ASTM_B_70_-_90_2013.pdf

Designation:B7090(Reapproved 2013)Standard Test Method forChange of Resistance With Temperature of MetallicMaterials for Electrical Heating1This standard is issued under the fixed designation B70;the number immediately following the designation indicates the year of originaladoption or,in the case of revision,the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon()indicates an editorial change since the last revision or reapproval.1.Scope1.1 This test method covers the determination of the changeof resistance with temperature of metallic materials for elec-trical heating,and is applicable over the range of servicetemperatures.1.2 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.1.3 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 become familiarwith all hazards including those identified in the appropriateMaterial Safety Data Sheet(MSDS)for this product/materialas provided by the manufacturer,to establish appropriatesafety and health practices,and determine the applicability ofregulatory limitations prior to use.2.Significance and Use2.1 The change in resistance with temperature for heatingelement materials is a major design factor and may influencematerial selection.The measurement of this change is essentialto ensure that heating elements perform as designed.This testmethod was designed to minimize the effect different manu-facturing processes have on resistance change,thereby yieldingresults that are reproducible.3.Test Specimen and Leads3.1 The test specimen shall be prepared from material as leftby the manufacturing process,and in a form suitable formeasuring its resistance in an electric furnace.When theresistance is to be measured with a Kelvin bridge,potentiometer,digital ohmmeter,or equivalent,a current leadshall be welded to each end of the specimen in such a mannerthat there will be no change of current distribution in thespecimen during measurements.Potential leads,one at eachend,shall be attached by welding,at a distance from thecorresponding current lead not less than one tenth of the lengthof the specimen between the potential leads.3.2 When the resistance is to be measured with a Wheat-stone bridge,only the current leads are required.The resistanceof the leads in this case shall not exceed 1%of the resistanceof the specimen and the leads shall be made of the same typeof alloy as the test specimen.For both methods ofmeasurement,the leads shall have a length within the heatedzone of the furnace of at least 50 times their minimumtransverse dimension,in order to avoid disturbance of thetemperature of the specimen by conduction of heat to thecolder parts of the furnace.4.Electric Furnace4.1 The furnace for heating the specimen shall be of such atype that the temperature can be controlled over the range fromroom temperature to the maximum desired.It shall be soconstructed that the specimens and the thermocouples can bemaintained at a uniform and constant temperature at desiredpoints within the working range.The specimen and thermo-couples shall be so shielded as to prevent direct radiation fromhotter,or to colder,parts of the furnace.4.2 In order to test the uniformity of the temperature in theregion to be occupied by the test specimen,a typical specimenand thermocouple shall be prepared and mounted in the centerof this region.The furnace shall then be heated to its maximumtemperature and maintained at this temperature until equilib-rium is reached.The specimen shall then be moved in thefurnace in the direction of the maximum temperature gradientthrough a distance equal to the maximum dimension of thelargest specimen and thermocouple assembly which is to beused in this furnace.The temperature of the typical specimen inthis position shall not differ from that in the normal position bymore than 10C.5.Resistance Measurements5.1 A Kelvin bridge,potentiometer,digital ohmmeter,orequivalent shall be used when measuring specimens havingresistances less than 10.A Wheatstone bridge may be usedwith specimens having resistances greater than 10.Theresistance of the specimen shall be measured with an accuracy1This test method is under the jurisdiction of ASTM Committee B02 onNonferrous Metals and Alloys and is the direct responsibility of SubcommitteeB02.10 on Thermostat Metals and Electrical Resistance Heating Materials.Current edition approved May 1,2013.Published May 2013.Originallyapproved in 1927.Last previous edition approved in 2007 as B70 90(2007).DOI:10.1520/B0070-90R13.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 of 0.1%.The measuring current shall be so small that theresistance of a specimen is not changed thereby as much as0.1%.This condition may be determined experimentally orcalculated from the power expended