terminology A 340):3.2 Definitions of Terms Specific to This Standard:3.2.1 attenuation ratiothe ratio of the measured voltage inthe pickup coil before and after the insertion of the shield undertest, E1/E2.3.3 Abbreviations
8、:3.3.1 Aac ammeter (usually root mean square (rms) butmay be peak reading type).3.3.2 E1voltage in the pickup coil, after establishing thefield of test but before insertion of the shield into the testposition.3.3.3 E2highest voltage observed in the pickup coil, at thefield of test, while rotating th
9、e test shield in the test position.3.3.4 Hpmagnetic field strength, Oe A/m.3.3.5 Ipac magnetizing current, peak amperes.3.3.6 Iac magnetizing current, rms amperes.3.3.7 lclength of demagnetizing air coil, cm m.3.3.8 Nnumber of turns per coil section (one half of totalHelmholtz coil turns).3.3.9 N1nu
10、mber of turns in demagnetizing coil.3.3.10 Rmean radius of Helmholtz coil, cm m.3.3.11 Velectronic voltmeter (average volts 3p/2=2).4. Summary of Test Method4.1 FrequencyTests under this test method are normallyconducted at a test frequency of 60 Hz.NOTE 1Other test frequencies are used by mutual ag
11、reement. Ingeneral, as test frequencies are increased, it will be necessary to makecorresponding changes in pickup or test coils and in the size of the powersupply.4.2 Magnetic Field StrengthTests may be conducted atany desired magnetic field strength. The standard test level forac magnetic fields i
12、s 0.5 or 2.0 peak Oe 40 or 160 peak A/mand in the presence of the earths magnetic field (see 9.2).4.3 InstrumentationThe instruments described are for useat 60 Hz. At higher frequencies, electronic voltmeters andpreamplifiers should also be satisfactory. It may be necessaryto change the rms ammeter
13、and power supplies for use athigher test frequencies. It may also be desirable to reduce thenumber of area turns in the pickup coil at these frequencies.4.4 Field AttenuationThe usable range of attenuationratios is determined by the strength of the starting field and thesensitivity of the available
14、instrumentation. Measurements1This test method is under the jurisdiction of ASTM Committee A06 onMagnetic Properties and is the direct responsibility of SubcommitteeA06.01 on TestMethods.Current edition approved May 1, 2007. Published June 2007. Originallyapproved in 1974. Last previous edition appr
15、oved in 2002 as A 698/A 698M02.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International,
16、 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.shall not be made when signal voltages cannot be clearlydistinguished from amplifier noise or other transient voltages.5. Significance and Use5.1 This test method provides an easy, accurate, and repro-ducible method
17、 for determination of shielding factors (attenu-ation ratios) in simple alternating magnetic fields.5.2 Since the sensing or pickup coil is of finite size, themeasured shielding factor tends to be the average value for thespace enclosed by the coil. Due care is required when inter-preting results wh
18、en the coil is located near an opening in theshield.5.3 This test method is suitable for design, specificationacceptance, service evaluation, quality assurance, and researchpurposes on magnetic shields.5.4 Provided geometrically identical shields are compared,this test method is also suitable for ev
19、aluation and grading ofmagnetic shielding materials.6. Apparatus6.1 Helmholtz Coil:6.1.1 The standard Helmholtz coil shall have an insidediameter of 48 in. 1.22 m. It shall consist of two identical coilsections of 48 turns each, wound on separate forms that arerigidly fastened together or as two sep
20、arate 48-turn coilswound on a single cylindrical form. The frame shall beconstructed of nonmagnetic materials. A satisfactory wire sizeis No. 14 varnish-insulated copper wire wound into concen-trated multi-layer coils of approximately square cross section.The two coils shall be mounted to have a com
21、mon axis withthe spacing between coil centers equal to the coil radius.The electrical connections between coil sections shall be suchthat they are connected in series with their fields aiding to formthe Helmholtz coil (Figs. 1 and 2).6.1.2 The standard Helmholtz coil will measure shields upto 16 in.
22、 0.40 m in length and 12 in. 0.30 m in diameter. Iflarger shields are to be measured, the Helmholtz coil diametershall be at least three times the length of the test specimen orfour times its diameter, or both. By mutual agreement, smallspecimens may be measured in smaller Helmholtz coils pro-viding
23、 the ratio of coil to specimen sizes described above ismaintained. The framework shall be constructed to allow easyaccess for insertion of the shield test specimen. The pickup coilshould be fastened to a support that maintains its position andorientation at the center of the Helmholtz coil system. T
24、he testfixture also requires an adjustable specimen support thatpermits perpendicular positioning in relation to the axis of thecoil and permits 360 rotation. Also the test fixture shall beequipped with a centering adjustment which holds the shieldspecimen in place at the center of the Helmholtz coi
25、l systemduring the test. The axis of the test specimen shall be 90 to thedirection of the field. No magnetic material shall be used in theconstruction of the test apparatus, and the use of electricallyconducting materials should also be avoided wherever pos-sible.6.2 Pickup CoilThe pickup coil shall
26、 be wound to have avalue of area turns that is adequate to provide sufficient voltagefor good instrument scale deflections when the coil is in theweakest field to be measured. In general, for broad-rangerequirements, the pickup coil should have an area turns productof approximately 50 000-cm2turns.
27、For best reproducibility,the coil should be covered with a thin nonmagnetic electricallyconductive foil shield (copper, aluminum, and so forth) whichshould be connected to the system common point or to ground.Care must be taken to make sure the edges of this shieldmaterial are insulated in such a ma
28、nner as to prevent a shortedturn effect around the flux linking the coil area. Coil leads tothe instrument shall be enclosed in a grounded electrostaticshield which may also be connected to the foil of the coilshield. The pickup coil should be positioned at the center of theHelmholtz coil, equidista
29、nt from the two coil sections, andoriented so its axis coincides with that of the Helmholtz coil.Either or both the diameter and length of the pickup coil shallbe equal to or less than one half of the diameter of the shieldtest specimen and should be less than one tenth of theHelmholtz coil radius.
30、The pickup coils listed in Table 1 arerecommended for use with various size shields.6.3 AmmeterA true rms ammeter, having rated full-scaleaccuracy of at least 1 %, may be used to set the magnetizingcurrent in the Helmholtz coil. The peak value of magnetizingcurrent is calculated from the measured va
31、lue of rms amperes.When desired, the ammeter may be one which reads directly inpeak amperes.6.4 Electronic VoltmeterA wide voltage range, averageresponding type, calibrated p/2=2 3 average (sometimesFIG. 1 Shield Testing ApparatusFIG. 2 Diagram of Connections for the Shield TestA 698/A 698M 072calle
32、d rms calibrated), electronic voltmeter having a ratedfull-scale accuracy of at least 1 % shall be used to measureinduced voltage in the pickup coil. This meter shall have aminimum impedance of 1 MV to avoid excessive loadingeffects on the pickup coil. It shall be accurately calibrated at thefrequen
33、cy of test.6.5 PreamplifierWhen specimens having high attenuationare tested or when pickup coils with relatively small values ofarea turns are used, the electronic voltmeter may be unable toresolve the low value of pickup voltage obtained from insidethe shield. In such cases, a linear high-impedance
34、 preamplifierhaving an extremely low noise level voltage in its output isplaced between the pickup coil and the electronic voltmeter toextend its low voltage range. This preamplifier should have aninput impedance of at least 1 MV at the test frequency. Thepreamplifier should be connected into the ci
35、rcuit for voltagemeasurement, both with and without the shield in test position.A calibrated attenuator may be used if necessary.6.6 Power SupplyCommercial 60-Hz power sources aresatisfactory providing they have good regulation and maintainan accurate sinusoidal voltage. Current waveform distortion
36、isacceptable in forms or amounts that do not cause the measuredpeak value of current to deviate by more than 5 % from thepeak value obtained by calculation from the measured rmsexciting current and by using sinusoidal waveform constants.For satisfactory tests, the current waveform distortion mustrem
37、ain constant for the duration of each test. Twisted pair-typeleads should be used for all connections within the powersupply and between the power supply and the Helmholtz coils.6.7 Power Control EquipmentVariable-ratio autotrans-formers, induction regulators, oscillators, power amplifiers, orother
38、devices are suitable for adjusting voltages needed toprovide the desired Helmholtz coil currents. The use ofrheostat-type controls is not recommended. All power supplyparts must be located as far as practicable from the Helmholtzcoils and must be energized during all tests.6.8 Hall ProbesIn certain
39、instances such as when lowattenuation ratios are present or when very small shields are tobe tested, transverse Hall probes may be used instead of pickupcoils. The active sensing element dimensions must conform tothe same size restrictions as pickup coils (see 6.2).7. Test Specimens7.1 Magnetic shie
40、lds of any shape may be evaluated forattenuation under this test method providing their geometricshape is one that can partially surround the pickup coil. Anysize shield may be tested when the requirements specified in7.2 and 7.3 are met.7.2 The shield shall be large enough to fit around a pickupcoi
41、l of sufficient area turns and must maintain the coil to shieldsize relationship specified in 6.2.7.3 Large magnetic shields of any size may be testedproviding the ratio of Helmholtz coil to magnetic shield size ismaintained in accordance with the requirements of 6.2.8. Standard Test Values8.1 The s
42、tandard test conditions shall be an alternating field,at a frequency of 60 Hz and at a magnetic field strength, Hp,of0.5 or 2.0 Oe 39.8 or 159 A/m, which is calculated from ameasured peak value of current or from the assumed peakvalue of the measured rms current using the relationship Ip= I=2 . By m
43、utual agreement, tests may be conducted at otherfrequencies and field strengths.9. Procedures9.1 Orient the test set to the direction that gives the lowestpossible spurious voltage in the test pickup coil as a result ofstray magnetic fields (Note 2). Measure this residual voltage onthe electronic vo
44、ltmeter with and without the test specimen inplace and before power is applied to the Helmholtz coils. Asatisfactory test location is one in which the stray pickupvoltage is very small relative to either of the test voltages. (Therecommended ratio of test to stray voltage is 100:1.) Take careto avoi
45、d coupling stray pickup directly into the electronicvoltmeter amplifier through its metal case. It should be isolatedfrom equipment such as oscilloscopes or other test instruments.They should be remotely located and turned off during mea-surement where interference occurs.NOTE 2Some high-permeabilit
46、y magnetic shields are very susceptibleto physical damage and must be handled very carefully to avoiddegradation of magnetic properties.9.2 Demagnetize the shield before test (see 9.3). Afterdemagnetization, begin the test at the lowest value of magneticfield strength (Note 3). With the shield speci
47、men remotelylocated, apply excitation to the Helmholtz coil and increase ituntil the desired value of magnetizing current has beenreached. Then read the electronic voltmeter and record thevoltage as E1for that value of magnetic field strength. Reducethe excitation to zero field, then insert the shie
48、ld test specimenover the pickup coil and center it in the Helmholtz coil. Slowlyrestore and readjust the current to the same magnetic fieldstrength, then obtain the attenuated value of pickup voltage E2by slowly rotating the test shield around its axis (pickup coiland Helmholtz coil remaining statio
49、nary). Record the highestvalue of voltage pickup as E2. Nonsymmetrical or partial shieldenclosures should be oriented in the direction of use. Removethe shield from the Helmholtz coil. The next higher value ofmagnetic field strength is now established and the testingprocedure shall be repeated.NOTE 3During the test, care must be taken to ensure that the shield isthe only magnetic material present. Trouble may result from metal stools,benches, desks, carts, spare test specimens, tools, laboratory instruments,file cabinets, or other metal dev
