IEC 61788-23:2021

IEC 61788-23 ®
Edition 2.0 2021-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Superconductivity –
Part 23: Residual resistance ratio measurement – Residual resistance ratio
of cavity-grade Nb superconductors
Supraconductivité –
Partie 23: Mesurage du rapport de résistance résiduelle – Rapport de résistance
résiduelle des supraconducteurs de Nb à cavités
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
either IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC
copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or
your local IEC member National Committee for further information.
Droits de reproduction réservés. Sauf indication contraire, aucune partie de cette publication ne peut être reproduite
ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie
et les microfilms, sans l'accord écrit de l'IEC ou du Comité national de l'IEC du pays du demandeur. Si vous avez des
questions sur le copyright de l'IEC ou si vous désirez obtenir des droits supplémentaires sur cette publication, utilisez
les coordonnées ci-après ou contactez le Comité national de l'IEC de votre pays de résidence.
IEC Central Office Tel.: +41 22 919 02 11
3, rue de Varembé info@iec.ch
CH-1211 Geneva 20 www.iec.ch
Switzerland
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.
About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigendum or an amendment might have been published.
IEC publications search - webstore.iec.ch/advsearchform IEC online collection - oc.iec.ch
The advanced search enables to find IEC publications by a Discover our powerful search engine and read freely all the
variety of criteria (reference number, text, technical publications previews. With a subscription you will always
committee, …). It also gives information on projects, replaced have access to up to date content tailored to your needs.
and withdrawn publications.
Electropedia - www.electropedia.org
IEC Just Published - webstore.iec.ch/justpublished
The world's leading online dictionary on electrotechnology,
Stay up to date on all new IEC publications. Just Published
containing more than 22 000 terminological entries in English
details all new publications released. Available online and
and French, with equivalent terms in 18 additional languages.
once a month by email.
Also known as the International Electrotechnical Vocabulary
(IEV) online.
IEC Customer Service Centre - webstore.iec.ch/csc
If you wish to give us your feedback on this publication or
need further assistance, please contact the Customer Service
Centre: sales@iec.ch.
A propos de l'IEC
La Commission Electrotechnique Internationale (IEC) est la première organisation mondiale qui élabore et publie des
Normes internationales pour tout ce qui a trait à l'électricité, à l'électronique et aux technologies apparentées.
A propos des publications IEC
Le contenu technique des publications IEC est constamment revu. Veuillez vous assurer que vous possédez l’édition la
plus récente, un corrigendum ou amendement peut avoir été publié.
Recherche de publications IEC - IEC online collection - oc.iec.ch
webstore.iec.ch/advsearchform Découvrez notre puissant moteur de recherche et consultez
La recherche avancée permet de trouver des publications IEC gratuitement tous les aperçus des publications. Avec un
en utilisant différents critères (numéro de référence, texte, abonnement, vous aurez toujours accès à un contenu à jour
comité d’études, …). Elle donne aussi des informations sur adapté à vos besoins.
les projets et les publications remplacées ou retirées.
Electropedia - www.electropedia.org
IEC Just Published - webstore.iec.ch/justpublished
Le premier dictionnaire d'électrotechnologie en ligne au
Restez informé sur les nouvelles publications IEC. Just
monde, avec plus de 22 000 articles terminologiques en
Published détaille les nouvelles publications parues.
anglais et en français, ainsi que les termes équivalents dans
Disponible en ligne et une fois par mois par email.
16 langues additionnelles. Egalement appelé Vocabulaire
Electrotechnique International (IEV) en ligne.
Service Clients - webstore.iec.ch/csc
Si vous désirez nous donner des commentaires sur cette
publication ou si vous avez des questions contactez-nous:
sales@iec.ch.
IEC 61788-23 ®
Edition 2.0 2021-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Superconductivity –
Part 23: Residual resistance ratio measurement – Residual resistance ratio
of cavity-grade Nb superconductors
Supraconductivité –
Partie 23: Mesurage du rapport de résistance résiduelle – Rapport de résistance
résiduelle des supraconducteurs de Nb à cavités
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 17.220; 29.050 ISBN 978-2-8322-1011-5
– 2 – IEC 61788-23:2021 © IEC 2021
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 . 14
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 . 20
B.4 Uncertainty evaluation example for IEC TC 90 standards . 22
Annex C (informative) Uncertainty evaluation for resistance ratio measurement of Nb
superconductors . 24

C.1 Evaluation of uncertainty . 24
C.1.1 Room temperature measurement uncertainty . 24
C.1.2 Cryogenic measurement uncertainty . 25
C.1.3 Estimation of uncertainty for typical experimental conditions . 27
C.2 Inter-laboratory comparison summary . 28
Bibliography . 29

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 . 27
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 . 21
Table B.4 – Standard uncertainties of two output signals . 21
Table B.5 – Coefficients of variation of two output signals . 21
Table C.1 – Uncertainty of measured parameters . 27
Table C.2 – RRR values obtained by inter-laboratory comparison using liquid helium . 28

– 4 – IEC 61788-23:2021 © IEC 2021
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SUPERCONDUCTIVITY –
Part 23: Residual resistance ratio measurement –
Residual resistance ratio of cavity-grade Nb superconductors

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as "IEC Publication(s)"). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC 61788-23 has been prepared by IEC technical committee 90: Superconductivity. It is an
International Standard.
This second edition cancels and replaces the first edition published in 2018. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) The scope of this standard was modified to restrict the range of residual resistance ratio to
that encountered by providers of material for superconducting radio-frequency cavities.
b) The references to technical material were updated and corrected.

The text of this International Standard is based on the following documents:
FDIS Report on voting
90/478/FDIS 90/482/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/standardsdev/publications.
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 website under 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 61788-23:2021 © IEC 2021
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 RRR 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 millimetres 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 this 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. Note that systematic
differences of up to 10 % are produced by these other methods, which is larger than the target
uncertainty of this document. It is therefore important 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 Clause C.2.

SUPERCONDUCTIVITY –
Part 23: Residual resistance ratio measurement –
Residual resistance ratio of cavity-grade 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 150 < r < 600. The test method is valid for specimens with rectangular or round
RRR
2 2
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
r
RRR
ratio of resistance at room temperature to the resistance just above the superconducting
transition
r = R / R
(1)
RRR 1 2
where
R is the resistance at 293 K;
R is the resistance just above the superconducting transition, at ~10 K.
– 8 – IEC 61788-23:2021 © IEC 2021

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 r is obtained as
RRR
follows: Figure 1 shows schematically resistance versus temperature data and the graphical procedure used to
determine the value of R . In Figure 1, 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 .
2 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 T in [2].
c
Note 3 to entry: Some standards or documented techniques, e.g. [3], [4], [5], [6], define r with the value of 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.
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.

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 are 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 is 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 aluminium, 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
aluminium alloys, and electrical grades of silver alloys. These provide high thermal conductivity
and serve to remove thermal gradients during measurement. The specimen shall be insulated
from the mandrel. Possible insulating materials include polyethylene terephthalate, polyester,
and polytetrafluoroethylene, which may be applied as foils, tapes, or coatings. Glass-fibre
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 centre 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 61788-23:2021 © IEC 2021
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 when 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. Alternatively, 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. The heater should be distributed
along the mandrel and excessive power settings should be avoided. For instance, a point source
of 1 W heat input operating at the centre 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
−1 −1
is 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
can 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 shall be protected and immobilized against a
support piece during transport. Operations to de-burr samples shall 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 that coating the entire specimen is avoided.
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.
Installation and instrumentation of the specimen shall not apply excessive force, bending strain,
tensile strain, or torsion 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. It is important 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 can
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 in accordance with the requirements in Clause 4. The resulting
voltage U shall be recorded together with I and T . The resistance shall be determined by
1 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 [7] over the interval 273 K to 300 K.
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 61788-23:2021 © IEC 2021
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.
After the boil-off rate is suitable for measurements, a voltage measurement U shall be
recorded while the sample is immersed in liquid helium. The sample is likely to 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 U and U
0 0
shall be acquired for forward and reverse current polarity, respectively. Any differences 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−
(3)
U =
− +
where it should be noted that the sign of U is opposite that of U ; i.e. Formula (3) indicates
2 2
an 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 the temperature T
determined by the thermometer attached to the mandrel or base plate. Graphical aids and data
analysis software are acceptable tools for plotting the resistance versus temperature 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 analysed 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+
(6)
< 3%
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) edition of IEC 61788-23 followed.

– 14 – IEC 61788-23:2021 © IEC 2021
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.
9.4 Test conditions
The following test conditions shall be included in the test report:
a) room temperature T ;
b) transport currents I and I ;
1 2
c) voltages U and U , noting that U varies with temperature and therefore requires reporting
1 2 2
as a table or graph;
d) resistances R and R ;
1 2
+ −
e) voltages U , U , U or validation, Formula (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 ± 19,2 ( k = 2 ), where u is
RRR RRR RRR
the combined standard uncertainty in accordance with Annex C. Alternatively, r may be
RRR
quoted as a minimum 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. Results should be expressed as three significant figures if not
otherwise specified.
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 r = 300, then a resistance of 750 / 300 = 2,5 µΩ can be expected at ~10 K. Thus,
RRR
given the scope described in Clause 1, a resistance of 1,5 µΩ to 5,0 µΩ is observed.
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 RRR among different specimens. This
corresponds to a current density of approximately 0,1 A/mm .
f) As an alternative guideline, assuming a voltage of 1 µV at 10 K is produced by 1 A
−1
measuring current for r = 300, then L/A = 1 µΩ × 300 / 15 µΩ cm = 20 cm . If the
RRR
sample width w is the same as its
...