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ASTM B70-2019 pdf free download

ASTM B70-2019 pdf free download.Standard Test Method for Change of Resistance With Temperature of Metallic Materials for Electrical Heating
1. Scope
1.1 This test method covers the determination of the change of resistance with temperature of metallic materials for elec- trical heating, and is applicable over the range of service temperatures. 1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard. 1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appro- priate safety, health, and environmental practices and deter- mine the applicability ofregulatory limitations prior to use. 1.4 This international standard was developed in accor- dance with internationally recognized principles on standard- ization established in the Decision on Principles for the Development of International Standards, Guides and Recom- mendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Significance and Use
2.1 The change in resistance with temperature for heating element materials is a major design factor and may influence material selection. The measurement of this change is essential to ensure that heating elements perform as designed. This test method was designed to minimize the effect different manu- facturing processes have on resistance change, thereby yielding results that are reproducible.
3. Test Specimen and Leads
3.1 The test specimen shall be prepared from material as left by the manufacturing process, and in a form suitable for measuring its resistance in an electric furnace. When the resistance is to be measured with a Kelvin bridge, potentiometer, digital ohmmeter, or equivalent, a current lead shall be welded to each end of the specimen in such a manner that there will be no change of current distribution in the specimen during measurements. Potential leads, one at each end, shall be attached by welding, at a distance from the corresponding current lead not less than one tenth ofthe length of 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 resistance of the leads in this case shall not exceed 1 % of the resistance of the specimen, and the leads shall be made of the same type of alloy as the test specimen. For both methods of measurement, the leads shall have a length within the heated zone of the furnace of at least 50 times their minimum transverse dimension, in order to avoid disturbance of the temperature of the specimen by conduction of heat to the colder parts of the furnace.
4. Electric Furnace
4.1 The furnace for heating the specimen shall be of such a type that the temperature can be controlled over the range from room temperature to the maximum desired. It shall be so constructed that the specimens and the thermocouples can be maintained at a uniform and constant temperature at desired points within the working range. The specimen and thermo- couples shall be so shielded as to prevent direct radiation from hotter, or to colder, parts of the furnace.4.2 In order to test the uniformity of the temperature in the region to be occupied by the test specimen, a typical specimen and thermocouple shall be prepared and mounted in the center ofthis region. The furnace shall then be heated to its maximum temperature and maintained at this temperature until equilib- rium is reached. The specimen shall then be moved in the furnace in the direction of the maximum temperature gradient through a distance equal to the maximum dimension of the largest specimen and thermocouple assembly which is to be used in this furnace. The temperature ofthe typical specimen in this position shall not differ from that in the normal position by more than 10 °C.

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