Insulating liquids - Test methods for the determination of interfacial tension of insulating liquids - Determination with the ring method

IEC 62961:2018 establishes the measurement of the interfacial tension between insulating liquid and water by means of the Du Noüy ring method close to equilibrium conditions. In order to obtain a value that provides a realistic expression of the real interfacial tension, a measurement after a surface age of approximately 180 s is recorded.

Isolants liquides - Méthodes d'essai pour la détermination de la tension interfaciale des isolants liquides - Détermination par la méthode à l'anneau

L'IEC 62961:2018 spécifie le mesurage de la tension interfaciale entre un isolant liquide et l'eau par la méthode à l'anneau de Du Noüy proche des conditions d'équilibre. Pour obtenir une valeur, qui donne une expression plus réaliste de la véritable tension interfaciale, un mesurage après une maturation de l'interface d'environ 180 s est enregistré.

General Information

Status
Published
Publication Date
19-Sep-2018
Current Stage
PPUB - Publication issued
Completion Date
20-Sep-2018
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IEC 62961
Edition 1.0 2018-09
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside

Insulating liquids – Test methods for the determination of interfacial tension of

insulating liquids – Determination with the ring method
Isolants liquides – Méthodes d'essai pour la détermination de la tension
interfaciale des isolants liquides – Détermination par la méthode à l'anneau
IEC 62961:2018-09(en-fr)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC 62961
Edition 1.0 2018-09
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside

Insulating liquids – Test methods for the determination of interfacial tension of

insulating liquids – Determination with the ring method
Isolants liquides – Méthodes d'essai pour la détermination de la tension
interfaciale des isolants liquides – Détermination par la méthode à l'anneau
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.040.10 ISBN 978-2-8322-6037-1

Warning! Make sure that you obtained this publication from an authorized distributor.

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® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 3 ----------------------
– 2 – IEC 62961:2018  IEC 2018
CONTENTS

FOREWORD ........................................................................................................................... 4

INTRODUCTION ..................................................................................................................... 6

1 Scope .............................................................................................................................. 8

2 Normative references ...................................................................................................... 8

3 Terms and definitions ...................................................................................................... 8

4 Principle .......................................................................................................................... 8

5 Apparatus ........................................................................................................................ 9

5.1 Tensiometer ............................................................................................................ 9

5.2 Ring ...................................................................................................................... 10

5.3 Measuring vessel .................................................................................................. 10

6 Preparation of apparatus ............................................................................................... 10

6.1 Cleaning of the measuring vessel ......................................................................... 10

6.2 Cleaning of the ring ............................................................................................... 11

6.3 Water used for the test.......................................................................................... 11

7 Procedure ...................................................................................................................... 11

7.1 General ................................................................................................................. 11

7.2 Calibration and taring ............................................................................................ 11

7.3 Determination of the surface tension of water used for the test ............................. 12

7.4 Determination of interfacial tension between water and insulating liquid................ 12

8 Test report ..................................................................................................................... 12

9 Precision ....................................................................................................................... 13

9.1 Repeatability ......................................................................................................... 13

9.2 Reproducibility ...................................................................................................... 13

Annex A (informative) Determination of the interfacial tension of insulation liquids by

the drop volume method ....................................................................................................... 14

A.1 General ................................................................................................................. 14

A.2 Principle of the method ......................................................................................... 14

A.2.1 Basics ........................................................................................................... 14

A.2.2 Effect of adsorption (surface age) on the values obtained .............................. 15

A.3 Apparatus ............................................................................................................. 15

A.4 Procedure ............................................................................................................. 15

A.4.1 Preparation of apparatus ............................................................................... 15

A.4.2 Calibration ..................................................................................................... 15

A.4.3 Preparation of the test sample ....................................................................... 15

A.4.4 Determination ................................................................................................ 16

A.4.5 Evaluation/expression of results .................................................................... 16

A.4.6 Correlation of results obtained with drop volume method to results

obtained with ring method .............................................................................. 16

A.5 Precision ............................................................................................................... 17

A.6 Test report ............................................................................................................ 17

Annex B (informative) Investigative tests for differentiating between aged insulating

liquids ................................................................................................................................... 18

B.1 General ................................................................................................................. 18

B.2 Application ............................................................................................................ 19

Bibliography .......................................................................................................................... 20

---------------------- Page: 4 ----------------------
IEC 62961:2018  IEC 2018 – 3 –

Figure 1 – Typical development of interfacial tension values of new and service aged

mineral insulating liquids ......................................................................................................... 6

Figure 2 – Typical development of interfacial tension values of a new and a service

aged ester insulating liquid ..................................................................................................... 7

Figure 3 – Dimensions of platinum-iridium alloy ring in mm ................................................... 10

Figure B.1 – Plot of the data from Table B.1 according to Kezdy-Swinbourne method .......... 19

Table 1 – Repeatability (r) as a % for the measurement of interfacial tension at

approximately 180 s with both manual and motor driven instruments .................................... 13

Table 2 – Reproducibility (R) as a % for the measurement of interfacial tension at

approximately 180 s with both manual and motor driven instruments .................................... 13

Table A.1 – Comparison of interfacial values by measurement at 180 s and at 300 s to

400 s between the drop volume and ring methods ................................................................. 17

Table B.1 – Interfacial tension measured in constant equal time intervals ............................. 18

Table B.2 – Comparison of interfacial tension values by measurement at 180 s with

equilibrium values according to Kezdy-Swinbourne method .................................................. 19

---------------------- Page: 5 ----------------------
– 4 – IEC 62961:2018  IEC 2018
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INSULATING LIQUIDS – TEST METHODS FOR
THE DETERMINATION OF INTERFACIAL TENSION OF INSULATING
LIQUIDS – DETERMINATION WITH THE RING METHOD
FOREWORD

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is

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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.

International Standard IEC 62961 has been prepared by IEC technical committee 10: Fluids

for electrotechnical applications.
The text of this standard is based on the following documents:
FDIS Report on voting
10/1062/FDIS 10/1066/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.

---------------------- Page: 6 ----------------------
IEC 62961:2018  IEC 2018 – 5 –

The committee has decided that the contents of this document will remain unchanged until the

stability date indicated on the IEC website 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 'colour inside' logo on the cover page of this publication indicates

that it contains colours which are considered to be useful for the correct

understanding of its contents. Users should therefore print this document using a

colour printer.
---------------------- Page: 7 ----------------------
– 6 – IEC 62961:2018  IEC 2018
INTRODUCTION

Interfacial tension (IFT) of insulating liquid against water has been used for a long time as a

criterion for ageing evaluation. Statistical values that are used as orientation values and for

their interpretation have been published in IEC 60422 [1] .

The interfacial tension of insulating liquids changes with time depending on the type and

nature of the ageing products. This process is more pronounced with aged than with new

insulating liquids. It is well known that the interfacial tension of insulating liquids depends on

the interfacial concentration of the surface active amphiphilic aged products at the time of

measuring (dynamic interfacial tension), see Figure 1. The adsorption procedures, and thus

the attaining of a state of equilibrium, can take several minutes or even hours. With the

so-called static measuring methods – e.g. the Du Noüy ring [2]– measurements are repeated

on the same sample surface until no further change occurs.
49 19
Aged mineral
New mineral
insulating liquid insulating liquid
48,5 18,5
48 18
47,5 17,5
47 17
46,5 16,5
46 16
0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700
Time (s) Time (s)
IEC IEC

a) Typical development of interfacial tension b) Typical development of interfacial tension

values of a new inhibited mineral insulating liquid values of a service aged mineral insulating liquid

Figure 1 – Typical development of interfacial tension values
of new and service aged mineral insulating liquids
____________
Numbers in square brackets refer to the Bibliography.
IFT new mineral insulating liquid (mN/m)
IFT aged mineral insulating liquid (mN/m)
---------------------- Page: 8 ----------------------
IEC 62961:2018  IEC 2018 – 7 –
New ester
Aged ester
0 200 400 600 800 1 000
Time (s)
IEC
Figure 2 – Typical development of interfacial tension values
of a new and a service aged ester insulating liquid

The interfacial tension of insulating liquids measured by the existing method ASTM D971 [3],

working in non-equilibrium modus, provides only a single value within quite a short time (60 s)

and hence might be quite different from the static interfacial value, particularly in the case of

aged insulating liquids. In addition, the error of the time measurement might become a more

important aspect than the performance of the measurement itself. These weaknesses of

ASTM D971 could be generally compensated by replacing it with EN 14210 [4]. However, for

the practical work in the laboratory, the requirement of repeating tests until "static" conditions

are obtained can increase the test time dramatically.

The scope of this document is to find a compromise between the less accurate but fast

ASTM D971 method and the precise, but time consuming EN 14210 procedure. Experience of

the round robin tests shows clearly that the slope of the time-dependent interfacial tension

curve decreases significantly over a period of 180 s in the case of both mineral insulating

liquids (Figure 1 a), Figure 1 b)) and insulating synthetic and natural esters (Figure 2). A

measurement is carried out after a surface age of approximately 180 s in order to obtain a

value that provides a more realistic expression of the real interfacial tension, and that is less

sensitive to the timing of the measurement taken, and does not overly increase the test time.

The proposed surface age of 180 s allows the distinction between differently aged ester

liquids, which is not possible with ASTM D971.

The drop volume method for the determination of interfacial tension can deliver similar results

as the ring method if adapted concerning the surface age. This method is described in

Annex A.

Experience and results of round robin tests have shown that the deviation of tests repeated

after 10 min is less than 1 mN/m per min. Such tests can be necessary in case of further

comparative investigations of aged mineral and ester insulating liquids, and are described in

Annex B.
IFT ester liquids (mN/m)
---------------------- Page: 9 ----------------------
– 8 – IEC 62961:2018  IEC 2018
INSULATING LIQUIDS – TEST METHODS FOR
THE DETERMINATION OF INTERFACIAL TENSION OF INSULATING
LIQUIDS – DETERMINATION WITH THE RING METHOD
1 Scope

This document establishes the measurement of the interfacial tension between insulating

liquid and water by means of the Du Noüy ring method close to equilibrium conditions. In

order to obtain a value that provides a realistic expression of the real interfacial tension, a

measurement after a surface age of approximately 180 s is recorded.
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.
ISO 862, Surface active agents – Vocabulary

ISO 3675, Crude petroleum and liquid petroleum products – Laboratory determination of

density – Hydrometer method

ISO 12185, Crude petroleum and petroleum products – Determination of density – Oscillating

U-tube method
EN 14370, Surface active agents – Determination of surface tension
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 862 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
interfacial tension
tension at the interface between two phases

Note 1 to entry: The SI unit of interfacial tension is the Newton per metre (N/m). In practice, the submultiple

millinewton per metre (mN/m) is used.
4 Principle

The maximum force, F, necessary to pull or to force a ring of perimeter πD out of the interface

between insulating liquid and water in the direction of the insulating liquid is measured. The

interfacial tension, σ, is obtained by calculation, where the following approximate equation (1)

serves as the base:
---------------------- Page: 10 ----------------------
IEC 62961:2018  IEC 2018 – 9 –
max
(1)
σ= f
2πD
where
σ is the interfacial tension expressed as mN/m;
F is the maximum force exerted on the ring when pulled out of the liquid, in mN;
max
D is the mean diameter of the ring, in m;

f is a correction factor, taking into account that the measured maximum force includes

the additional volume of liquid extracted together with the ring because of the finite

diameter of the wire and of the lamella overlap inside the ring immediately prior to

detachment. Extrapolation formulae have been reported by Zuidema and Waters [5]
and others.

For the ring dimensions valid for this document and described in EN 14370, the following

simplified correction factor (2) is commonly used for interfacial tension tests in the range from

4 mN/m to 50 mN/m:
f= 0,725 + 4,014 ×10 × + 0,012 87 (2)
where
f is the correction factor according to Zuidema and Waters [5];
γ is the interfacial tension without correction in mN/m;

is the difference in the densities between water and insulating liquid at the measuring

temperature in g/cm . Density shall be measured in accordance with ISO 12185
(reference method), but ISO 3675 is accepted as well.

In automatic tensiometers with a built-in evaluation unit, software may perform the necessary

corrections without a direct report of the measured force.

To obtain exact values for surface or interfacial tension, it is necessary to measure the

maximum force on pulling the ring out of the liquid. Because of the great risk of detachment in

the case of the manual apparatus, extremely smooth manipulation is necessary since the

value obtained immediately prior to detachment of the film is not identical to the maximum

value. Automatic tensiometers can determine the maximum value electronically and reverse

the platform movement promptly prior to detachment of the film. This makes it possible to

obtain reliable, accurate time-consistent serial measurements without tearing the film.

5 Apparatus
5.1 Tensiometer

The tensiometer shall be designed for a ring and shall consist mainly of two parts:

• support for the sample vessel in the form of a small horizontal platform which can be

moved up and down;

• apparatus for measuring the force exerted on the ring; the uncertainty of measurement

shall not exceed ± 10 N, which corresponds to a maximum error of ± 0,1 mg weight
measurement.

Instead of a torsion balance as stated in ASTM D971, a lever balance or an electronic

balance (laboratory, analytical or microbalance) can be used. To obtain higher efficiency and

reproducibility, it is recommended to use an automatic tensiometer incorporating a balance,

motor driven platform and evaluation unit.
---------------------- Page: 11 ----------------------
– 10 – IEC 62961:2018  IEC 2018
5.2 Ring

The ring shall consist of a platinum/iridium wire with a thickness not greater than 0,4 mm and

a mean circumference of 60 mm (for example: inner diameter 18,7 mm, outer diameter

19,5 mm). It shall be suspended horizontally and connected to the tensiometer.

The dimensions of the ring made of platinum-iridium alloy are specified in EN 14370

(Figure 3).
Pt/lr 10
≤0,4
ø19,5
IEC
Figure 3 – Dimensions of platinum-iridium alloy ring in mm
5.3 Measuring vessel
Cylindrical glass vessel with a minimum diameter of 60 mm.

NOTE If a vessel with a diameter of less than 60 mm is used, wall effects can cause an error in the interfacial

tension measurement result.
6 Preparation of apparatus
6.1 Cleaning of the measuring vessel

The vessels shall be dedicated to IFT measurement only. Rinsing with solvents with

increasing polarity (such as heptane, acetone and/or 2-propanol, in this order) followed by a

final rinse with hot tap water and afterwards with deionized water or bi-distilled water has

been found suitable.
EXAMPLE An example of a step-by-step cleaning procedure is as follows:

• rinse three times with n-heptane (only if the vessel is being used after previous tests with insulating liquid),

then afterwards with 2-propanol (also in the case of unused beakers and after water testing). Rinse with hot

tap water and afterwards thoroughly with deionized water/bi-distilled water – see requirements for water in 6.3.

---------------------- Page: 12 ----------------------
IEC 62961:2018  IEC 2018 – 11 –

• a laboratory dishwasher with integrated deionized water with the required quality may be used if this provides

the required cleanliness. Ensure that all washing agents are removed completely before drying.

6.2 Cleaning of the ring
The ring shall be cleaned with a suitable solvent and then by flame cleaning.
A typical cleaning procedure is as follows:
– rinse three times with
...

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