IEC 61788-23:2018

Superconductivity - Part 23: Residual resistance ratio measurement - Residual resistance ratio of Nb superconductors

IEC 61788-23:2018 addresses a test method for the determination of the residual resistance ratio (RRR) of cavity-grade niobium. The test method should be valid for specimens with rectangular or round cross-section, cross-sectional area greater than 1 mm2 but less than 20 mm2, and a length not less than 10 nor more than 25 times the width or diameter.

Supraconductivité - Partie 23: Mesurage du rapport de résistance résiduelle – Rapport de résistance résiduelle des supraconducteurs de Nb

IEC 61788-23:2018 spécifie une méthode d'essai pour la détermination du rapport de résistance résiduelle (RRR) du niobium à cavités. Il convient que la méthode d’essai soit valide pour des éprouvettes à sections rectangulaires ou circulaires, de surface de section supérieure à 1 mm2 mais inférieure à 20 mm2, et dont la longueur n’est pas inférieure à 10 fois ni supérieure à 25 fois la largeur ou le diamètre.

General Information

• Status: Published
• Publication Date: 04-Jun-2018
• ICS: 17.220 - Electricity. Magnetism. Electrical and magnetic measurements
• ICS: 29.050 - Superconductivity and conducting materials
• ICS: 31.220.01 - Electromechanical components in general
• Technical Committee: TC 90 - Superconductivity
• Drafting Committee: WG 4 - TC 90/WG 4
• Current Stage: DELPUB - Deleted Publication
• Start Date: 23-Aug-2021
• Completion Date: 31-Jan-2019

Relations

• Revised: IEC 61788-23:2021 - Superconductivity - Part 23: Residual resistance ratio measurement - Residual resistance ratio of cavity-grade Nb superconductors
• Effective Date: 05-Sep-2023

Overview

IEC 61788-23:2018 is an international standard developed by the International Electrotechnical Commission (IEC) that specifies a test method for measuring the Residual Resistance Ratio (RRR) of niobium (Nb) superconductors. This standard focuses on cavity-grade niobium, widely used in superconducting radio-frequency cavities and superconducting wire applications. The RRR is a key indicator of metal purity and performance, defined as the ratio of electrical resistance at room temperature (around 293 K) to that at low temperature just above the superconducting transition (~4.2 K).

This measurement is critical for quality control, material procurement, and manufacturing processes involving superconducting Nb components. IEC 61788-23:2018 ensures consistency and reliability in measuring RRR, specifically for specimens with rectangular or round cross-sections, cross-sectional areas between 1 mm² and 20 mm², and lengths between 10 to 25 times the width or diameter.

Key Topics

• Scope and Applicability
  The standard applies to high-purity, cavity-grade niobium specimens used in superconducting technologies. It establishes procedures for specimens with specific geometric dimensions to guarantee accurate RRR measurement.

• Measurement Principle
  The principle involves precise electrical resistance measurements at two temperature points-room temperature and just above the superconducting transition. This yields the residual resistance ratio, which correlates directly with the purity and performance of Nb superconductors.

• Measurement Apparatus
  IEC 61788-23 outlines the required apparatus such as mandrels or base plates, cryostat setups, and specimen supports critical for maintaining stable temperature conditions and reliable resistance readings.

• Specimen Preparation and Data Acquisition
  Proper specimen preparation, including cleaning and mounting, is essential. The standard details data acquisition hardware requirements and analytical methods to calculate resistance values at both temperature points.

• Uncertainty Evaluation
  The document provides detailed guidance on assessing measurement uncertainty, including factors affecting resistance measurement accuracy at room and cryogenic temperatures. Uncertainty evaluation ensures traceability and confidence in the RRR values obtained.

• Test Reporting
  Standardized reporting protocols are included to capture specimen details, test conditions, equipment used, and final RRR values, facilitating transparency and reproducibility across laboratories and manufacturers.

Applications

• Superconducting Radio-Frequency Cavities
  IEC 61788-23:2018 supports quality assurance in manufacturing superconducting cavities used in particle accelerators and quantum devices, where Nb purity directly impacts performance.

• Material Procurement and Quality Control
  Suppliers and manufacturers of niobium undergo RRR testing to verify purity and adherence to strict superconductivity specifications, ensuring high-quality input materials.

• Research & Development
  RRR measurement is critical in R&D labs exploring advanced superconducting materials and technologies, enabling optimized material processing techniques.

• Superconducting Wire Manufacturing
  Although focused on cavity-grade Nb, the standard's principles are applicable to niobium used in superconducting wires, promoting consistency in electrical and superconducting properties.

Related Standards

• IEC 61788 Series
  Part of the larger IEC 61788 series on superconductivity, which covers various aspects and test methods related to superconducting materials and devices.

• ISO/IEC Directives, Part 2
  The standard is drafted in accordance with these directives, ensuring global harmonization in technical content and documentation.

• Other Nb Superconductor Test Methods
  Annexes in IEC 61788-23 provide comparisons with alternative test methods, including magnetic field suppression techniques and AC measurement methods for resistance, offering comprehensive insights into Nb characterization.

Summary

IEC 61788-23:2018 provides an authoritative, standardized approach for measuring the Residual Resistance Ratio of niobium superconductors, a key quality parameter influencing superconducting performance. Its rigorous methodology supports manufacturers, researchers, and quality controllers in delivering and verifying high-purity Nb materials used in critical superconducting applications. By adhering to this standard, industries benefit from reliable, reproducible, and internationally recognized RRR measurements that underpin the advancement of superconducting technologies.

Keywords: IEC 61788-23, residual resistance ratio, RRR measurement, niobium superconductor, superconductivity standard, cavity-grade niobium, superconducting radio-frequency cavities, niobium purity, superconducting wire, cryogenic resistance measurement, superconducting materials testing, IEC superconductivity standards.

Standards Content (Sample)

IEC 61788-23 ®
Edition 1.0 2018-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Superconductivity –
Part 23: Residual resistance ratio measurement – Residual resistance ratio of Nb
superconductors
Supraconductivité –
Partie 23: Mesurage du rapport de résistance résiduelle – Rapport de résistance
résiduelle des supraconducteurs de Nb

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IEC 61788-23 ®
Edition 1.0 2018-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Superconductivity –
Part 23: Residual resistance ratio measurement – Residual resistance ratio of Nb

superconductors
Supraconductivité –
Partie 23: Mesurage du rapport de résistance résiduelle – Rapport de résistance

résiduelle des supraconducteurs de Nb

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 17.220; 29.050 ISBN 978-2-8322-5719-7

– 2 – IEC 61788-23:2018 © IEC 2018
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Principle . 8
5 Measurement apparatus . 9
5.1 Mandrel or base plate . 9
5.2 Cryostat and support of mandrel or base plate . 9
6 Specimen preparation . 10
7 Data acquisition and analysis . 11
7.1 Data acquisition hardware . 11
7.2 Resistance (R ) at room temperature . 11
7.3 Residual resistance (R ) just above the superconducting transition . 11
7.4 Validation of the residual resistance measurement . 13
7.5 Residual resistance ratio . 13
8 Uncertainty of the test method . 13
9 Test report . 13
9.1 General . 13
9.2 Test information . 13
9.3 Specimen information . 13
9.4 Test conditions . 14
9.5 RRR value . 14
Annex A (Informative) Additional information relating to the measurement of RRR . 15
A.1 Considerations for specimens and apparatus . 15
A.2 Considerations for specimen mounting orientation . 16
A.3 Alternative methods for increasing temperature of specimen above
superconducting transition temperature . 16
A.3.1 General . 16
A.3.2 Heater method . 16
A.3.3 Controlled methods. 16
A.4 Other test methods . 16
A.4.1 General . 16
A.4.2 Measurement of resistance versus time . 17
A.4.3 Comparison of ice point and room temperature . 17
A.4.4 Extrapolation of the resistance to 4,2 K . 17
A.4.5 Use of magnetic field to suppress superconductivity at 4,2 K . 18
A.4.6 AC techniques . 18
Annex B (informative) Uncertainty considerations . 19
B.1 Overview. 19
B.2 Definitions. 19
B.3 Consideration of the uncertainty concept . 19
B.4 Uncertainty evaluation example for TC 90 standards . 21

Annex C (informative) Uncertainty evaluation for resistance ratio measurement of Nb
superconductors . 23
C.1 Evaluation of uncertainty . 23
C.1.1 Room temperature measurement uncertainty . 23
C.1.2 Cryogenic measurement uncertainty . 24
C.1.3 Estimation of uncertainty for typical experimental conditions . 26
C.2 Round robin test summary . 26
Bibliography . 28

Figure 1 – Relationship between temperature and resistance near the superconducting
transition . 8
Figure A.1 – Determination of the value of R from a resistance versus time plot . 17
Figure C.1 – Graphical description of the uncertainty of regression related to the
measurement of R . 25
Table B.1 – Output signals from two nominally identical extensometers . 20
Table B.2 – Mean values of two output signals . 20
Table B.3 – Experimental standard deviations of two output signals . 20
Table B.4 – Standard uncertainties of two output signals . 21
Table B.5 – Coefficient of variations of two output signals . 21
Table C.1 – Uncertainty of measured parameters . 26
Table C.2 – RRR values obtained by round robin test . 27

– 4 – IEC 61788-23:2018 © IEC 2018
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SUPERCONDUCTIVITY –
Part 23: Residual resistance ratio measurement –
Residual resistance ratio of Nb superconductors

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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indispensable for the correct application of this publication.
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rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61788-23 has been prepared by IEC technical committee 90:
Superconductivity.
The text of this International Standard is based on the following documents:
FDIS Report on voting
90/400/FDIS 90/403/RVD
Full information on the voting for the approval of this International Standard can be found in the
report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 61788 series, published under the general title Superconductivity,
can be found on the IEC website.

The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'color inside' logo on the cover page of this publication indicates that
it contains colors which are considered to be useful for the correct understanding of its
contents. Users should therefore print this document using a color printer.

– 6 – IEC 61788-23:2018 © IEC 2018
INTRODUCTION
High-purity niobium is the chief material used to make superconducting radio-frequency cavities.
Similar grades of niobium may be used in the manufacture of superconducting wire.
Procurement of raw materials and quality assurance of delivered products often use the residual
resistance ratio (RRR) to specify or assess the purity of a metal. RRR is defined for
non-superconducting metals as the ratio of electrical resistance measured at room temperature
(293 K) to the resistance measured for the same specimen at low temperature (~4,2 K). The
low-temperature value is often called the residual resistance. Higher purity is associated with
higher values of RRR.
Niobium presents special problems due to its transformation to a superconducting state at ~9 K,
so DC electrical resistance is effectively zero below this temperature. The definition above
would then yield an infinite value for RRR. This document describes a test method to determine
the residual resistance value by using a plot of the resistance to temperature as the test
specimen is gradually warmed through the superconducting transition in the absence of an
applied magnetic field. This results in a determination of the residual resistance at just above
superconducting transition, ~10 K, from which RRR is subsequently determined.
International standards also exist to determine the residual resistance ratio of superconducting
wires. In contrast to superconducting wires, which are usually a composite of a superconducting
material and a non-superconducting material and the RRR value is representative of only the
non-superconducting component, here the entire specimen is composed of superconducting
niobium. Frequently, niobium is procured as a sheet, bar, tube, or rod, and not as a wire. For
such forms, test specimens will likely be a few millimeters in the dimensions transverse to
electric current flow. This difference is significant when making electrical resistance
measurements, since niobium samples will likely be much longer than that for the same
length-to-diameter ratio as a wire, and higher electrical current may be required to produce
sufficient voltage signals. Guidance for sample dimensions and electrical connections is
provided in Annex A. Test apparatus should also take into consideration aspects such as the
orientation of a test specimen relative to the liquid helium surface, accessibility through ports on
common liquid helium dewars, design of current contacts, and minimization of thermal gradients
over long specimen lengths. These aspects distinguish the present document from similar wire
standards.
Other test methods have been used to determine RRR. Some methods use a measurement at a
temperature other than 293 K for the high resistance value. Some methods use extrapolations at
4,2 K in the absence of an applied magnetic field for the low resistance value. Other methods
use an applied magnetic field to suppress superconductivity at 4,2 K. A comparison between this
document and some other test methods is presented in Annex A. It should be noted that
systematic differences of up to 10 % are produced by these other methods, which is larger than
the target uncertainty of this document. Care should therefore be taken to apply this document
or the appropriate corrections listed in Annex A according to the test method used.
Whenever possible, this test method should be transferred to vendors and collaborators who
also perform RRR measurements. To promote consistency, the results of inter-laboratory
comparisons are described in Annex C.

SUPERCONDUCTIVITY –
Part 23: Residual resistance ratio measurement –
Residual resistance ratio of Nb superconductors

1 Scope
This part of IEC 61788 addresses a test method for the determination of the residual resistance
ratio (RRR), r , of cavity-grade niobium. This method is intended for high-purity niobium
RRR
grades with 15 < r < 600. The test method should be valid for specimens with rectangular or
RRR
2 2
round cross-section, cross-sectional area greater than 1 mm but less than 20 mm , and a
length not less than 10 nor more than 25 times the width or diameter.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60050-815, International Electrotechnical Vocabulary – Part 815: Superconductivity
(available at: www.electropedia.org)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-815 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
residual resistance ratio
RRR
ratio of resistance at room temperature to the resistance just above the superconducting
transition
(1)
r = R / R
RRR 1 2
where is the resistance at 293 K and is the resistance just above the superconducting
R R
1 2
transition, at ~10 K.
– 8 – IEC 61788-23:2018 © IEC 2018
A
(b)
R
(a)
T *
Temperature
c
IEC
Figure 1 – Relationship between temperature and resistance near
the superconducting transition
Note 1 to entry: In this document, the room temperature is defined as 20 °C = 293 K, and is obtained as follows:
r
RRR
Figure 1 shows schematically resistance versus temperature data and the graphical procedure used to determine the
value of R . In this figure, the region of maximum slope is extrapolated upward in resistance, as shown by line (a),
and the region of minimum slope at temperatures above the transition temperature is extrapolated downward in
temperature, as shown by line (b). The intersection of these extrapolations at point A determines the value of R as
*
well as a temperature value T .
c
*
Note 2 to entry: The value T is similar to the transition value defined in [1] , and should not be confused with the
c
*
value defined at the midpoint of the transition, called in [2].
T
c
Note 3 to entry: Some standards or documented techniques, e.g. [3][4][5], define with the value of
r R
RRR 1
determined at a temperature other than 293 K, or the value of R determined at a temperature below the
superconducting transition. The user of this document should be alert for such differences in definition.
Note 4 to entry: This note applies to the French language only.
4 Principle
The 4-point DC electrical resistance technique shall be performed both at room temperature and
at cryogenic temperature. The test may be done either as a function of temperature or as a
function of time with increasing temperature.
The relative combined standard uncertainty of this method is 3 % with coverage factor 2.
Measurements shall have the following attributes:
a) Measuring current is sufficiently high to provide voltage signals of the order of 1 µV. For
-2
electrical safety, maximum current density should never exceed 1 A mm .
___________
Numbers in square brackets refer to the Bibliography.
Resistance
b) Contact resistance for current leads is sufficiently low to avoid excessive heating of the
sample. Typical cryogenic measurement conditions require power dissipation at contacts to
be less than 1 mW.
c) Sample sizes shall be sufficiently large to minimize effects from cutting and handling
damage. Typical samples are 1 mm to 3 mm in cross-section dimension and > 5 mm in
cross-sectional area.
d) Sample length shall be at least 10 times and not more than 25 times the width or diameter.
Annex A discusses considerations for sample dimensions and measuring current.
5 Measurement apparatus
5.1 Mandrel or base plate
A straight mandrel or base plate shall be used to support the specimen. Possible materials of
construction include pure copper, pure aluminum, pure silver, electrical grades of Cu-Zr,
Cu-Cr-Zr, Cu-Be, and other copper alloys, electrical grades of Al-Mg, Al-Ag, and other aluminum
alloys, and electrical grades of silver alloys. These provide high thermal conductivity and serve
to remove thermal gradients during measurement. Care should be taken to insulate the
specimen from the mandrel. Possible insulating materials include polyethylene terephthalate,
polyester, and polytetrafluoroethylene, which may be applied as foils, tapes, or coatings.
Glass-fiber reinforced epoxy or other composite materials with good thermal conductivity at
cryogenic temperature may also be used.
The base plate should have a clean and smooth surface finish. There should be no burrs, ridges,
seams, or other asperities that may affect the specimen. High purity niobium specimens are soft
and are susceptible to indentation by surface flaws, and such indentations may alter the sample
and invalidate the resistance measurement.
The mandrel or base plate shall support the entire length and width of the specimen. Mandrel or
base plate geometry should not impose a bending strain of more than 0,2 % on the sample.
A thermometer accurate to 0,1 K is helpful but not required. The mandrel or base plate may
incorporate a mounting for a cryogenic thermometer directly against the body of the mandrel or
base plate and near the center of the test specimen.
Practical base plates are at least 30 mm in length to accommodate assembly of pieces and
handling of samples by human hands. Multiple samples may be mounted against a single base
plate.
5.2 Cryostat and support of mandrel or base plate
The apparatus shall make provisions for mechanical support of the mandrel or base plate. In
addition, such support shall provide electrical leads to carry currents for samples and
thermometers, and measure their voltages. For R and R measurements, the support shall
1 2
permit current to flow through only the sample, so that the entire resulting voltage measured is
only that generated by the sample.
The support structure shall permit measurement of both R and R without dismounting or
1 2
remounting the test specimen. Measurement of R shall require the use of a cryostat, which
shall, moreover, integrate with the support.

– 10 – IEC 61788-23:2018 © IEC 2018
The cryostat shall include a liquid helium reservoir at the bottom of a substantial vertical column.
A support structure shall accommodate the raising and lowering of the sample into or out of the
helium bath. In addition, anchoring of the sample position, either while immersed in liquid helium
or suspended above the surface of the liquid at an arbitrary height, shall be provided. Such
suspension permits the equilibration of temperature during measurement and slow increase of
temperature with height above the helium bath. Alternately, immersion of the sample into the
bath followed by reduction of the bath level via boil-off or pressurized transfer can also be used
to vary temperature.
A heater may be employed to warm the mandrel or base plate. Care should be taken to distribute
the heater along the mandrel and avoid excessive power settings. For instance, a point source
of 1 W heat input operating at the center of a 1 cm mandrel upon which a 5 cm sample is
mounted could produce thermal gradients of 2,5 K along the sample if the thermal conductivity is
-1 -1
100 W m K .
Proper cryogenic techniques shall be followed for the construction of the cryostat and apparatus.
This includes the use of low thermal conductivity materials such as thin-walled stainless steel
tubes, composite materials, ceramics, and insulation, to prevent excessive boil-off due to heat
conduction from the surroundings. A can or shield may surround the base plate or mandrel with
mounted sample to improve thermal stability. Provisions for pressure relief and vacuum isolation
of the liquid helium should be incorporated with the apparatus.
6 Specimen preparation
High-purity niobium is quite malleable, and even the slightest force can produce deformation of
the material. Since dislocations are one source of electron scattering, specimen deformation
may inadvertently contribute to the residual resistivity and affect the test result. Therefore,
special protocols shall be observed when preparing the specimen. Cutting techniques shall
avoid heat and strain to the extent possible. Discharge machining, fluid-jet cutting, or low-speed
conventional machining are acceptable and widely-used techniques for applications using
high-purity niobium. Specimens cut from larger pieces should be protected and immobilized
against a support piece during transport. Operations to de-burr samples should be done with
great care to not bend, excessively heat or otherwise damage the sample. Light sanding with
fine paper is one acceptable approach.
Specimens should be rectangular or circular bars with uniform cross-section. Long sides of the
specimen shall be parallel. Any twisting or curvature shall be avoided to ensure that bending or
torsion is not applied to the test specimen during mounting to the mandrel or base plate.
Specimens that form an arc or a “U” shape are acceptable provided that the entire curvature can
be supported on a plane, without applying torsion to the bent specimen.
The specimen shall be clean and have no trace of residues from cutting fluids or any other
surface contaminants. Degreasing with solvents, followed by ultrasonic cleaning using a mild
water-based detergent, followed by rinsing with distilled or ultra-pure water, then drying in air, is
preferred for cleaning residues. Chemical etching to clean the surface poses a risk of
introducing contaminants, especially hydrogen and oxygen, and should be avoided. Gentle
mechanical polishing of the regions where voltage taps and current leads attach is usually
sufficient to remove surface oxides. Coating these regions with indium foil or another metal, for
example by evaporation or sputtering, is an acceptable method to protect polished contacts
provided care is taken to avoid coating the entire specimen.
The test specimen shall be a single piece and shall not include any joints or splices.
A mechanical method shall be used to affix the test specimen to the mandrel or base plate.
Special care shall be taken during the installation and instrumentation of the specimen to ensure
that there is no excessive force, bending strain, tensile strain, or torsion applied to the
specimen.
The test specimen shall be instrumented with current contacts near each end of the specimen
and a pair of voltage contacts over the central portion between the current contacts (i.e. a
4-point measurement technique). The voltage contacts shall be separated from the current
contacts by a distance no smaller than the largest dimension (width, thickness, or diameter)
perpendicular to the specimen length.
7 Data acquisition and analysis
7.1 Data acquisition hardware
Modern power supplies can be computer controlled and come with a variety of features that
permit remote control of the current output. Use of such power supplies is not required but could
greatly enable automation of the data acquisition. Pulsed modes permit application of current
only when voltage signals are being acquired, thereby removing heat generated in the sample
during the off cycle. If pulsed current application is used, the pulse duration shall include ample
periods for voltage signals to settle and be filtered.
Some power supplies incorporate an internal shunt to regulate the output current. If such a
power supply is used, the internal shunt shall be calibrated periodically with an external shunt
and voltage measurement.
The test set-up may establish an arbitrary baseline voltage U , which might be detectable when
the sample is in the superconducting state and the power supply is off. The value U can drift
over time due to changes in the thermal environment and other effects. More complex hardware
includes compensation for drift and automatic nulling such that the time average of U is 0.
Digital voltage meters are not required but greatly enhance the data acquisition. Besides
compensation for drift and voltage nulling, filtering and internal compensation for
thermally-induced voltages can improve the accuracy of the voltage measurement. Filtering
should average voltage signals for a time at least as long as the thermal time constant of the
apparatus at low temperature, typically of the order of 0,1 s to 10 s. Care should be taken to
understand how voltages are corrected for drift and thermal effects. Sensitive voltage meters,
especially nanovolt meters, require a pre-amplifier that needs to be at thermal equilibrium, which
may require several hours of operation in advance of the measurement.
Data acquisition via computer greatly facilitates the recording and reporting of data.
7.2 Resistance (R ) at room temperature
The ambient temperature T of the measurement laboratory shall be measured. A specimen
current I shall be applied per the requirements in Clause 4. The resulting voltage U shall be
1 1
recorded together with I and T . The resistance shall be determined by
1 1
U
RT=1−−0,0037 293 (2)
( )

I
with T in units of kelvin. The coefficient 0,003 7 reflects the experimentally observed rate of
change of resistance with temperature given in [5].
7.3 Residual resistance (R ) just above the superconducting transition
The measurement of R shall be made with the sample still mounted on the mandrel or base
plate for the measurement of R .
– 12 – IEC 61788-23:2018 © IEC 2018
The specimen shall be placed in a cryostat as specified in 5.2. The specimen shall be slowly
lowered into a liquid helium bath and cooled to liquid helium temperature. While a vigorous
boil-off of liquid helium will accompany the initial cool down, removal of heat from the mandrel,
especially if it is shielded, can require a time period of more than 5 min. Current may be applied,
and voltage may be monitored during this period, but no measurement shall be made until the
vigorous boil-off of liquid helium has subsided.
shall be
After the boil-off rate is suitable for measurements, a voltage measurement U
recorded while the sample is immersed in liquid helium. The sample should be in the
superconducting state under these conditions. Current I shall then be applied per
requirements of Clause 4 and with considerations of Clause 5. Voltage readings shall be
+ −
acquired for forward and reverse current polarity, and respectively. Any differences
U U
0 0
+ −
between U , U , and U shall be recorded.
0 0 0
The specimen shall then be gradually warmed so that a transition from the superconducting
state into the normal state occurs gradually. An apparatus that conforms to Clause 6 will permit
gradual warming of the specimen by raising the level of the mandrel above the level of the liquid
+ −
helium bath, for example. Two voltages U and U shall be measured almost simultaneously
2 2
with the application of the same measuring current I with forward and reverse polarity,
respectively. The current shall not be applied when measurements are not being recorded. The
voltage U shall be determined by
+−
UU−
U = (3)
− +
where it should be noted that the sign of U is opposite that of U ; i.e. Formula (3) indicates an
2 2
average of the two numbers approximately equal in magnitude. A resistance R shall be
determined from the voltage by
U
R = (4)
I
As the sample is warmed, values R shall be recorded as a function of either the temperature T
determined by the thermometer attached to the mandrel or base plate, or the time t. Graphical
aids and data analysis software are acceptable tools for plotting the resistance versus
temperature curve or resistance versus time curve and performing extrapolations.
A resistance versus temperature curve shall thus be obtained as in Figure 1. The resistance
versus temperature curve shall be continuously recorded until a temperature of at least 15 K is
reached. The resistance versus temperature curve shall be analyzed by drawing a line through
the region of steepest slope near the midpoint of the resistance rise, line (a) on Figure 1, and
extrapolating this line sufficiently above the value of R recorded at 15 K. A second line shall be
drawn through the region of the resistance versus temperature curve above the transition, line (b)
in Figure 1, and this line shall be extrapolated to sufficiently lower temperature such that it
intersects with line (a). The intersection is labelled as point A in Figure 1. The value of resistance
R corresponding to intersection point A shall be recorded, along with the value of temperature
*
T corresponding to intersection point A.
c
7.4 Validation of the residual resistance measurement
The determination of R shall be valid if all the following criteria are met: Interfering voltages
shall be such that
+−
UU−
(5)
< 3%
IR
+ −
Thermal drift or scatter shall be such that, for consecutive values U and U recorded with
2 2
*
temperature near T ,
c
+−
UU+
2 2
< 3% (6)
IR
The ambient temperature shall be such that
283 K < T < 303 K (7)
7.5 Residual resistance ratio
The RRR shall be calculated using Formula (1) and recorded.
8 Uncertainty of the test method
Based on the outcome of inter-laboratory comparison, discussed fully in Clause C.2, a typical
uncertainty across laboratories of 0,3 % to 1,3 % has been obtained.
9 Test report
9.1 General
A test report shall be provided to summarize the findings of the RRR test procedure.
9.2 Test information
The following shall be included to record the test information:
a) date and time of the measurement;
b) operator name;
c) version of the procedure followed.
9.3 Specimen information
The following information pertinent to the specimen shall be included in the test report:
a) vendor’s heat treatment, fabrication, or other tracking information such as a purchase order
number;
b) sheet or piece identification number, if any;
c) specimen shape and orientation relative to the helium bath.

– 14 – IEC 61788-23:2018 © IEC 2018
9.4 Test conditions
The following test conditions shall be included in the test report:
a) room temperature ;
T
b) transport currents and ;
I I
1 2
c) voltages U and U , noting that U varies with temperature and therefore requires
1 2 2
reporting as a table or graph;
d) resistances R and R
1 2
+ −
e) voltages U , U , U or validation condition (5);
0 0 0
The following additional information may be included in the test report:
f) voltage tap distance L;
g) specimen dimensions and cross-sectional area A;
−1 −1
h) resistivity ρ = R AL and ρ = R AL .
11 22
9.5 RRR value
The RRR value shall be quoted as ru± , for example 300 ± 15 ( k = 2 ), where u is the

RRR RRR RRR
combined standard uncertainty per Annex C. Alternately, r may be quoted as a minimum
RRR
value, for example 285 minimum, to denote the lower limit of the confidence interval represented
by the uncertainty. It is not necessary to report the uncertainty for a single measurement.
Additional information relating to the measurement of RRR is given in Annex A. Annex B
describes definitions and an example of uncertainty in measurement. Uncertainty evaluation in
the reference test method of RRR for composite superconductors is given in Annex C.

Annex A
(Informative)
Additional information relating to the measurement of RRR

A.1 Considerations for specimens and apparatus
The requirements in Clause 4 imply several general guidelines for preparing specimens and the
configuration of the measurement apparatus:
a) Niobium sheet stock is typically 2 mm to 5 mm thick. This implies a typical cross-sectional
2 2
area A of a sample of ~10 mm = 0,1 cm if a bar is machined with width approximately the
same as the sheet thickness.
b) Voltage tap separation depends on the apparatus dimensions, but cannot be longer than
about 80 % of the length of the niobium bar cut from sheets. To conserve liquid helium, this
is about 10 cm maximum, so a voltage tap separation L of 2 cm to 5 cm is reasonable.
c) Given that the resistivity ρ of pure niobium at 293 K is approximately 15 µΩ-cm, a typical
resistance of the niobium bar is R = ρL/A = 15 µΩ-cm × 5 cm / 0,1 cm = 750 µΩ.
d) If = 300, then a resistance of 750 / 300 = 2,5 µΩ can be expected at ~10 K. Thus, given
r
RRR
the scope described in Clause 1, a resistance of 1 µΩ to 20 µΩ should be expected.
e) To produce a measurement signal of ~ 1 µV at 10 K, as required by Clause 4, a current of
1 µV / 2,5 µΩ = 0,4 A will be required. Thus, a target of 1 A measuring current should be used
to provide ample allowance for variations in RR
...