ASTM_D_3215_-_93.pdf

Designation:D 3215 93An American National StandardStandard Test Method forMeasurement of the Specific Electrical Impedance ofElectrical-Grade Magnesium Oxide for Use in Sheathed-TypeHeating Elements1This standard is issued under the fixed designation D 3215;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(e)indicates an editorial change since the last revision or reapproval.1.Scope1.1 This test method covers the determination of specificelectrical impedance of electrical-grade magnesium oxide byexternally heating a test cell to elevated temperatures usingtypical materials.1.2 The values stated in SI units are to be regarded as thestandard.The values given in parentheses are for informationonly.1.3 This standard does not purport to address all of thesafety problems,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 ASTM Standards:B 344 Specification for Drawn or Rolled Nickel-Chromiumand Nickel-Chromium-Iron Alloys for Electrical HeatingElements2D 2755 Test Method of Sampling and Reduction to TestWeight of Electrical Grade Magnesium Oxide33.Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 measured impedance,Z,nthe ratio of the root meansquare of the voltage applied between two electrodes that are incontact with the specimen to the root mean square of thecurrent distributed throughout the volume of the specimen(neglecting phase effects).3.1.2 shape factor,F,nthe ratio of the test cell length tothe logarithm of the ratio of the test-cell diameters,relating thecurrent flow per unit length to the logarithmic mean potentialgradient across the insulation.3.1.3 specific electrical impedance,Zs,nthe ratio of theroot mean square of an alternating potential gradient parallel tothe current in the material to the root mean square of thecurrent density,and equal to the product of the measuredimpedance Z and a geometrical shape factor F.(The impedanceis essentially resistive and phase effects are ignored).4.Summary of Test Method4.1 The specimen of magnesium oxide is poured into alength of tubing around a centrally located rod,and theassembly is compacted to form a test cell simulating a sheathedheating element.The test cell is placed in a heated furnace.Voltage is applied across the magnesium oxide in the test cellin series with a known resistance.The leakage current ismeasured in milliamperes.The leakage current and the geo-metric factor are used to calculate the specific impedance.5.Significance and Use5.1 The specific electrical impedance can be used to indicatewhether or not the magnesium oxide provides sufficient im-pedance to perform satisfactorily in high-temperature heatingelements.6.Apparatus6.1 Tube Furnace,4horizontal,having a uniformly hot zoneat least 178 mm(7 in.)long.A maximum temperature spreadof 65.5C(610F)between three thermocouples is accept-able.6.2 Tube Loading Devices(see Fig.1 for suggested design).6.3 Swaging Machine.56.4 Swaging Dies,12.06 mm(0.475 in.),11.43 mm(0.450in.),and 11.18 mm(0.440 in.).6.5 Metal Trimming Device.6.6 Vibrator.66.7 Spot Welding Machine.6.8 Thermocouples,three,Type R,No.24 B&S gage,1This test method is under the jurisdiction of ASTM Committee D-9 onElectrical and Electronic Insulating Materials and is the direct responsibility ofSubcommittee D09.14 on Electric Heating Unit Insulation.Current edition approved March 15,1993.Published May 1993.Originallypublished as D 3215 73.Last previous edition D 3215 83.2Annual Book of ASTM Standards,Vol 03.04.3Annual Book of ASTM Standards,Vol 10.02.4A Lindberg Single Zone Tube Furnace,Type 59545,12 in.,500 to 1500C,available from Lindberg Co.,304 Hart St.,Watertown,WI 53094,with TemperatureControl using Type R Thermocouple and suitable Ceramic Tube and Plugs,orequivalent,has been found suitable for this purpose.5Size 4 swaging machines available from either the Torrington Co.,59 Field St.,Torrington,CN 06790,or the Fenn Mfg.Co.,200 Fenn Rd.,Newington,CN 06111,or their equivalent,are satisfactory for this test method.6The L&A vibrator,Model F-769,Type B,Size 3,available from the PressedSteel Co.,705 N.Pennsylvania Ave.,Wilkes-Barre,PA 18703,or its equivalent,issatisfactory.1Copyright ASTM,100 Barr Harbor Drive,West Conshohocken,PA 19428-2959,United States.NOTICE:This standard has either been superseded and replaced by a new version or discontinued.Contact ASTM International(www.astm.org)for the latest information.0.510-mm(0.020 in.)diameter with appropriate potentiom-eter.76.9 Test Circuit,(see Fig.2),capable of measuring tempera-ture from 500 to 1500C and leakage current from 0 to 19.99mA,comprised of the following:6.9.1 Essentially Sinusoidal Power Source,with a mi