ASTM B193-20

Standard Test Method for Resistivity of Electrical Conductor Materials

ABSTRACT
This test method covers the determination of the electrical resistivity of metallic electrical conductor material. Weight resistivity accuracy may be adversely affected by possible inaccuracies in the assumed density of the conductor. The definition of resistivity and the equations for calculating volume resistivity and weight resistivity are given. Resistance shall be measured using an apparatus with a circuit configuration and instrumentation that has a resistance measurement capability of the prescribed accuracy. The test specimen requirements and the test procedure including: (1) determination of dimensions (such as length and cross-section), weight, and density, and (2) resistance measurement are detailed. The formula for calculating the corrected resistance, when the measurement is made at any temperature other than a reference temperature, is given. No statement of precision has been made and no work has been planned to develop such a statement. This test method has no bias because the value for resistivity is determined solely in terms of this test method.
SCOPE
1.1 This test method covers the determination of the electrical resistivity of metallic electrical conductor material. It provides for an accuracy of ±0.30 % on test specimens having a resistance of 0.00001 Ω (10 μΩ) or more. Weight resistivity accuracy may be adversely affected by possible inaccuracies in the assumed density of the conductor.  
1.2 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 appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.3 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Oct-2020
• ICS: 29.050 - Superconductivity and conducting materials
• Technical Committee: B01 - Electrical Conductors
• Drafting Committee: B01.02 - Methods of Test and Sampling Procedure

Relations

• Refers: ASTM B3-13(2024) - Standard Specification for Soft or Annealed Copper Wire
• Effective Date: 01-Apr-2024
• Refers: ASTM B941-24 - Standard Specification for Heat Resistant Aluminum-Zirconium Alloy Wire for Electrical Purposes
• Effective Date: 01-Apr-2024
• Refers: ASTM B33-10(2020)e1 - Standard Specification for Tin-Coated Soft or Annealed Copper Wire for Electrical Purposes
• Effective Date: 01-Jan-2020
• Refers: ASTM B227-15(2019) - Standard Specification for Hard-Drawn Copper-Clad Steel Wire
• Effective Date: 01-Oct-2019
• Refers: ASTM B2-13(2018) - Standard Specification for Medium-Hard-Drawn Copper Wire
• Effective Date: 01-Oct-2018
• Refers: ASTM B3-13(2018) - Standard Specification for Soft or Annealed Copper Wire
• Effective Date: 01-Oct-2018
• Refers: ASTM B1-13(2018) - Standard Specification for Hard-Drawn Copper Wire
• Effective Date: 01-Oct-2018
• Refers: ASTM B9-76(2017)e1 - Standard Specification for Bronze Trolley Wire
• Effective Date: 01-Apr-2017
• Refers: ASTM B9-76(2017) - Standard Specification for Bronze Trolley Wire
• Effective Date: 01-Apr-2017
• Refers: ASTM B47-95a(2017) - Standard Specification for Copper Trolley Wire
• Effective Date: 01-Apr-2017
• Refers: ASTM B298-12(2017) - Standard Specification for Silver-Coated Soft or Annealed Copper Wire
• Effective Date: 01-Apr-2017
• Refers: ASTM B498/B498M-08(2016) - Standard Specification for Zinc-Coated (Galvanized) Steel Core Wire for Use in Overhead Electrical Conductors
• Effective Date: 01-Oct-2016
• Refers: ASTM B355-11(2016) - Standard Specification for Nickel-Coated Soft or Annealed Copper Wire
• Effective Date: 01-Oct-2016
• Refers: ASTM B227-15 - Standard Specification for Hard-Drawn Copper-Clad Steel Wire
• Effective Date: 01-Apr-2015
• Refers: ASTM B33-10(2014) - Standard Specification for Tin-Coated Soft or Annealed Copper Wire for Electrical Purposes
• Effective Date: 01-Sep-2014

Overview

ASTM B193-20 - Standard Test Method for Resistivity of Electrical Conductor Materials establishes a reliable method for determining the electrical resistivity of metallic conductor materials. This international test method, developed by ASTM International, provides guidelines for accurate measurement and calculation of both volume resistivity and weight resistivity, accommodating a wide range of metallic conductor geometries such as wires, rods, bars, strips, tubes, and shapes.

The standard is essential for professionals involved in the manufacture, quality control, and testing of electrical conductor materials, ensuring uniformity and comparability in testing procedures. The method covers specimen preparation, dimension and weight determination, resistance measurement, temperature correction, and reporting requirements.

Key Topics

• Electrical Resistivity Determination
  Details procedures for measuring the resistivity of metallic conductors with resistance of 0.00001 Ω (10 μΩ) or higher with an accuracy of ±0.30%.

• Volume and Weight Resistivity Calculations
  Provides formulas for calculating resistivity based on specimen cross-sectional area or weight, in both metric and customary units.

• Specimen Preparation
  Outlines requirements for acceptable test specimens, ensuring accurate results by mandating uniform cross-section, absence of joints and visible defects, and proper surface condition.

• Measurement Methodology
  Describes apparatus configuration, measurement techniques including use of Kelvin-type bridges or potentiometers, and precautions for lead arrangements.

• Temperature Correction
  Presents a correction formula to adjust resistance measurements to a standard reference temperature, accounting for the temperature coefficient of resistance.

• Reporting Requirements
  Recommends specific data points to be reported for referee and routine tests, including material identification, dimensions, measurement methodology, and calculated resistivity values.

Applications

The ASTM B193-20 test method is widely used in industries requiring precise determination of electrical properties for conductor materials. Its applications include:

• Quality Control in Manufacturing
  Ensures the electrical performance of copper, aluminum, and specialty alloy wires or cables meets industry and regulatory requirements.

• Material Comparison and Selection
  Facilitates comparative assessment of resistivity for selecting optimal conductor materials based on performance criteria.

• Product Certification and Compliance
  Supports compliance with international standards and specifications by providing a consistent measurement framework.

• Research and Development
  Aids in the development and evaluation of new metallic conductor materials, including those with special coatings or composite structures.

• Electrical and Electronics Engineering
  Assists engineers in designing components and systems by enabling accurate assessment of conductor material properties.

Related Standards

ASTM B193-20 is referenced alongside several key specifications and standards in the field of electrical conductors, including:

• ASTM B1 - Hard-Drawn Copper Wire
• ASTM B3 - Soft or Annealed Copper Wire
• ASTM B9 - Bronze Trolley Wire
• ASTM B33 - Tin-Coated Copper Wire for Electrical Purposes
• ASTM B47 - Copper Trolley Wire
• ASTM B105 - Hard-Drawn Copper Alloy Wires for Electric Conductors
• ASTM B230 - Aluminum 1350–H19 Wire for Electrical Purposes
• ASTM B298 - Silver-Coated Copper Wire
• ASTM B355 - Nickel-Coated Copper Wire

NIST Handbook 100 – Copper Wire Tables is also commonly referenced for conductivity and density values.

ASTM B193-20 aligns with internationally recognized standardization principles as established by the World Trade Organization Technical Barriers to Trade (TBT) Committee, ensuring global relevance and acceptance.

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Keywords: ASTM B193-20, electrical resistivity, metallic conductor materials, volume resistivity, weight resistivity, resistance measurement, electrical conductor standards, quality control, temperature correction, wire testing.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: B193 − 20
Standard Test Method for
Resistivity of Electrical Conductor Materials
This standard is issued under the fixed designation B193; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope Wire for Electrical Purposes
B47 Specification for Copper Trolley Wire
1.1 This test method covers the determination of the elec-
B105 Specification for Hard-Drawn CopperAlloy Wires for
trical resistivity of metallic electrical conductor material. It
Electric Conductors
provides for an accuracy of 60.30 % on test specimens having
B227 Specification for Hard-Drawn Copper-Clad Steel Wire
a resistance of 0.00001 Ω (10 µΩ) or more. Weight resistivity
B230 Specification forAluminum 1350–H19 Wire for Elec-
accuracy may be adversely affected by possible inaccuracies in
trical Purposes
the assumed density of the conductor.
B298 Specification for Silver-Coated Soft orAnnealed Cop-
1.2 This standard does not purport to address all of the
per Wire
safety concerns, if any, associated with its use. It is the
B355 SpecificationforNickel-CoatedSoftorAnnealedCop-
responsibility of the user of this standard to establish appro-
per Wire
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use. B396 Specification forAluminum-Alloy 5005-H19 Wire for
1.3 This international standard was developed in accor- Electrical Purposes (Withdrawn 2003)
dance with internationally recognized principles on standard-
B398 Specification for Aluminum-Alloy 6201-T81 Wire for
ization established in the Decision on Principles for the
Electrical Purposes [Metric](Discontinued 1999-Replaced
Development of International Standards, Guides and Recom-
by B 398/B398M) B0398_B0398M
mendations issued by the World Trade Organization Technical
B415 Specification for Hard-Drawn Aluminum-Clad Steel
Barriers to Trade (TBT) Committee.
Wire
B498/B498M Specification for Zinc-Coated (Galvanized)
2. Referenced Documents
Steel Core Wire for Use in Overhead Electrical Conduc-
2.1 ASTM Standards:
tors
A111 Specification for Zinc-Coated (Galvanized) “Iron”
B502 Specification for Aluminum-Clad Steel Core Wire for
Telephone and Telegraph Line Wire
Use in Overhead Electrical Aluminum Conductors
A326 Specification for Zinc-Coated (Galvanized) High Ten-
B566 Specification for Copper-Clad Aluminum Wire
sile Steel Telephone and Telegraph Line Wire (Withdrawn
B606 Specification for High-Strength Zinc-Coated (Galva-
1990)
nized) Steel Core Wire for Aluminum and Aluminum-
B1 Specification for Hard-Drawn Copper Wire
Alloy Conductors, Steel Reinforced
B2 Specification for Medium-Hard-Drawn Copper Wire
B609 Specification for Aluminum 1350 Round Wire, An-
B3 Specification for Soft or Annealed Copper Wire
nealed and Intermediate Tempers, for Electrical Purposes
B9 Specification for Bronze Trolley Wire
B800 Specification for 8000 Series Aluminum Alloy Wire
B33 Specification for Tin-Coated Soft or Annealed Copper
for Electrical Purposes—Annealed and Intermediate Tem-
pers
This test method is under the jurisdiction of ASTM Committee B01 on
B802 Specification for Zinc-5% Aluminum-Mischmetal
Electrical Conductors and is the direct responsibility of Subcommittee B01.02 on
Alloy-Coated Steel Core Wire forAluminum Conductors,
Methods of Test and Sampling Procedure.
Current edition approved Nov. 1, 2020. Published November 2020. Originally Steel Reinforced (ACSR)[Metric](Discontinued 1998-
approved in 1944. Last previous edition approved in 2019 as B193 – 19. DOI:
Replaced by B 802/B802M) B0802_B0802M
10.1520/B0193-20.
B803 Specification for High-Strength Zinc–5 % Aluminum-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Mischmetal Alloy-Coated Steel Core Wire for Use in
Standards volume information, refer to the standard’s Document Summary page on
Overhead Electrical Conductors
the ASTM website.
3 B941 Specification for Heat ResistantAluminum-Zirconium
The last approved version of this historical standard is referenced on
www.astm.org. Alloy Wire for Electrical Purposes
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B193 − 20
B957 Specification for Extra-High-Strength and Ultra-High- 5.2.5 No joints or splices.
Strength Zinc-Coated (Galvanized) Steel Core Wire for
Overhead Electrical Conductors
6. Procedure
B958 Specification for Extra-High-Strength and Ultra-High-
6.1 Make all determinations of the dimensions and weight
Strength Class A Zinc–5% Aluminum-Mischmetal Alloy-
of the test specimen using instruments accurate to 60.05 %. In
Coated Steel Core Wire for Use in Overhead Electrical
order to assure this accuracy in measuring the length between
Conductors
potential contacts, the surface in contact with the test specimen
2.2 NIST Document:
shall be a substantially sharp knife-edge when using a Kelvin-
NBS Handbook 100 —Copper Wire Tables
type bridge or a potentiometer.
3. Resistivity
6.2 The cross-sectional dimensions of the specimen may be
determined by micrometer measurements, and a sufficient
3.1 Resistivity (Explanatory Note 1) is the electrical resis-
number of measurements shall be made to obtain the mean
tance of a body of unit length, and unit cross-sectional area or
cross section to within 60.10 %. In case any dimension of the
unit weight.
specimen is less than 0.100 in. and cannot be measured to the
3.2 VolumeResistivity is commonly expressed in ohms for a
requiredaccuracy,determinethecross-sectionfromtheweight,
theoretical conductor of unit length and cross-sectional area; in
density, and length of the specimen.
inch-pound units in Ω·cmil/ft and in acceptable metric units in
6.3 When the density is unknown, determine the density by
Ω·mm /m. It may be calculated by the following equation:
weighing a specimen first in air and then in a liquid of known
ρ 5 A/L R
~ !
v
density at the test temperature, which shall be room tempera-
where:
turetoavoiderrorsduetoconvectioncurrents.Exercisecarein
removing all air bubbles from the specimen when weighing it
ρ = volume resistivity, Ω·cmil/ft or Ω·mm /m,
v
A = cross-sectional area, cmil or mm , in the liquid. Calculate the density from the following equa-
L = gage length, used to determine R,ftorm,and
tion:
R = measured resistance, Ω.
δ 5 W 3d / W 2 W
~ ! ~ !
a a l
3.3 Weight Resistivity is commonly expressed in ohms for a
where:
theoretical conductor of unit length and weight. The method
δ = density of the specimen, g/cm ;
for calculating weight resistivity, based on resistance, length,
W = weight of the specimen in air, g;
a
and weight measurements, of a test specimen is given in
W = weight of the specimen in the liquid, g; and
l
Explanatory Note 2.
d = density of the liquid at the test temperature, g/cm .
4. Apparatus
6.4 When potential leads are used, make sure the distance
4.1 Resistance shall be measured with a circuit configura- between each potential contact and the corresponding current
contact is at least equal to 1 ⁄2 times the cross-sectional
tion and instrumentation that has a resistance measurement
capability of 60.15 % accuracy. perimeter of the specimen. Make sure the yoke resistance
(between reference standard and test specimen) is appreciably
5. Test Specimen
smaller than that of either the reference standard or the test
5.1 The test specimen may be in the form of a wire, strip, specimen unless a suitable lead compensation method is used,
rod, bar, tube, or shape. It shall be of uniform cross section or it is known that the coil and lead ratios are sufficiently
throughout its length within 60.75 % of the cross-sectional balanced so that variation in yoke resistance will not decrease
the bridge accuracy below stated requirements.
area. Wherever possible it shall be the full cross section of the
material it represents, if the full cross section is such that the
6.5 Make resistance measurements to an accuracy of
uniformity of the cross-sectional area can be accurately deter-
60.15 %. To ensure a correct reading, allow the reference
mined.
standardandthetestspecimentocometothesametemperature
5.2 The test specimen shall have the following characteris- as the surrounding medium. (If the reference standard is made
tics: of manganin it is possible to obtain correct readings with the
5.2.1 A resistance of at least 0.00001 Ω (10 µΩ) in the test test specimen at reference temperatures other than room
length between potential contacts, temperature). In all resistance measurements, the measuring
5.2.2 A test length of at least 1 ft or 300 mm, current raises the temperature of the medium. Therefore, take
5.2.3 A diameter, thickness, width, or other dimension care to keep the magnitude of the current low, and the time of
suitable to the limitations of the resistance measuring its use short enough so that the change in resistance cannot be
instrument, detected with the galvanometer. To eliminate errors due to
5.2.4 No surface cracks or defects visible to the unaided
contact potential, take two readings, one direct and one with
normal eye, and substantially free from surface oxide, dirt, and current reversed, in direct succession. Check tests are recom-
grease, and
mended whereby the specimen is turned end for end, and the
test repeated. Surface cleaning of the specimen at current and
potential contact points may be necessary to obtain good
Available from National Institute of Standards and Technology (NIST), 100
Bureau Dr., Stop 1070, Gaithersburg, MD 20899-1070, http://www.nist.gov. electrical contact.
B193 − 20
7. Temperature Correction 8.1.5.1 If by micrometer, the average values of micrometer
readings, or
7.1 When the measurement is made at any other than a
8.1.5.2 If by weighing, a record of length, weight, any
reference temperature, the resistance may be corrected for
density determinations that may be made, and calculated
moderate temperature differences to what it would be at the
cross-sectional areas.
reference temperature, as follows:
8.1.6 Weight, if used,
R
t
8.1.7 Method of measuring resistance,
R 5
T
11α t 2 T
~ !
T
8.1.8 Value of resistance,
8.1.9 Reference temperature,
where:
8.1.10 Calculated value of resistivity at the reference
R = resistance at reference temperature T,
T
temperature, and
R = resistance as measured at temperature t,
t
8.1.11 P
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: B193 − 19 B193 − 20
Standard Test Method for
Resistivity of Electrical Conductor Materials
This standard is issued under the fixed designation B193; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope
1.1 This test method covers the determination of the electrical resistivity of metallic electrical conductor material. It provides for
an accuracy of 60.30 % on test specimens having a resistance of 0.00001 Ω (10 μΩ) or more. Weight resistivity accuracy may
be adversely affected by possible inaccuracies in the assumed density of the conductor.
1.2 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 appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.3 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
A111 Specification for Zinc-Coated (Galvanized) “Iron” Telephone and Telegraph Line Wire
A326 Specification for Zinc-Coated (Galvanized) High Tensile Steel Telephone and Telegraph Line Wire (Withdrawn 1990)
B1 Specification for Hard-Drawn Copper Wire
B2 Specification for Medium-Hard-Drawn Copper Wire
B3 Specification for Soft or Annealed Copper Wire
B9 Specification for Bronze Trolley Wire
B33 Specification for Tin-Coated Soft or Annealed Copper Wire for Electrical Purposes
B47 Specification for Copper Trolley Wire
B105 Specification for Hard-Drawn Copper Alloy Wires for Electric Conductors
B227 Specification for Hard-Drawn Copper-Clad Steel Wire
B230 Specification for Aluminum 1350–H19 Wire for Electrical Purposes
B298 Specification for Silver-Coated Soft or Annealed Copper Wire
B355 Specification for Nickel-Coated Soft or Annealed Copper Wire
B396 Specification for Aluminum-Alloy 5005-H19 Wire for Electrical Purposes (Withdrawn 2003)
B398 Specification for Aluminum-Alloy 6201-T81 Wire for Electrical Purposes [Metric](Discontinued 1999-Replaced by B
398/B398M) B0398_B0398M
This test method is under the jurisdiction of ASTM Committee B01 on Electrical Conductors and is the direct responsibility of Subcommittee B01.02 on Methods of
Test and Sampling Procedure.
Current edition approved Oct. 1, 2019Nov. 1, 2020. Published October 2019November 2020. Originally approved in 1944. Last previous edition approved in 20162019
as B193 – 16.B193 – 19. DOI: 10.1520/B0193-19.10.1520/B0193-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B193 − 20
B415 Specification for Hard-Drawn Aluminum-Clad Steel Wire
B498/B498M Specification for Zinc-Coated (Galvanized) Steel Core Wire for Use in Overhead Electrical Conductors
B502 Specification for Aluminum-Clad Steel Core Wire for Use in Overhead Electrical Aluminum Conductors
B566 Specification for Copper-Clad Aluminum Wire
B606 Specification for High-Strength Zinc-Coated (Galvanized) Steel Core Wire for Aluminum and Aluminum-Alloy
Conductors, Steel Reinforced
B609 Specification for Aluminum 1350 Round Wire, Annealed and Intermediate Tempers, for Electrical Purposes
B800 Specification for 8000 Series Aluminum Alloy Wire for Electrical Purposes—Annealed and Intermediate Tempers
B802 Specification for Zinc-5% Aluminum-Mischmetal Alloy-Coated Steel Core Wire for Aluminum Conductors, Steel
Reinforced (ACSR)[Metric](Discontinued 1998-Replaced by B 802/B802M) B0802_B0802M
B803 Specification for High-Strength Zinc–5 % Aluminum-Mischmetal Alloy-Coated Steel Core Wire for Use in Overhead
Electrical Conductors
B941 Specification for Heat Resistant Aluminum-Zirconium Alloy Wire for Electrical Purposes
B957 Specification for Extra-High-Strength and Ultra-High-Strength Zinc-Coated (Galvanized) Steel Core Wire for Overhead
Electrical Conductors
B958 Specification for Extra-High-Strength and Ultra-High-Strength Class A Zinc–5% Aluminum-Mischmetal Alloy-Coated
Steel Core Wire for Use in Overhead Electrical Conductors
2.2 NIST Document:
NBS Handbook 100 —Copper Wire Tables
3. Resistivity
3.1 Resistivity (Explanatory Note 1) is the electrical resistance of a body of unit length, and unit cross-sectional area or unit weight.
3.2 Volume Resistivity is commonly expressed in ohms for a theoretical conductor of unit length and cross-sectional area; in
inch-pound units in Ω·cmil/ft and in acceptable metric units in Ω·mm /m. It may be calculated by the following equation:
ρ 5 ~A/L!R
v
where:
ρ = volume resistivity, Ω·cmil/ft or Ω·mm /m,
v
A = cross-sectional area, cmil or mm ,
L = gage length, used to determine R, ft or m, and
R = measured resistance, Ω.
3.3 Weight Resistivity is commonly expressed in ohms for a theoretical conductor of unit length and weight. The method for
calculating weight resistivity, based on resistance, length, and weight measurements, of a test specimen is given in Explanatory
Note 2.
4. Apparatus
4.1 Resistance shall be measured with a circuit configuration and instrumentation that has a resistance measurement capability of
60.15 % accuracy.
5. Test Specimen
5.1 The test specimen may be in the form of a wire, strip, rod, bar, tube, or shape. It shall be of uniform cross section throughout
its length within 60.75 % of the cross-sectional area. Wherever possible it shall be the full cross section of the material it
represents, if the full cross section is such that the uniformity of the cross-sectional area can be accurately determined.
5.2 The test specimen shall have the following characteristics:
5.2.1 A resistance of at least 0.00001 Ω (10 μΩ) in the test length between potential contacts,
5.2.2 A test length of at least 1 ft or 300 mm,
Available from National Institute of Standards and Technology (NIST), 100 Bureau Dr., Stop 1070, Gaithersburg, MD 20899-1070, http://www.nist.gov.
B193 − 20
5.2.3 A diameter, thickness, width, or other dimension suitable to the limitations of the resistance measuring instrument,
5.2.4 No surface cracks or defects visible to the unaided normal eye, and substantially free from surface oxide, dirt, and grease,
and
5.2.5 No joints or splices.
6. Procedure
6.1 Make all determinations of the dimensions and weight of the test specimen using instruments accurate to 60.05 %. In order
to assure this accuracy in measuring the length between potential contacts, the surface in contact with the test specimen shall be
a substantially sharp knife-edge when using a Kelvin-type bridge or a potentiometer.
6.2 The cross-sectional dimensions of the specimen may be determined by micrometer measurements, and a sufficient number of
measurements shall be made to obtain the mean cross section to within 60.10 %. In case any dimension of the specimen is less
than 0.100 in. and cannot be measured to the required accuracy, determine the cross-section from the weight, density, and length
of the specimen.
6.3 When the density is unknown, determine the density by weighing a specimen first in air and then in a liquid of known density
at the test temperature, which shall be room temperature to avoid errors due to convection currents. Exercise care in removing all
air bubbles from the specimen when weighing it in the liquid. Calculate the density from the following equation:
δ5 W 3d / W 2 W
~ ! ~ !
a a l
where:
δ = density of the specimen, g/cm ;
W = weight of the specimen in air, g;
a
W = weight of the specimen in the liquid, g; and
l
d = density of the liquid at the test temperature, g/cm .
6.4 When potential leads are used, make sure the distance between each potential contact and the corresponding current contact
is at least equal to 1 ⁄2 times the cross-sectional perimeter of the specimen. Make sure the yoke resistance (between reference
standard and test specimen) is appreciably smaller than that of either the reference standard or the test specimen unless a suitable
lead compensation method is used, or it is known that the coil and lead ratios are sufficiently balanced so that variation in yoke
resistance will not decrease the bridge accuracy below stated requirements.
6.5 Make resistance measurements to an accuracy of 60.15 %. To ensure a correct reading, allow the reference standard and the
test specimen to come to the same temperature as the surrounding medium. (If the reference standard is made of manganin it is
possible to obtain correct readings with the test specimen at reference temperatures other than room temperature). In all resistance
measurements, the measuring current raises the temperature of the medium. Therefore, take care to keep the magnitude of the
current low, and the time of its use short enough so that the change in resistance cannot be detected with the galvanometer. To
eliminate errors due to contact potential, take two readings, one direct and one with current reversed, in direct succession. Check
tests are recommended whereby the specimen is turned end for end, and the test repeated. Surface cleaning of the specimen at
current and potential contact points may be necessary to obtain good electrical contact.
7. Temperature Correction
7.1 When the measurement is made at any other than a reference temperature, the resistance may be corrected for moderate
temperature differences to what it would be at the reference temperature, as follows:
R
t
R 5
T
11α ~t 2 T!
T
where:
R = resistance at reference temperature T,
T
R = resistance as measured at temperature t,
t
B193 − 20
α = known or given temperature coefficient of resistance of the specimen being measured at reference temperature T,
T
T = reference temperature, and
t = temperature at which measurement is made.
NOTE 1—The parameter α , in the above equation, varies with conductivity and temperature. For copper of 100 % conductivity and a reference
T
temperature of 20°C, its value is 0.00393. Values at other conductivities and temperatures will be found in NBS Handbook 100.100 . Table 12 lists
temperature coefficients for the common electrical conductor materials.
8. Report
8.1 For referee tests, report the following information:
8.1.1 Identification of test specimen,
8.1.2 Kind of material,
8.1.3 Test temperature,
8.1.4 Test length of specimen,
8.1.5 Method of obtaining cross-sectional area:
8.1.5.1 If by micrometer, the average values of micrometer readings, or
8.1.5.2 If by weighing, a record of length, weight, any density determinations that may be made, and calculated cross-sectional
areas.
8.1.6 Weight, if used,
8.1.7 Method of measuring resistance,
8.1.8 Value of resistance,
8.1.9 Reference temperature,
8.1.10 Calculated value of resistivity at the reference temperature, and
TABLE 1 Resistivity and Conductivity Conversion
NOTE 1—These factors are applicable only to resistivity and conductivity values corrected to 20°C (68°F). They are applicable for any temperature
when used to convert between volume units only or between weight units only. Values of density, δ, for the common electrical conductor materials, are
listed in Table 2.
Given N→
Volume Resistivity at 20°C Weight Resistivity
...