Standard Test Methods For DC Resistance Or Conductance Of .

D257 – 07designing an insulator for a specific application. The change ofresistivity or conductivity with temperature and humidity maybe great (1, 2, 3, 4),3 and must be known when designing foroperating conditions. Volume resistivity or conductivity determinations are often used in checking the uniformity of aninsulating material, either with regard to processing or to detectconductive impurities that affect the quality of the material andthat may not be readily detectable by other methods.5.5 Volume resistivities above 1021 V·cm (1019 V·m), calculated from data obtained on specimens tested under usuallaboratory conditions, are of doubtful validity, considering thelimitations of commonly used measuring equipment.5.6 Surface resistance or conductance cannot be measuredaccurately, only approximated, because some degree of volumeresistance or conductance is always involved in the measurement. The measured value is also affected by the surfacecontamination. Surface contamination, and its rate of accumulation, is affected by many factors including electrostaticcharging and interfacial tension. These, in turn, may affect thesurface resistivity. Surface resistivity or conductivity can beconsidered to be related to material properties when contamination is involved but is not a material property of electricalinsulation material in the usual sense.6. Electrode Systems6.1 The electrodes for insulating materials should be of amaterial that is readily applied, allows intimate contact with thespecimen surface, and introduces no appreciable error becauseof electrode resistance or contamination of the specimen (5).The electrode material should be corrosion-resistant under theconditions of test. For tests of fabricated specimens such asfeed-through bushings, cables, etc., the electrodes employedare a part of the specimen or its mounting. Measurements ofinsulation resistance or conductance, then, include the contaminating effects of electrode or mounting materials and aregenerally related to the performance of the specimen in actualuse.6.1.1 Binding-Post and Taper-Pin Electrodes, Fig. 1 andFig. 2, provide a means of applying voltage to rigid insulatingmaterials to permit an evaluation of their resistive or conductive properties. These electrodes simulate to some degree theactual conditions of use, such as binding posts on instrumentpanels and terminal strips. In the case of laminated insulatingmaterials having high-resin-content surfaces, somewhat lowerinsulation resistance values may be obtained with taper-pinthan with binding posts, due to more intimate contact with thebody of the insulating material. Resistance or conductancevalues obtained are highly influenced by the individual contactbetween each pin and the dielectric material, the surfaceroughness of the pins, and the smoothness of the hole in thedielectric material. Reproducibility of results on differentspecimens is difficult to obtain.3The boldface numbers in parentheses refer to the list of references appended tothese test methods.FIG. 1 Binding-Post Electrodes for Flat, Solid SpecimensFIG. 2 Taper-Pin Electrodes6.1.2 Metal Bars in the arrangement of Fig. 3 were primarily devised to evaluate the insulation resistance or conductance of flexible tapes and thin, solid specimens as a fairlysimple and convenient means of electrical quality control. Thisarrangement is somewhat more satisfactory for obtainingapproximate values of surface resistance or conductance whenthe width of the insulating material is much greater than itsthickness.Copyright by ASTM Int'l (all rights reserved); Wed Jul 11 19:48:55 EDT 20123Downloaded/printed byUniversity of California Berkeley Library pursuant to License Agreement. No further reproductions authorized.

D257 – 07FIG. 3 Strip Electrodes for Tapes and Flat, Solid SpecimensFIG. 5 Tubular Specimen for Measuring Volume and SurfaceResistances or Conductances6.1.3 Silver Paint, Fig. 4, Fig. 5, and Fig. 6, is availablecommercially with a high conductivity, either air-drying orlow-temperature-baking varieties, which are sufficiently porous to permit diffusion of moisture through them and therebyallow the test specimen to be conditioned after the applicationof the electrodes. This is a particularly useful feature instudying resistance-humidity effects, as well as change withtemperature. However, before conductive paint is used as anelectrode material, it should be established that the solvent inthe paint does not attack the material so as to change itsFIG. 4 Flat Specimen for Measuring Volume and SurfaceResistances or Conductanceselectrical properties. Reasonably smooth edges of guard electrodes may be obtained with a fine-bristle brush. However, forcircular electrodes, sharper edges can be obtained by the use ofa ruling compass and silver paint for drawing the outline circlesof the electrodes and filling in the enclosed areas by brush. Anarrow strip of masking tape may be used, provided thepressure-sensitive adhesive used does not contaminate thesurface of the specimen. Clamp-on masks also may be used ifthe electrode paint is sprayed on.6.1.4 Sprayed Metal, Fig. 4, Fig. 5, and Fig. 6, may be usedif satisfactory adhesion to the test specimen can be obtained.Thin sprayed electrodes may have certain advantages in thatthey are ready for use as soon as applied. They may besufficiently porous to allow the specimen to be conditioned, butthis should be verified. Narrow strips of masking tape orclamp-on masks must be used to produce a gap between theguarded and the guard electrodes. Use a tape that is known notto contaminate the gap surface.6.1.5 Evaporated Metal may be used under the same conditions given in 6.1.4.6.1.6 Metal Foil, Fig. 4, may be applied to specimensurfaces as electrodes. The usual thickness of metal foil usedfor resistance or conductance studies of dielectrics ranges from6 to 80 µm. Lead or tin foil is in most common use, and isusually attached to the test specimen by a minimum quantity ofpetrolatum, silicone grease, oil, or other suitable material, as anadhesive. Such electrodes shall be applied under a smoothingpressure sufficient to eliminate all wrinkles, and to work excessadhesive toward the edge of the foil where it can be wiped offwith a cleansing tissue. One very effective method is to use ahard narrow roller (10 to 15 mm wide), and to roll outward onthe surface until no visible imprint can be made on the foil withthe roller. This technique can be used satisfactorily only onspecimens that have very flat surfaces. With care, the adhesivefilm can be reduced to 2.5 µm. As this film is in series with theCopyright by ASTM Int'l (all rights reserved); Wed Jul 11 19:48:55 EDT 20124Downloaded/printed byUniversity of California Berkeley Library pursuant to License Agreement. No further reproductions authorized.

D257 – 07FIG. 6 Conducting-Paint Electrodesspecimen, it will always cause the measured resistance to betoo high. This error may become excessive for the lowerresistivity specimens of thickness less than 250 µm. Also thehard roller can force sharp particles into or through thin films(50 µm). Foil electrodes are not porous and will not allow thetest specimen to condition after the electrodes have beenapplied. The adhesive may lose its effectiveness at elevatedtemperatures necessitating the use of flat metal back-up platesunder pressure. It is possible, with the aid of a suitable cuttingdevice, to cut a proper width strip from one electrode to forma guarded and guard electrode. Such a three-terminal specimennormally cannot be used for surface resistance or conductancemeasurements because of the grease remaining on the gapsurface. It may be very difficult to clean the entire gap surfacewithout disturbing the adjacent edges of the electrode.6.1.7 Colloidal Graphite, Fig. 4, dispersed in water or othersuitable vehicle, may be brushed on nonporous, sheet insulating materials to form an air-drying electrode. Masking tapes orclamp-on masks may be used (6.1.4). This electrode material isrecommended only if all of the following conditions are met:6.1.7.1 The material to be tested must accept a graphitecoating that will not flake before testing,6.1.7.2 The material being tested must not absorb waterreadily, and6.1.7.3 Conditioning must be in a dry atmosphere (Procedure B, Practice D6054), and measurements made in this sameatmosphere.6.1.8 Liquid metal electrodes give satisfactory results andmay prove to be the best method to achieving the contact to thespecimen necessary for effective resistance measurements. Theliquid metal forming the upper electrodes should be confinedby stainless steel rings, each of which should have its lower rimreduced to a sharp edge by beveling on the side away from theliquid metal. Fig. 7 and Fig. 8 show two possible electrodearrangements.6.1.9 Flat Metal Plates, Fig. 4, (preferably guarded) may beused for testing flexible and compressible materials, both atroom temperature and at elevated temperatures. They may becircular or rectangular (for tapes). To ensure intimate contactwith the specimen, considerable pressure is usually required.FIG. 7 Liquid Metal Electrodes for Flat, Solid SpecimensCopyright by ASTM Int'l (all rights reserved); Wed Jul 11 19:48:55 EDT 20125Downloaded/printed byUniversity of California Berkeley Library pursuant to License Agreement. No further reproductions authorized.

D257 – 07be out of the water and of such length that leakage along theinsulation is negligible. Refer to specific wire and cable testmethods for the necessity to use guard at each end of aspecimen. For standardization it is desirable to add sodiumchloride to the water so as to produce a sodium chlorideconcentration of 1.0 to 1.1 % NaCl to ensure adequate conductivity. Measurements at temperatures up to about 100 C havebeen reported as feasible.FIG. 8 Liquid Metal Cell for Thin Sheet MaterialPressures of 140 to 700 kPa have been found satisfactory (seematerial specifications).6.1.9.1 A variation of flat metal plate electrode systems isfound in certain cell designs used to measure greases or fillingcompounds. Such cells are preassembled and the material to betested is either added to the cell between fixed electrodes or theelectrodes are forced into the material to a predeterminedelectrode spacing. Because the configuration of the electrodesin these cells is such that the effective electrode area and thedistance between them is difficult to measure, each cellconstant, K, (equivalent to the A/t factor from Table 1) can bederived from the following equation:K 5 3.6 p C 5 11.3 C(1)where:K has units of centimetres, andC has units of picofarads and is the capacitance of the electrode system withair as the dielectric. See Test Methods D150 for methods of measurementfor C.6.1.10 Conducting Rubber has been used as electrode material, as in Fig. 4, and has the advantage that it can quickly andeasily be applied and removed from the specimen. As theelectrodes are applied only during the time of measurement,they do not interfere with the conditioning of the specimen.The conductive-rubber material must be backed by properplates and be soft enough so that effective contact with thespecimen is obtained when a reasonable pressure is applied.NOTE 1—There is evidence that values of conductivity obtained usingconductive-rubber electrodes are always smaller (20 to 70 %) than valuesobtained with tinfoil electrodes (6). When only order-of-magnitudeaccuracies are required, and these contact errors can be neglected, aproperly designed set of conductive-rubber electrodes can provide a rapidmeans for making conductivity and resistivity determinations.6.1.11 Water is widely employed as one electrode in testinginsulation on wires and cables. Both ends of the specimen must7. Choice of Apparatus and Test Method7.1 Power Supply—A source of very steady direct voltage isrequired (see X1.7.3). Batteries or other stable direct voltagesupplies have been proven suitable for use.7.2 Guard Circuit—Whether measuring resistance of aninsulating material with two electrodes (no guard) or with athree-terminal system (two electrodes plus guard), considerhow the electrical connections are made between the testinstrument and the test specimen. If the test specimen is atsome distance from the test instrument, or the test specimen istested under humid conditions, or if a relatively high (1010 to1015 ohms) specimen resistance is expected, spurious resistance paths can easily exist between the test instrument and testspecimen. A guard circuit is necessary to minimize interferencefrom these spurious paths (see also X1.9).7.2.1 With Guard Electrode—Use coaxial cable, with thecore lead to the guarded electrode and the shield to the guardelectrode, to make adequate guarded connections between thetest equipment and test specimen. Coaxial cable (again with theshield tied back to the guard) for the unguarded lead is notmandatory here (or in 7.2.2), although its use provides somereduction in background noise (see also Fig. 9).7.2.2 Without Guard Electrode—Use coaxial cable, with thecore lead to one electrode and the shield terminated about 1 cmfrom the end of the core lead (see also Fig. 10).7.3 Direct Measurements—The current through a specimenat a fixed voltage is measured using any equipment that has therequired sensitivity and accuracy (610 % is usually adequate).Current-measuring devices available include electrometers, d-camplifiers with indicating meters, and galvanometers. Typicalmethods and circuits are given in Appendix X3. When themeasuring device scale is calibrated to read ohms directly nocalculations are required for resistance measurements.7.4 Comparison Methods—A Wheatstone-bridge circuitmay be used to compare the resistance of the specimen withthat of a standard resistor (see Appendix X3).7.5 Precision and Bias Considerations:7.5.1 General—As a guide in the choice of apparatus, thepertinent considerations are summarized in Table 2, but it is notimplied that the examples enumerated are the only onesapplicable. This table is not intended to indicate the limits ofsensitivity and error of the various methods per se, but ratheris intended to indicate limits that are distinctly possible withmodern apparatus. In any case, such limits can be achieved orexceeded only through careful selection and combination of theapparatus employed. It must be emphasized, however, that theerrors considered are those of instrumentation only. Errors suchas those discussed in Appendix X1 are an entirely differentmatter. In this latter connection, the last column of Table 2 liststhe resistance that is shunted by the insulation resistanceCopyright by ASTM Int'l (all rights reserved); Wed Jul 11 19:48:55 EDT 20126Downloaded/printed byUniversity of California Berkeley Library pursuant to License Agreement. No further reproductions authorized.

D257 – 07TABLE 1 Calculation of Resistivity or ConductivityAType of Electrodes or SpecimenCircular (Fig. 4)RectangularSquareTubes (Fig. 5)CablesVolume Resistivity, V-cmVolume Conductivity, S/cmARrv 5t vtGgv 5A vrv 5ARt vgv 5tGA vA 5rv 5ARt vgv 5tGA vA (a g) (b g)rv 5ARt vgv 5tGA vA (a g) 2rv 5ARt vgv 5tGA vA pD0(L g)rv 52pLRvD2lnD1Surface Resistivity,V (per square)Pp s 5 RsgCircular (Fig. 4)RectangularSquareTubes (Figs. 5 and 6)Effective Area of Measuring Electrodep D1 1 g! 24D2D12pLRvlngv 5Surface Conductivity,S (per square)ggs 5 GsPEffective Perimeterof Guarded Electrodeps 5PRg sgs 5gGP sP pD0ps 5PRg sgs 5gGP sP 2(a b 2g)ps 5PRg sgs 5gGP sP 4(a g)ps 5PRg sgs 5gGP sP 2p D2Nomenclature:A the effective area of the measuring electrode for the particular arrangement employed,P the effective perimeter of the guarded electrode for the particular arrangement employed,Rv measured volume resistance in ohms,Gv measured volume conductance in siemens,Rs measured surface resistance in ohms,Gs measured surface conductance in siemens,t average thickness of the specimen,D0, D1, D2, g, L dimensions indicated in Fig. 4 and Fig. 6 (see Appendix X2 for correctionto g),a, b, lengths of the sides of rectangular electrodes, andln natural logarithm.AAll dimensions are in centimetres.between the guarded electrode and the guard system for thevarious methods. In general, the lower such resistance, the lessprobability of error from undue shunting.NOTE 2—No matter what measurement method is employed, thehighest precisions are achieved only with careful evaluation of all sourcesof error. It is possible either to set up any of these methods from thecomponent parts, or to acquire a completely integrated apparatus. Ingeneral, the methods using high-sensitivity galvanometers require a morepermanent installation than those using indicating meters or recorders. Themethods using indicating devices such as voltmeters, galvanometers, d-camplifiers, and electrometers require the minimum of manual adjustmentand are easy to read but the operator is required to make the reading at aparticular time. The Wheatstone bridge (Fig. X1.4) and the potentiometermethod (Fig. X1.2 (b)) require the undivided attention of the operator inkeeping a balance, but allow the setting at a particular time to be read atleisure.7.5.2 Direct Measurements:7.5.2.1 Galvanometer-Voltmeter—The maximum percentage error in the measurement of resistance by thegalvanometer-voltmeter method is the sum of the percentageerrors of galvanometer indication, galvanometer readability,and voltmeter indication. As an example: a galvanometerhaving a sensitivity of 500 pA/scale division will be deflected25 divisions with 500 V applied to a resistance of 40 GV(conductance of 25 pS). If the deflection can be read to thenearest 0.5 division, and the calibration error (including AyrtonShunt error) is 62 % of the observed value, the resultantgalvanometer error will not exceed 64 %. If the voltmeter hasan error of 62 % of full scale, this resistance can be measuredwith a maximum error of 66 % when the voltmeter reads fullscale, and 610 % when it reads one-third full scale. Thedesirability of readings near full scale are readily apparent.7.5.2.2 Voltmeter-Ammeter—The maximum percentage error in the computed value is the sum of the percentage errorsin the voltages, Vx and Vs, and the resistance, Rs. The errors inVs and Rs are generally dependent more on the characteristicsCopyright by ASTM Int'l (all rights reserved); Wed Jul 11 19:48:55 EDT 20127Downloaded/printed byUniversity of California Berkeley Library pursuant to License Agreement. No further reproductions authorized.

D257 – 07FIG. 9 Connections to Guarded Electrode for Volume and SurfaceResistivity Measurements (Volume Resistance hook-up shown)FIG. 10 Connections to Unguarded Electrodes for Volume andSurface Resistivity Measurements (Surface Resistance Hook-UpShown)of the apparatus used than on the particular method. The mostsignificant factors that determine the errors in Vs are indicatorerrors, amplifier zero drift, and amplifier gain stability. Withmodern, well-designed amplifiers or electrometers, gain stability

1.2 These test methods are not suitable for use in measuring the electrical resistance/conductance of moderately conductive materials. Use Test Method D4496 to evaluate such materials. 1.3 This standard describes several general alternative methodologies for measuring resistance (or conductance).File Size: 421KBPage Count: 18Explore furtherASTM D257 - 14(2021)e1 Standard Test Methods for DC .www.astm.orgSurface Resistivity, Volume Resistivity, ASTM D257, IEC .www.intertek.com(PDF) Designation: D257 -07 Standard Test Methods for DC .www.researchgate.netDielectric Strength ASTM D149, IEC 60243www.intertek.comASTM D257-99 PDF - State of PDFstateofflux.infoRecommended to you b