1、Designation: B 70 90 (Reapproved 2007)Standard Test Method forChange of Resistance With Temperature of MetallicMaterials for Electrical Heating1This standard is issued under the fixed designation B 70; the number immediately following the designation indicates the year of originaladoption or, in the
2、 case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscriptepsilon (e) 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
3、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,
4、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
5、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 h
6、eating 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 manufactu
7、ring process, and in a form suitable formeasuring its resistance in an electric furnace. When theresistance is to be measured with a Kelvin bridge, potentiom-eter, digital ohmmeter, or equivalent, a current lead shall bewelded to each end of the specimen in such a manner that therewill be no change
8、of current distribution in the specimen duringmeasurements. Potential leads, one at each end, shall beattached by welding, at a distance from the correspondingcurrent lead not less than one tenth of the length of thespecimen between the potential leads.3.2 When the resistance is to be measured with
9、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 of measure-ment, the leads shall have a length within t
10、he heated zone ofthe furnace of at least 50 times their minimum transversedimension, in order to avoid disturbance of the temperature ofthe specimen by conduction of heat to the colder parts of thefurnace.4. Electric Furnace4.1 The furnace for heating the specimen shall be of such atype that the tem
11、perature 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 shie
12、lded 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
13、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 asse
14、mbly 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 havin
15、gresistances less than 10 V . A Wheatstone bridge may be usedwith specimens having resistances greater than 10 V. Theresistance of the specimen shall be measured with an accuracyof 0.1 %. The measuring current shall be so small that the1This test method is under the jurisdiction of ASTM Committee B0
16、2 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, 2007. Published May 2007. Originallyapproved in 1927. Last previous edition approved in 2001 as B 70 90 (2001).1Cop
17、yright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.resistance of a specimen is not changed thereby as much as0.1 %. This condition may be determined experimentally orcalculated from the power expended and the surface of thespecimen.6. Test
18、Current6.1 To determine experimentally that the test current is nottoo large, bring the specimen to a temperature (Note 1) wherethere is a relatively large uniform change of resistance withtemperature. Apply the test current and maintain it until theresistance of the specimen has become constant. Th
19、en increasethe current by 40 % and maintain it at this value until theresistance has again become constant. If the change in resis-tance is greater than 0.1 %, the test current is too large and shallbe reduced until the foregoing limitations are reached.NOTE 1In the case of nickel-chromium alloy 400
20、C is a suitabletemperature.6.2 The test current has a negligible effect on the resistancemeasurement when the power lost in its passage through thespecimen is less than 0.01 W/cm2of the effective free surfaceof the specimen. For straight specimens and those which are socoiled or bent that the distan
21、ce between the adjacent convolu-tions is greater than five times the maximum transversedimension of the cross section of the specimen, consider thefree surface to be that surface area of the portion of thespecimen between the potential leads. When, as for the sake ofsaving space in the furnace, the
22、specimen is wound into a spiralor helix, or bent back and forth upon itself in such a mannerthat the distance between adjacent convolutions is less than fivetimes the maximum transverse dimension of the cross sectionof the specimen, consider the free surface to be that surfacearea of the cylindrical
23、 or prismatic volume enclosing the coiledor convoluted specimen. Calculate the power loss due to themeasuring current from the following equation:W 5 I2Rmwhere:W = power loss, W,I = measuring current A, andRm= resistance at maximum test temperature, V.7. Procedure7.1 Mount the test specimen in the f
24、urnace, bring thetemperature of the furnace to the maximum specified tempera-ture of test for the alloy in question, and hold at this value untilthe resistance of the specimen remains constant except for therelatively slow changes due to oxidation. Then lower thetemperature of the furnace in steps o
25、f approximately 100C toroom temperature. Take measurements at each point whentemperature and resistance have become stationary. Note thetime at which each reading is taken. Define each point by themean of at least one pair of resistance readings for which thecurrent through the sample has been rever
26、sed between read-ings. This is necessary in order to eliminate the effects ofthermal electromotive forces.7.2 Determine the temperature by means of calibratedthermocouples, in conjunction with a potentiometer or pyrom-eter of such construction as to ensure an accuracy correspond-ing to a temperature
27、 uncertainty not exceeding 10C.8. Temperature-Resistance Curve8.1 Plot a curve showing the change of resistance withtemperature using the final room-temperature resistance valueas a base. Note the time interval between successive readingson the curve sheet. Consider the curve thus obtained withdesce
28、nding temperature as defining the true temperature-resistance characteristics of the material tested.9. Singular Points9.1 If there are indications that the curve is not smooth atany point, carry the specimen through the temperature cycleagain, and take temperature and resistance readings at interva
29、lsof approximately 25C in the suspected region.10. Precision and Bias10.1 The reproducibility of the change in resistance withtemperature depends primarily on the uniformity of tempera-ture of the sample and secondarily on the rate of temperaturedecrease. For nickel-chromium alloys the faster the de
30、crease intemperature, the lower the change in resistance.10.2 The precision of this test method is within 62%.10.3 The bias cannot be determined for reasons detailed in10.1.11. Keywords11.1 coefficient of resistance; heating elements; resistancechange; resistors; temperature-resistanceASTM Internati
31、onal takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standard are expressly advised that determination of the validity of any such patent rights, and the riskof infringement of such rights, are entirely thei
32、r own responsibility.This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years andif not revised, either reapproved or withdrawn. Your comments are invited either for revision of this standard or for additional standardsand should b
33、e addressed to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of theresponsible technical committee, which you may attend. If you feel that your comments have not received a fair hearing you shouldmake your views known to the ASTM Committee on Standard
34、s, at the address shown below.This standard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,United States. Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the aboveaddress or at 610-832-9585 (phone), 610-832-9555 (fax), or serviceastm.org (e-mail); or through the ASTM website(www.astm.org).B 70 90 (2007)2
