# ASTM E1629-12

Designation: E1629 − 12Standard Practice forDetermining the Impedance of Absolute Eddy-CurrentProbes1This standard is issued under the fixed designation E1629; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (´) indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This practice covers a procedure for determining theimpedance of absolute eddy-current probes (bridge-type, air orferrite core, wire wound, shielded, or unshielded) used forfinding material defects in electrically conducting material.This practice is intended to establish a uniform methodology tomeasure the impedance of eddy-current probes prior to receiptof these probes by the purchaser or the specifier.1.2 Limitations—This practice does not address the charac-terization or measurement of the impedance of differential, a-ccoupled, or transmit/receive types of probes. This practice doesnot address the use of magnetic materials in examinationprobes. This practice shall not be used as a basis for selectionof the best probe for a particular application or as a means bywhich to calibrate or standardize a probe for a specificexamination. This practice does not address differences in theimpedance values that can be obtained when the probe andmaterial are in relative motion, as in a rotating probe, since theprocedure described here requires the probe and material to bestationary.1.3 Units—The values stated in SI units are to be regardedas the standard.The values given in parentheses are mathemati-cal conversions to inch-pound units that are provided forinformation only and are not considered standard.1.4 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 regulatory limitations prior to use.2. Referenced Documents2.1 The following document forms a part of this practice tothe extent specified herein:2.2 ASTM Standards:E1316 Terminology for Nondestructive Examinations23. Terminology3.1 Definitions—Definitions of terms relating to electromag-netic examination are given in Terminology E1316.3.2 Definitions of Terms Specific to This Standard:3.2.1 eddy-current reference standard— for the purposes ofthe method described in this practice, a rectangular block madeof an aluminum alloy (see 6.1.2) to which an active eddy-current probe is applied. The eddy-current reference standardcan also be referred to as an eddy-current test block.3.3 Mathematical Symbols:3.3.1 j—a symbol used in electrical engineering to represent=21 . It is associated with the restriction to the flow ofelectrical current caused by capacitors and coils.3.3.2 N— any number.3.3.3 | –N|—the magnitude of N, regardless whether N ispositive, negative, or a vector quantity.3.3.4 =N— the square root of N.3.3.5 (–N)2—N squared, that is, N × N.3.3.6 ∆N— delta N, the change or difference in N.3.4 Abbreviations:3.4.1 tan—used for the tangent function. The tan−1, arctangent or inverse tangent function, returns a value that is ameasure of an angle and can be in either degrees or radians.When using a calculator to determine the tan−1, care should betaken to determine whether the answer is in degrees or radianssince the numerical values that represent the same angle aredifferent.3.4.2 cos—used for the cosine function.3.4.3 sin—used for the sine function.4. Significance and Use4.1 Eddy-current probes may be used for the nondestructiveexamination of parts or structures made of electrically conduct-ing materials. Many of these examinations are intended to1This practice is under the jurisdiction of ASTM Committee E07 on Nonde-structive Testing and is the direct responsibility of Subcommittee E07.07 onElectromagnetic Method.Current edition approved Nov. 1, 2012. Published November 2012. Originallyapproved in 1994. Last previous edition approved in 2007 as E1629 - 07. DOI:10.1520/E1629-12.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.*A Summary of Changes section appears at the end of this standardCopyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1discover material defects, such as cracks, that may cause thepart or structure to be unsafe or unfit for service. Eddy-currentprobes that fail to meet the performance level requirements ofthis practice shall not be used for the examination of materialor hardware unless the probe is qualified by some other systemor an agreement has been reached by the probe manufacturerand the purchaser, or both.5. General Practice5.1 Use of Eddy-Current Reference Standards—The eddy-current reference standards described in this practice shall notbe used for purposes other than measuring the impedance ofeddy-current probes as specified in this practice. They shall notbe used for standardizing an examination or for determiningsensitivity to flaws.5.2 Responsibility—Initial determination of the impedancevalues of eddy-current probes shall be performed by the probemanufacturer in accordance with this practice. The results shallbe delivered with the probe and maintained by the organizationusing the probe. The results should include a description of themeasurement configuration and impedance values as listed inX1.4. While the retesting of probes may be performed usingthis practice, only the results obtained before the probe is in useshould be compared to the initial impedance values. Theimpedance results should not be compared to the initial valuesafter a probe has been used.6. Specific Practice6.1 Method—Impedance measurements shall be made on analuminum alloy reference block with a machined slot thatconforms to the requirements of this practice. The operatingfrequency (as specified by the probe manufacturer) may varyfor each probe examined, depending on the specific probegeometry, required skin depth, matching impedance, desiredsignal strength, and application. A commercial impedancemeasuring instrument that conforms to 6.1.1 shall be used tomake the measurements. The measurements will be recordedon the worksheet (Appendix X1) to calculate the probeimpedance. The calculated values shall be compared to theacceptable criteria (6.3) to determine probe acceptability.6.1.1 Equipment—The instrument shall be either a commer-cial impedance measuring instrument or an LCR meter with anoscillator capable of driving a current in the probe at theprobe’s operating frequency. The output shall display the probeimpedance in either polar coordinates, providing a magnitudeand a phase angle, or rectangular coordinates, providingresistive and reactive components of the impedance, or bothformats, that is, in polar and rectangular coordinates. Thisinstrument shall be certified as having been calibrated inaccordance with the manufacturer’s specifications, with thecalibration sticker indicating the calibration date and therequired interval or next calibration date.6.1.2 Reference Standards—The reference standard shall befabricated from 7075-T6 aluminum alloy 1.9-cm (0.75-in.)thick, with slotted holes for testing bolt hole probes and aslotted flat section for testing surface probes.All surfaces of thereference standard shall be polished to an average finish ≤ 15µm (591 µin.). The conductivity of the aluminum alloy shouldbe between 30 and 35 % IACS.6.1.2.1 The slotted flat section shall have side measurementsof at least six times the coil diameter or 5.1 by 5.1 cm (2 by 2in.), whichever is larger. The slot shall be machined across theblock’s surface and shall measure at least 5.0 cm (2-in.) long.The slot cross section shall measure 0.1 6 0.01 mm (0.004 60.0004 in.) wide and 0.5 6 0.025 mm (0.02 6 0.001 in.) deep.6.1.2.2 The holes shall be made for all of the nominal sizesof bolt hole probes to be examined. The edges of the holes shallbe spaced 1.9-cm (0.75-in.) apart from each other and from thereference standard edges. The slot shall run the entire length ofthe hole and will be 0.1 6 0.01-mm (0.004 6 0.0004-in.) wideand 0.5 6 0.025-mm (0.02 6 0.001-in.) deep.6.1.2.3 Fig. 1 shows a representative reference standardwith the holes used for testing two different sizes of bolt holeprobes. The length of the block (x + 5.1 cm) depends on thenumber and size of the test holes required by the user, as wellas the amount of clearance required between each hole and thereference standard’s edges.6.2 Measurement Procedure:6.2.1 Measurements—Impedance values can be expressedin different ways. Impedances are most commonly given inFIG. 1 Representative Eddy-Current Reference StandardE1629 − 122either rectangular or polar coordinates. In polar form, theimpedance is expressed as a magnitude, |Z|, with a correspond-ing phase angle, θ, and often appears as |Z| /θ. In rectangularform the impedance is expressed as a combination of a resistivecomponent, R, and a reactive or imaginary (denoted by j)component, X. This form often appears as R 6 jX. Fig. 2 showstwo points measured in both forms and the resulting impedancechange (∆Z) calculation. This is typical of the way in whichimpedance changes are measured on actual eddy-currentprobes using the approach specified in this practice. Theimpedance of a probe is measured first off the slot and then onthe slot in this method, and the difference between these twomeasurements is calculated. An error may occur in the calcu-lations if appropriate coordinate conversions are not made(addition and subtraction are performed in the case of rectan-gular coordinates and multiplication and division in the case ofpolar coordinates).6.2.1.1 All performance tests shall be conducted within thetemperature range from 15 to 27°C (60 to 80°F). The probe testfrequency shall depend on the rated operating frequency of theparticular probe under test (see X1.4.3.3).6.2.2 Probe Impedance in Air—Attach the probe to theimpedance measuring instrument, and position the probe atleast 50.8 mm (2 in.) away from any electrically conductingmaterial or hardware, or both. Measure the impedance andrecord the impedance values on the worksheet. Compare themeasurement to the values listed in 6.3.1.6.2.3 Average Off-Slot Probe Impedance— Place the probeon the surface of or in the hole in the reference standard, asappropriate. For a surface probe, place the probe on fourdifferent positions on the face of the reference standard. Thecenter of each position shall be at least four coil diameters fromany edge, slot, or hole. For a bolt hole probe, rotate the probeface in the hole to four different positions that are away fromthe slot and the top and bottom of the hole. Measure theimpedance and record the four impedance values on theworksheet in either polar or rectangular coordinates. Ifnecessary, convert the polar values to Roffand Xoff(resistiveand reactive components). Calculate the magnitude of each ofthe measurements and record them on the worksheet. Deter-mine the variation (scatter) in the magnitudes of the measure-ments as defined in X1.3.1.3 and compare it to the require-ments given in 6.3.2. When acceptable values are obtained,average the four values to calculate Roff avgand Xoff avg. Recordthe averages on the worksheet.6.2.4 Maximum On-Slot Impedance—Position the probeface on the slot to obtain a maximum impedance reading.Perform this procedure four times, and record the four resultingimpedances in rectangular coordinates, Ronand Xon, on theworksheet.Average the four values to calculate Ron avgand Xonavg. Record the averages on the worksheet.6.2.5 Probe Performance Criterion—The probe impedancechange caused by the slot,∆Z, is the magnitude of Ron avg− Roffavg and Xon avg− Xoff avg. The fractional change is the absolutevalue of ∆ Z divided by |Zoff avg|. Appendix X1 contains theformulas for calculating the values of |∆Z| and| Zoff avg|. Recordthe calculated values on the worksheet and compare them tothe requirements of 6.3.3 to determine acceptance.6.3 Acceptance Criteria—Acceptance of a probe beingcharacterized by this practice requires that it meet all of thefollowing criteria:6.3.1 Probe Impedance in Air—Unless otherwise specified,the magnitude of the probe impedance in air shall be between20 and 1000 Ω, and the phase shall be between 70 and 90 deg.An impedance value below 20 Ω indicates the possibility of ashort circuit in the probe coil, and a value above 1000 Ωindicates a possible open circuit.6.3.1.1 The magnitude of the impedance in air shall bewithin 10 % of the value specified for that type of probe by theprobe manufacturer and be within the input impedance rangespecified for the measuring instrument.6.3.2 Measurement Scatter—A variation greater than 4 %among the off-slot impedance measurements indicates that thevalues are too scattered. The measurements must be repeatedusing greater care in holding the surface probe more securelyor fitting the bolt hole probe more snugly in the hole. Repeatedhigh variation indicates that an analysis of the system should beperformed using different examiners or known acceptableprobes. The probe is unacceptable if the measurement scattercannot be reduced to the acceptable value.6.3.3 Probe Impedance Ratio—These ratios will be deter-mined by agreement between the eddy-current probe manufac-turer and the probe purchaser.7. Keywords7.1 absolute eddy-current probes; eddy-current probes; im-pedance; nondestructive testingFIG. 2 Rectangular and Polar Coordinates and Resulting ∆ZE1629 − 123APPENDIXES(Nonmandatory Information)X1. WORKSHEET FOR CALCULATION OF RESULTSX1.1 GeneralX1.1.1 Scope—This appendix provides mathematical for-mulas and a worksheet for recording measurements andcalculating results. It is recommended that a blank worksheetcontaining relevant fields such as identified in X1.4 be pre-pared for recording information and data from measurementsin accordance with this practice, and this blank worksheet bephotocopied for each probe tested. Appendix X2 contains anexample of a completed worksheet.X1.2 General PracticeX1.2.1 These measurements should be performed using aninstrument that displays impedance in polar or rectangularcoordinates, or both. To perform the necessary calculations,both forms of complex impedance (polar and rectangular) arenecessary. Mathematical conversion will be necessary if thetest instrument does not display the impedance in both forms.X1.2.1.1 Polar to Rectangular Conversion—The conver-sions from polar (|Z| / θ) to rectangular (R 6 jX) can beperformed using the following formulas:R 5?Z? cos θ X 5?Z? sin θ (X1.1)The use of a calculator with trigonometric functions (cos andsin) is recommended.X1.2.1.2 Rectangular to Polar Co