This standard is applicable to the measurement of the change of resistance of electric heating alloys with temperature, and can also be used in situations where the temperature range of electric heating alloys is exceeded. JB/T 7900-2002 Change of resistance of high resistance electric heating alloys with temperature JB/T7900-2002 Standard download decompression password: www.bzxz.net

ICS29.120.20
Mechanical Industry Standard of the People's Republic of China
JB/T7900-2002
Replaces JB/T7900-1995
Standard test method for change of resistance with temperature of high resistance alloys for electrical heating (ASTM B70-90 [Reapproved 1995], MOD) Issued on December 27, 2002
Implementation on April 1, 2003
Issued by the State Economic and Trade Commission of the People's Republic of China Foreword,
Significance and application
Specimen and leads.
Resistance measurement
Measuring current
Test steps.
Temperature-resistance curve
Abnormal points.
10 Test report
Table of contents: No.
JB/T7900-2002
Appendix A (Informative Appendix) Comparison of the article numbers of this standard with those of ASTMB7090 (confirmed in 1995) Appendix B (Informative Appendix) Schematic diagram of resistance measurement circuit, Appendix C (Informative Appendix) Accuracy and deviation Figure B.1 Schematic diagram of single-arm bridge method circuit
Figure B.2 Schematic diagram of double-arm bridge method circuit
Table A.1 Comparison of the article numbers of this standard with those of ASTMB7090 (confirmed in 1995) Table 3
JB/T7900—2002
This standard modifies the American Society for Testing and Materials standard ASTMB7090 (confirmed in 1995) "Test method for change of resistance of electric heating alloys with temperature".
This standard replaces JB/T7900-1995 "Test Method for Change of Resistance of High Resistance Electric Heating Alloy with Temperature". This standard is redrafted based on ASTMB70-90 (confirmed in 1995). "Test Report" (Chapter 10), "Resistance Measurement Circuit Diagram" (Appendix B: Resistance Measurement Circuit Diagram) are added, "Keywords" are deleted, and Appendix A lists the comparison table of the chapter and article numbers of this standard and ASTMB7090 (confirmed in 1995). Compared with JB/T7900-1995, the main changes of this standard are as follows: the writing format conforms to GB/T1.1-2000 "Guidelines for Standardization Work Part 1: Structure and Writing Rules of Standards"; the scope of application is adjusted from 20℃～1300℃ to room temperature to above the use temperature of electric heating alloys (1995 version Section - suspension, this version 1.1); the distance between the voltage terminal and the current terminal is adjusted from not less than 2 times the maximum transverse dimension of the specimen to not less than 1/10 of the specimen length (1995 version 4.4, this version 3.2);
The calculation method of the free surface after the specimen is coiled or bent is added (this version 6.2.2): -The resistance temperature factor is no longer calculated and reported (1995 version 1.1): Appendix A, Appendix B, and Appendix C of this standard are all informative appendices. This standard is proposed by the China Machinery Industry Federation. This standard is under the jurisdiction of the National Technical Committee for Electrical Alloy Standardization. This standard is drafted by the Shanghai Electric Science Research Institute. The main drafter of this standard is Yu Jie.
The previous versions of the standards replaced by this standard are: -JB1096-1967, GB40681983, JB/T7900-1995. 1 Scope
Test method for change of resistance of high resistance electric heating alloy with temperature JB/T7900-2002
1.1 This standard is applicable to the measurement of change of resistance of electric heating alloy with temperature, and can also be used in situations exceeding the temperature range of use of electric heating alloy. 1.2 This standard does not cover all safety factors. Before using this standard, the user is responsible for establishing an appropriate safety system. 2 Significance and application
The change of resistance of electric heating alloy with temperature is an important design parameter, which involves the selection of materials and must be considered when designing electric heating elements or resistors. This standard can be used to analyze the impact of the manufacturing process on the change of resistance with temperature to achieve the purpose of consistent product performance.
3 Specimens and Leads
3.1 The test specimens shall be prepared from the materials left over from the manufacturing process and shall be shaped to be suitable for measuring resistance in an electric furnace. 3.2 When measuring resistance with a Kelvin bridge (double-arm bridge method), the current leads shall be welded to the ends of the specimen to prevent changes in the current distribution through the specimen during the test. The voltage leads shall be welded to both ends of the specimen, with the distance between the voltage and current terminals not less than 1/10 of the length of the specimen between the voltage terminals.
3.3 When measuring resistance with a Wheatstone bridge (single-arm bridge method), only the current leads shall be used, and the lead resistance shall not exceed 1% of the specimen resistance. 3.4 When measuring resistance, the leads shall exceed the length of the heating zone of the electric furnace and be at least 50 times their own lateral dimensions to prevent heat conduction to the cooler parts of the furnace from interfering with the test temperature. 3.5 To prevent the generation of thermoelectric potential, the leads shall be made of the same alloy as the specimen. 4 Electric furnace
4.1 The electric furnace for heating the sample should be able to effectively control the range from room temperature to the required maximum temperature. Its structure should ensure that the sample and thermocouple maintain a uniform and stable temperature at each required point within the working range. Protective measures should be taken for the sample and thermocouple to prevent the heating zone of the electric furnace from directly conducting heat outward.
4.2 The uniformity of the temperature in the sample placement area should be verified: select a typical sample and thermocouple and place them in the center of the heating zone, heat them to the highest test temperature, and maintain this temperature until equilibrium is reached. Then move the typical sample in the direction of the maximum temperature gradient in the electric furnace, and the moving distance should be the same as the maximum size of the sample and thermocouple placed in the electric furnace. The temperature difference between any two points in the moving area of ​​the typical sample should be less than 10°C.
5 Resistance measurement
See Appendix B for the circuit diagram of measuring resistance. When the resistance of the sample is less than 102, the Kelvin bridge should be used to measure the resistance: when the resistance of the sample is greater than 102, the Wheatstone bridge can be used to measure the resistance. The measurement accuracy of the resistance is 0.1%. The measurement current should make the change rate of the sample resistance not greater than 0.1%. It can be determined by experimental measurement or by calculating the surface power loss of the sample. 6 Measuring current
6.1 The measuring current can be determined by the following test method: Heat the sample to a certain temperature (for Ni-Cr alloy, 400℃ is a suitable temperature), at which time the resistance changes relatively large and uniformly with temperature. Apply a certain initial measurement current and keep it until the sample resistance stabilizes, then increase the current by 40% and keep this value until the sample resistance stabilizes again. If the difference between the two resistance measurements is greater than JB/T7900-2002
0.1% of the measured value, it means that the initial measurement current is too large. The initial test current should be adjusted until the above difference is less than 0.1%, and this current value can be selected as the measurement current.
6.2 When the effective free surface power loss of the specimen is less than 0.01Wlcm\, the effect of the measuring current on the resistance measurement can be ignored. The formula for calculating power loss-measuring current is as follows:
Where:
P-power loss, unit is W
measuring current, unit is A;
R.-resistance value at the highest test temperature, unit is 2. ..(1)
6.2.1 The specimen can be wound into a coil. The pitch of the coil should be greater than 5 times the maximum transverse dimension of the specimen cross section. Its effective free surface is the same as that of the straight strip specimen, which is the entire surface area of ​​the specimen between the two voltage terminals. 6.2.2 The specimen can also be bent forward or backward or coiled into a spiral shape. The interval between adjacent turns should be less than 5 times the maximum transverse dimension of the specimen cross section. Its effective free surface is the outer surface of the cylinder or prism formed after the specimen is bent or coiled. 7 Test steps
7.1 Place the specimen in an electric furnace and heat it to the maximum value of the specified test temperature. Maintain this temperature until the resistance of the specimen tends to be stable (excluding the relatively slow change in resistance caused by oxidation). Then gradually reduce the temperature of the electric furnace to room temperature, and measure once every 100°C when the temperature and resistance reach stability. In order to eliminate the influence of thermoelectric potential, each time the resistance is measured, a positive and negative measuring current should be passed through the specimen, and the average of the two resistance measurements should be taken as the measured value. 7.2 Use a calibrated thermocouple and potentiometer to measure the temperature. The temperature measurement accuracy error shall not exceed 10°C. 8 Temperature-resistance curve
Use the final room temperature resistance value as the base value to draw a curve of resistance versus temperature, and pay attention to the relative position between two consecutive points on the curve. By analyzing the curve, we can get the law of resistance decreasing with temperature and determine the temperature-resistance characteristics of the material under test. 9 Abnormal points
If there are abnormal points that are not smooth on the curve, we should follow 7.1 requires that the resistance and temperature be measured again at a temperature interval of about 25°C within the temperature range.
10 Test report
The contents of the test report are as follows:
Alloy material brand:
Sample number, specification and state:
Temperature resistance curve and actual measurement data: Measurement method:
Measurement current.
Appendix A
(Informative Appendix)
JB/T7900—2002
Comparison of the chapter and article numbers of this standard with those of ASTMB70—90 (confirmed in 1995) Table A.1 gives the comparison of the chapter and article numbers of this standard with those of ASTMB70—90 (confirmed in 1995). Table A.1 Comparison between the chapter and clause numbers of this standard and those of ASTM B70-90 (confirmed in 1995) Chapter and clause numbers of this standard
Chapter 2
3.1, 3.2
Chapter 5
6.2, 6.2.1, 6.2.2
Chapter 8
Chapter 9
Appendix C
Single-arm bridge
Appendix B
(Informative Appendix)
ASTM 70--90 (confirmed in 1995)Chapter No. 2.1
Chapter 11
Resistance Measurement Schematic diagram of measurement circuit
Electric medical furnace
Measuring plate end
Thermometer
Schematic diagram of single-arm bridge method circuit
Temperature controller
JB/T7900—2002
Double-charge bridge
Measuring end
Measurement only
Figure B.2 Schematic diagram of double-arm bridge method circuit
Appendix C
(Informative Appendix)
Accuracy and deviation
Temperature controller
C.1 The repeatability of resistance change with temperature mainly depends on the uniformity of the test temperature and the rate of cooling. For Ni--Cr alloy, the faster the rate of temperature reduction, the smaller the rate of change of resistance. C.2 The accuracy of this test method is ±2%.
People's Republic of China
Mechanical Industry Standard
Test Method for Change of Resistance of High Resistance Heating Alloy with Temperature JB/T7900—2002
Published and distributed by Machinery Industry Press
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Format 890mm×1240mm1/16×0.75 Printing Sheet·13,000 Words 2003 First edition, first printing, April 2017
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