IEC 61788-27:2025

Superconductivity - Part 27: Twist pitch measurement of practical superconducting wires - Twist pitch measurement of Nb‑Ti/Cu and Nb‑Sn/Cu composite superconductors

IEC 61788-27:2025 specifies a test method for the twist pitch measurement of Nb‑Ti/Cu and Nb‑Sn/Cu composite superconductors by an untwisting method.
The test method is applicable to Nb‑Ti/Cu and Nb‑Sn/Cu composite superconducting wires with monolithic structures, which have either a round cross section with a diameter ranging from 0,2 mm to 2 mm or a rectangular cross section that is equivalent in area to the round cross‑sectional wires. These wires possess a filament diameter ranging from 6 µm to 200 µm, a twist pitch between 5 mm and 50 mm, and a matrix of copper or copper alloy. This document uses nitric acid to remove the matrix (copper or copper alloy), so the surface of the composite superconducting wire can be plated with a material that is dissolvable by nitric acid.
Though uncertainty can increase, the method can apply to Nb‑Ti/Cu or Nb‑Sn/Cu composite superconducting wires when the parameters of cross-sectional area, filament diameter and twist pitch are out of the limit.
The test method specified in this document is expected to apply to other types of composite superconducting wires after some appropriate modifications.

Supraconductivité - Partie 27: Mesurage du pas de torsade de fils supraconducteurs pratiques - Mesurage du pas de torsade des composites supraconducteurs Nb‑Ti/Cu et Nb‑Sn/Cu

L'IEC 61788-27:2025 spécifie une méthode d'essai pour le mesurage du pas de torsade des composites supraconducteurs Nb‑Ti et Nb‑Sn/Cu par une méthode de détorsadage.
La méthode d'essai s'applique aux fils supraconducteurs composites Nb‑Ti/Cu et Nb‑Sn/Cu à structure monolithique, de section ronde d'un diamètre compris entre 0,2 mm et 2 mm ou de section rectangulaire dont la surface est équivalente à celle des fils à section ronde. Ces fils possèdent un diamètre de filament compris entre 6 µm et 200 µm, un pas de torsade compris entre 5 mm et 50 mm, et une matrice de cuivre ou d'alliage de cuivre. Le présent document utilise l'acide nitrique pour éliminer la matrice (cuivre ou alliage de cuivre), de sorte que la surface du fil supraconducteur composite puisse être revêtue d'un matériau dissoluble par l'acide nitrique.
Même si l'incertitude peut augmenter, la méthode peut s'appliquer aux fils supraconducteurs composites Nb‑Ti/Cu ou Nb‑Sn/Cu lorsque les paramètres de section, de diamètre de filament et de pas de torsade sont en dehors des limites.
Il est prévu que la méthode d'essai spécifiée dans le présent document s'applique à d'autres types de fils supraconducteurs composites après quelques modifications appropriées.

General Information

• Status: Published
• Publication Date: 24-Feb-2025
• ICS: 29.050 - Superconductivity and conducting materials
• Technical Committee: TC 90 - Superconductivity
• Drafting Committee: WG 6 - TC 90/WG 6
• Current Stage: PPUB - Publication issued
• Start Date: 25-Feb-2025
• Completion Date: 24-Jan-2025

Standards Content (Sample)

IEC 61788-27 ®
Edition 1.0 2025-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Superconductivity –
Part 27: Twist pitch measurement of practical superconducting wires – Twist
pitch measurement of Nb‑Ti/Cu and Nb‑Sn/Cu composite superconductors

Supraconductivité –
Partie 27: Mesurage du pas de torsade de fils supraconducteurs pratiques –
Mesurage du pas de torsade des composites supraconducteurs Nb‑Ti/Cu et
Nb‑Sn/Cu
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IEC 61788-27 ®
Edition 1.0 2025-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Superconductivity –
Part 27: Twist pitch measurement of practical superconducting wires – Twist

pitch measurement of Nb‑Ti/Cu and Nb‑Sn/Cu composite superconductors

Supraconductivité –
Partie 27: Mesurage du pas de torsade de fils supraconducteurs pratiques –

Mesurage du pas de torsade des composites supraconducteurs Nb‑Ti/Cu et

Nb‑Sn/Cu
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.050  ISBN 978-2-8327-0133-1

– 2 – IEC 61788-27:2025 © IEC 2025
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 8
4 Principle . 9
5 Reagents and auxiliary materials . 10
6 Apparatus and tools . 10
7 Specimen preparation . 10
7.1 Requirements . 10
7.2 Cleaning . 10
7.3 Drying . 10
7.4 Removing matrix copper and sanding barrier (apply to Nb-Sn/Cu specimen
of external stabilizer type only) . 11
8 Measurement procedure . 11
8.1 Specimen fixing . 11
8.2 Specimen dissolution . 11
8.3 Cleaning and drying . 11
8.4 Measurement . 12
9 Calculation of results . 12
10 Uncertainty of measurement . 12
11 Test report . 13
11.1 Specimen . 13
11.2 Results . 13
Annex A (normative) Measurement – Direct measurement method . 14
A.1 Method . 14
A.2 Measurement procedure . 14
A.2.1 Specimen dissolution . 14
A.2.2 Cleaning and drying . 14
A.2.3 Measurement . 15
A.3 Uncertainty of measurement . 15
A.4 Test report . 15
Annex B (normative) Measurement – Image processing method . 16
B.1 Method . 16
B.2 Measurement procedure . 17
B.2.1 Specimen dissolution . 17
B.2.2 Cleaning and drying . 17
B.2.3 Photo taking . 17
B.2.4 Measuring the helix angle . 17
B.2.5 Result and rounding method . 18
B.3 Uncertainty of measurement . 18
B.4 Test report . 18
Annex C (informative) A typical design of the auxiliary tool for twist pitch
measurement by the untwisting method . 19
C.1 Apparatus of twist angle measurement . 19
C.2 Measurement procedure . 20

Annex D (informative) Uncertainty evaluation . 22
D.1 Untwisting method . 22
D.1.1 Mathematical model . 22
D.1.2 Measurement uncertainty in twist pitch L for Nb-Ti/Cu . 22
p
D.1.3 Measurement uncertainty in twist pitch L for Nb-Sn/Cu . 24
p
D.1.4 Round robin test . 26
D.1.5 Other influences of uncertainty . 27
D.2 Direct measurement method . 28
D.2.1 Mathematical model . 28
D.2.2 Procedures of standard uncertainty evaluation . 28
D.2.3 Specimen identification . 29
D.2.4 Evaluation of each standard uncertainty component . 29
D.2.5 Evaluation results of combined standard uncertainty in twist pitch L . 30
p
D.2.6 Expanded uncertainty in twist pitch L . 30
p
D.2.7 Round robin test . 30
D.3 Image processing method . 32
D.3.1 Mathematical model . 32
D.3.2 Procedures of standard uncertainty evaluation . 32
D.3.3 Specimen identification . 33
D.3.4 Evaluation of each standard uncertainty component . 33
D.3.5 Evaluation results of combined standard uncertainty in twist pitch L . 35
p
D.3.6 Expanded uncertainty in twist pitch L . 35
p
D.3.7 Round robin test . 35
Bibliography . 37

Figure 1 – Principle demonstration . 9
Figure B.1 – Principle demonstration for image processing method . 16
Figure B.2 – Measurement of helix angle . 18
Figure C.1 – A typical design of the auxiliary tool for twist pitch measurement . 20
Figure C.2 – Specimen fixing . 20
Figure C.3 – Specimen bending and dissolution . 20
Figure C.4 – Back to the initial state . 20
Figure C.5 – Rotate the knob to untwist filaments . 21
Figure C.6 – Judge the parallelism of filaments . 21
Figure C.7 – Measure the distance . 21

Table D.1 – Measurement results of dissolved length l (Nb-Ti/Cu) . 23
Table D.2 – Measurement results of untwist angle θ (Nb-Ti/Cu) . 23
Table D.3 – Measurement results of dissolved length l (Nb-Sn/Cu) . 25
Table D.4 – Measurement results of untwisted angle θ (Nb-Sn/Cu) . 25
Table D.5 – RRT results of Nb-Ti/Cu specimen . 26
Table D.6 – RRT results of Nb-Sn/Cu specimen . 27
Table D.7 – Measurement results and uncertainties of each institution for
Nb-Ti/Cu specimen . 27

– 4 – IEC 61788-27:2025 © IEC 2025
Table D.8 – Measurement results and uncertainties of each institution for
Nb-Sn/Cu specimen . 27
Table D.9 – Measured twist pitches for Nb-Ti/Cu specimen . 29
Table D.10 – RRT results of Nb-Ti/Cu specimen . 31
Table D.11 – RRT results of Nb-Sn/Cu specimen . 31
Table D.12 – Measurement results and uncertainties of each institution for
Nb-Ti/Cu specimen . 31
Table D.13 – Measurement results and uncertainties of each institution for
Nb-Sn/Cu specimen . 32
Table D.14 – Measured diameter (D ), helix angle (φ ) and obtained twist pitch (L ) for
i i pi
Nb-Sn/Cu specimen . 34
Table D.15 – RRT results of Nb-Ti/Cu specimen . 35
Table D.16 – RRT results of Nb-Sn/Cu specimen . 36
Table D.17 – Measurement results and uncertainties of each institution for
Nb-Ti/Cu specimen . 36
Table D.18 – Measurement results and uncertainties of each institution for
Nb-Sn/Cu specimen . 36

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SUPERCONDUCTIVITY –
Part 27: Twist pitch measurement of practical superconducting wires –
Twist pitch measurement of Nb-Ti/Cu and
Nb-Sn/Cu composite superconductors

FOREWORD
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IEC 61788-27 has been prepared by IEC technical committee 90: Superconductivity. It is an
International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
90/532/FDIS 90/540/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
– 6 – IEC 61788-27:2025 © IEC 2025
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/publications.
A list of all parts of 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, or
• revised.
INTRODUCTION
Twisting of multi-filamentary superconductors is an important step in the development of wires
with AC losses at an acceptable level for AC applications. The necessary twist pitch depends
on wire architecture, critical current density, matrix material, and external factors such as
temperature, frequency and applied magnetic field.
Therefore, twist pitch is a very important parameter in the design and application of composite
superconducting wires, which is often inspected in the last stage of fabrication. Due to the
different architectures of different composite superconductors, appropriate test methods should
be adopted for specific architectures.
This document specifies the untwisting method for measuring the twist pitch of Nb-Ti/Cu and
Nb-Sn/Cu composite superconductors [1] . As supplementary methods, the direct measurement
method and the image processing method [1] are specified in Annex A and Annex B,
respectively.
___________
Numbers in square brackets refer to the Bibliography.

– 8 – IEC 61788-27:2025 © IEC 2025
SUPERCONDUCTIVITY –
Part 27: Twist pitch measurement of practical superconducting wires –
Twist pitch measurement of Nb-Ti/Cu and
Nb-Sn/Cu composite superconductors

1 Scope
This part of IEC 61788 specifies a test method for the twist pitch measurement of Nb-Ti/Cu and
Nb-Sn/Cu composite superconductors by an untwisting method.
The test method is applicable to Nb-Ti/Cu and Nb-Sn/Cu composite superconducting wires with
monolithic structures, which have either a round cross section with a diameter ranging from
0,2 mm to 2 mm or a rectangular cross section that is equivalent in area to the round
cross-sectional wires. These wires possess a filament diameter ranging from 6 µm to 200 µm,
a twist pitch between 5 mm and 50 mm, and a matrix of copper or copper alloy. This document
uses nitric acid to remove the matrix (copper or copper alloy), so the surface of the composite
superconducting wire can be plated with a material that is dissolvable by nitric acid.
Though uncertainty can increase, the method can apply to Nb-Ti/Cu or Nb-Sn/Cu composite
superconducting wires when the parameters of cross-sectional area, filament diameter and twist
pitch are out of the limit.
The test method specified in this document is expected to apply to other types of composite
superconducting wires after some appropriate modifications.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1
twist
turns made by a filament around a central wire axis
[SOURCE: IEC 60050-815:2015 [2], 815-13-46, modified – In the definition, "or strand" has
been deleted and "conductor" has been replaced with "central wire".]

3.2
twist direction
direction of a filament in twist
Note 1 to entry: There are two types of twist direction: Z-twist (sometimes referred to as right-hand twist) when the
filament or strand is twisted in a clockwise direction and S-twist (sometimes referred to as left-hand twist) when the
filament or strand is twisted in an anticlockwise direction.
3.3
twist pitch
L
p
axial length in which a filament firstly returns to its original relative position in a twisted
superconducting wire
[SOURCE: IEC 60050-815:2015 [2], 815-13-47, modified – The preferred term "twist pitch
length" has been deleted. In the definition, "or strand" has been deleted and "conductor" has
been replaced with "superconducting wire".]
3.4
twist angle
untwist angle
θ
angle at which all filaments of a twisted superconducting wire return to parallel original axial
positions during the untwisting process
4 Principle
Low temperature superconducting wire is usually composed of tens to even thousands of
superconducting filaments embedded in copper and copper alloy matrix. After the
superconducting wire is twisted, the inner filaments are also regularly twisted with a specific
twist pitch. This document specifies an untwisting method to measure the twist pitch
(see Figure 1).
Figure 1 – Principle demonstration
The untwisting method can be briefly described as follows.
a) Fix the two ends of the specimen (Figure 1a).
b) Remove the copper or copper alloy matrix in the middle section of the specimen with HNO
solution (Figure 1b).
c) Measure the length of the dissolved zone (denoted as l, explanation see 8.4).
d) Rotate one end of the wire to untwist it until the filaments in the dissolved zone are parallel
(Figure 1c and Figure 1d). Record the rotated angle (denoted as θ).

– 10 – IEC 61788-27:2025 © IEC 2025
e) Calculate the twist pitch L using l and θ (see Formulae (1) and (2)).
p
The other two normative methods, the direct measurement method and the image processing
method, are specified in Annex A and Annex B, which are equivalent alternatives.
5 Reagents and auxiliary materials
The following reagents shall be prepared for specimen preparation:
– detergent and degreaser;
– anhydrous ethanol;
– nitric acid solution (40 % to 65 % mass percentage concentration is recommended).
Rubber gloves and acid-resistant tweezers shall be used when handling acid solution.
The following auxiliary materials and tools shall be prepared for specimen preparation:
– 300 to 800 mesh fine sandpaper (apply to Nb-Sn/Cu specimens only).
6 Apparatus and tools
The apparatus and tools required for measurement shall include the following:
– an auxiliary tool for twist pitch measurement, which has an angle scale with an accuracy of
5° or better (an example is given in Annex C);
– fume hood;
– calliper (accuracy 0,02 mm or better);
– oven or dryer.
7 Specimen preparation
7.1 Requirements
The untwisting method shall be used for twist pitches in the range of 5 mm to 50 mm to ensure
adequate accuracy for the measurement. The specimen length shall be longer than six times of
the nominal twist pitch.
The specimen shall be free from torsion and bending.
7.2 Cleaning
Detergent and degreaser shall be used to remove oil stains and other contamination from the
specimen. Then, the specimen shall be rinsed repeatedly with running water, and finally
dehydrated by anhydrous ethanol.
7.3 Drying
After cleaning, the specimen shall be dried completely in an appropriate manner. For example,
drying with hot air or placed into an oven (60 °C to 70 °C).

7.4 Removing matrix copper and sanding barrier (apply to Nb-Sn/Cu specimen of
external stabilizer type only)
Some Nb-Sn/Cu wires have a Nb or Ta external diffusion barrier to inhibit diffusion reaction
between Cu-Sn matrix and external copper stabilizer, and the barrier is insoluble in nitric acid.
When the external copper stabilizer is dissolved, this barrier prevents the inner copper from
being corroded by nitric acid, therefore the barrier shall be removed. The following steps are
recommended when removing the barrier; these steps are not applied to distributed barrier
wires.
a) Removing matrix copper: Immerse the Nb-Sn/Cu specimen in nitric acid solution and leave
the container in a fume hood for about 15 minutes until the copper sheath of the specimen
is completely dissolved. Take the specimen out by acid-resistant tweezers, rinse it with
running water, and then dehydrate it with anhydrous ethanol, and dry it.
For safety, wear long sleeves, trousers, and rubber gloves to prevent nitric acid from
splashing on the skin.
b) Inspecting barrier surface for barrier seam: If the barrier has a seam, inspect the surface of
the barrier which will follow the filament position underneath. If a seam is clearly visible, the
specimen needs no further preparation.
c) Sanding barrier: Take a fine sandpaper, and evenly sand the measuring section until the
internal coppery red is completely exposed.
Use a fume hood during sanding.
d) Cleaning and drying: Clean and dry the specimen in accordance with 7.2 and 7.3,
respectively.
8 Measurement procedure
8.1 Specimen fixing
If necessary, for Nb-Sn/Cu specimens with external diffusion barrier, the barrier shall be
removed in advance in accordance with 7.4. Both ends of the specimen shall be clamped or
pressed tightly to the auxiliary tool to straighten the specimen. During the fixing process,
distortion, bending, or any other actions that will affect the measurement results are not allowed.
8.2 Specimen dissolution
Immerse the middle part of the specimen, approximately two thirds of the total length, in nitric
acid solution and hold for about 30 minutes. This can be achieved by pushing the slider in the
auxiliary tool toward the centre, and thus bending the specimen at the same time. Details are
given in Annex C.
Take the dissolved part out and observe inner filaments. If there is still residual copper or
filament bonding, or the dissolved zone boundary is not clear, immerse the part into nitric acid
solution again. Ensure that the position of the specimen relative to the nitric acid surface does
not change so that boundaries of the dissolution zone are as clear as possible.
Use personal protective equipment and a fume hood while handling the specimen.
8.3 Cleaning and drying
When all the inner filaments are fully dispersed with no residual copper, take the dissolved part
out, with the whole specimen still fixed on the auxiliary tool. Clean and dry the specimen in
accordance with 7.2 and 7.3, respectively.

– 12 – IEC 61788-27:2025 © IEC 2025
8.4 Measurement
Restore the specimen in the auxiliary tool to the initial state, horizontal without bending, and
observe the twist direction. While fixing the axial position of the specimen, rotate one end of it
in the opposite direction to untwist it. Meanwhile, observe the filaments or barrier seam carefully,
and stop rotating when all filaments or barrier seam are nearly parallel to the wire axis.
Insert two pieces of thin plate, such as copper foils, with approximately 0,5 mm thickness and
10 mm to 30 mm width. Two pieces of plate are overlapped, without any filament between them.
They are both applied to pick up several filaments. One of them is kept at the start of the
dissolved zone, and the other slides towards the end of the zone. Tune the angle at the same
time until the picked filaments are completely untwisted, that is, those filaments are now parallel
to the wire axis of the specimen. Alternatively, if there is a visible seam on the barrier, use the
apparatus to untwist the wire until the seam runs parallel to the wire axis.
Record the total rotation angle, i.e. twist angle θ. It is recommended to repeat the previous step
several times to improve the measurement accuracy.
Measure the length of the picked filaments by a calliper, using the copper foils marking the
boundaries. It is recommended to carry out multiple measurements to improve the measurement
accuracy. It is worth noting that, by twist pitch definition, l is the length in twist state. However,
since the boundary of the dissolved zone is difficult to identify, this compromise is justified. On
one hand, an appropriate auxiliary tool can be designed to fix the position of both ends of the
specimen, as seen in Annex C, so that the distance of the dissolved part remains unchanged.
On the other hand, the untwisting process cannot guarantee that all filaments are completely
parallel. Thus, only the length or distance of the picked filaments is measured. The introduced
uncertainty is considered in the uncertainty evaluation (Annex D).
For another dissolution method, the specimen is dissolved, cleaned and dried in advance, and
then fixed to the tool for measurement. In this case, the dissolved specimen shall not be twisted
before and during fixing.
9 Calculation of results
The twist pitch shall be calculated and rounded to one decimal using Formulae (1) and (2):
l
L =
(1)
p
n
θ
n= (2)
where
L is the twist pitch (mm);
p
l is the length of the dissolved zone (mm);
n is the number of turns;
θ is the untwist angle (°).
10 Uncertainty of measurement
The twist pitch is obtained by measuring the dissolved zone length l and the untwist angle θ.
The target relative expanded standard uncertainty (RSU) of measurement shall not exceed
2 % (k = 2). Details of the uncertainty evaluation are described in Clause D.1.

11 Test report
11.1 Specimen
The following information of the specimen shall be reported:
a) type and classification of wire;
b) identification of specimen;
c) dimension (diameter, length and width).
The following information of the specimen shall be reported if possible:
d) manufacturer;
e) manufacturing process;
f) lot number;
g) nominal copper to non-copper volume ratio;
h) nominal filament diameter;
i) number of filaments;
j) photo of cross-section.
11.2 Results
The following information shall be reported:
a) twist pitch of each specimen;
b) twist direction;
c) method.
The following information shall be reported if necessary:
d) uncertainty.
– 14 – IEC 61788-27:2025 © IEC 2025
Annex A
(normative)
Measurement – Direct measurement method
A.1 Method
This Annex A describes the direct measurement method to measure the twist pitch. It is
applicable to Nb-Ti/Cu and Nb-Sn/Cu composite superconducting wires in round or rectangular
shape and with or without a Nb or Ta diffusion barrier surrounding the outmost superconducting
filament bundle. It is recommended to be applied for twist pitches in the range of 5 mm to
100 mm with high reproducibility and accuracy. This method can also be used when twist pitch
is over 100 mm, but the uncertainty becomes larger for longer twist pitches, for example twist
pitch up to 300 mm. It will be possible to apply this method for external stabilizer type (with
external barrier) Nb-Sn/Cu wires without removal of the barrier where the contours of the
filaments underneath can be seen, or where a seam is visible on the barrier after etching the
outer copper sheath.
A.2 Measurement procedure
A.2.1 Specimen dissolution
Cut a specimen with a length much larger than the nominal twist pitch (for twist pitches in the
range of 50 mm to 100 mm, a specimen length of 300 mm is recommended) with a clean pair
of nippers, and stick a label of the specimen identification on one end.
Wipe the specimen with gauze soaked with anhydrous ethanol using clean gloves.
Immerse the specimen into nitric acid solution in a graduated cylinder (e.g. 100 ml) leaving the
stuck end. It is important that the immersed length is greater than 1,5 times the expected twist
pitch.
Use personal protective equipment and a fume hood while handling the specimen.
Prepare a distilled-water-filled graduated cylinder of same size to rinse the specimen.
Dissolve the copper sheath, and confirm that the copper sheath is etched away, so that the
outermost filaments have started to be visible along the circumference of the specimen. The
etched metal removal shall be as low as possible to ensure an accurate twist pitch determination.
To ensure this grade of etching, the etching of the specimen shall be continuously checked by
visual inspection. For the external stabilizer type (with external barrier) Nb-Sn/Cu wire
specimens shall be prepared in accordance with 7.4, steps a) and b).
A.2.2 Cleaning and drying
Take the specimen out of the solution and put it in the graduated cylinder filled with distilled
water to rinse it well or rinse it quickly with running water.
Dry the specimen completely after rinsing, using anhydrous ethanol and hot air, oven or a non-
marking tissue.
A.2.3 Measurement
Follow and mark one or more visible filaments along the wire axis with a permanent marker.
This can be done using an optical microscope, if needed. For external stabilizer type Nb-Sn/Cu
wire with a barrier seam, the seam itself shall serve as a visual guide. No marking on the
specimen is necessary.
Measure the length of one or a few twist pitches with a ruler or calliper.
Calculate the twist pitch by dividing the measured length by the number of marked twist pitches.
L
n
L = (A.1)
p
n
where
L is the twist pitch;
p
L is the length of n twist pitches;
n
n is the measured number of twist pitches.
NOTE The measurement can be performed with multiple specimens, ideally more than 10 specimens, or multiple
measurements with one specimen, hopefully more than ten times.
A.3 Uncertainty of measurement
The target relative expanded standard uncertainty (RSU) of measurement shall not exceed 5 %
(k = 2). Details of the uncertainty evaluation are described in Clause D.2.
A.4 Test report
See Clause 11.
– 16 – IEC 61788-27:2025 © IEC 2025
Annex B
(normative)
Measurement – Image processing method
B.1 Method
This Annex B describes the image processing method to measure the twist pitch. It is applicable
to Nb-Ti/Cu and Nb-Sn/Cu composite superconducting wires in round or rectangular shape and
with or without a Nb or Ta diffusion barrier surrounding the outmost superconducting filament
bundle. It is recommended to be applied for twist pitches in the range of 5 mm to 300 mm. This
method can be used when twist pitch is over 50 mm, but the uncertainty becomes larger for
longer twist pitches. The method will show higher uncertainty with increasing twist pitch, as the
uncertainty for the angle reading increases with increasing twist pitch. The schematic diagram
is shown in Figure B.1. This method is especially advantageous for specimens with high
filament number count, specimens with very small wire diameter and specimens with Nb or Ta
barrier surrounding the filament pack. The specimen shall be kept straight throughout the
measuring process. The measurement shall be carried out on photos (such as scanning electron
microscope (SEM) photos, metallographic photos and so on). It would be beneficial to use a
photo of the specimen surface with high depth of focus to determine the twist pitch helix. Here,
helix angle of the filaments and the diameter of the wire in the photo after removal of the Cu
matrix (or diffusion barrier, in case of the external stabilizer Nb-Sn/Cu wire) are needed to
determine the twist pitch.
The twist pitch is calculated by Formula (B.1):
πD
L =
(B.1)
p
tan(φ)
where
φ is the helix angle (rad);
D is the diameter of the wire after removal of the Cu matrix (or diffusion barrier) (mm).

Figure B.1 – Principle demonstration for image processing method

It is recommended to measure the helix angle at two orientations of the specimen, which are
rotated 90° in correction to the centre line of the specimen. This is necessary to exclude
measurement error due to ovality of the specimen and for rectangular specimens. The twist
pitch would in such a case be: L = 0,5 (L (x-axis) + L (y-axis)).
p p p
B.2 Measurement procedure
B.2.1 Specimen dissolution
For specimen without barrier, the etching process shall be done in accordance with A.2.1. For
the external stabilizer type Nb-Sn/Cu wire, the specimen shall be prepared in accordance with
7.4, steps a) and b). The copper matrix shall be dissolved in nitric acid until the filament bundles
in twist or the barrier seam – in case of the external barrier Nb-Sn/Cu wire – are recognizable.
The specimen length shall be sufficient to enable specimen handling and to ensure an accurate
angle measurement in the full scope of the microscope used. Label the specimen and put it
straight in a beaker or cylinder filled with nitric acid.
Use personal protective equipment and a fume hood while handling the specimen.
B.2.2 Cleaning and drying
Take the specimen out and wash repeatedly with running water for at least two min
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