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.

Isolierflüssigkeiten - Prüfverfahren zur Bestimmung der Grenzflächenspannung von Isolierflüssigkeiten - Bestimmung durch die Ring-Methode

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.

Izolacijske tekočine - Preskusne metode za ugotavljanje površinske napetosti izolacijskih tekočin - Ugotavljanje z metodo s prstanom

Ta dokument vzpostavlja meritev površinske napetosti med izolacijsko tekočino in vodo na podlagi Du Noüyjeve metode s prstanom pri pogojih skorajšnje uravnoteženosti. Za namene pridobitve vrednosti, ki ponuja realističen prikaz dejanske površinske napetosti, je treba zabeležiti rezultate meritve po starosti površine približno 180 s.

General Information

Status
Published
Publication Date
22-Nov-2018
Current Stage
6060 - Document made available
Due Date
03-Jun-2019

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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Isolierflüssigkeiten - Prüfverfahren zur Bestimmung der Grenzflächenspannung von Isolierflüssigkeiten - Bestimmung der Grenzflächenspannung mittels RingmethodeIsolants liquides - Méthodes d'essai pour la détermination de la tension interfaciale des isolants liquides - Détermination par la méthode à l'anneauInsulating liquids - Test methods for the determination of interfacial tension of insulating liquids - Determination with the ring method29.040.01Izolacijski fluidi na splošnoInsulating fluids in generalICS:Ta slovenski standard je istoveten z:EN IEC 62961:2018SIST EN IEC 62961:2019en01-januar-2019SIST EN IEC 62961:2019SLOVENSKI

STANDARD
SIST EN IEC 62961:2019
EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM
EN IEC 62961
November 2018 ICS 29.040.10
English Version

Insulating liquids - Test methods for the determination of interfacial tension of insulating liquids - Determination with the ring method (IEC 62961:2018)

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)

Isolierflüssigkeiten - Prüfverfahren zur Bestimmung der Grenzflächenspannung von Isolierflüssigkeiten - Bestimmung der Grenzflächenspannung mittels Ringmethode (IEC 62961:2018) This European Standard was approved by CENELEC on 2018-10-25. CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the same status as the official versions. CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom. European Committee for Electrotechnical Standardization

Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung CEN-CENELEC Management Centre: Rue de la Science 23,

B-1040 Brussels © 2018 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.

Ref. No. EN IEC 62961:2018 E SIST EN IEC 62961:2019

EN IEC 62961:2018 (E) 2 European foreword The text of document 10/1062/FDIS, future edition 1 of IEC 62961, prepared by IEC/TC 10 "Fluids for electrotechnical applications" was submitted to the IEC-CENELEC parallel vote and approved by CENELEC as EN IEC 62961:2018. The following dates are fixed: • latest date by which the document has to be implemented at national level by publication of an identical national standard or by endorsement (dop) 2019-07-25 • latest date by which the national standards conflicting with the document have to be withdrawn (dow) 2021-10-25

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.

Endorsement notice The text of the International Standard IEC 62961:2018 was approved by CENELEC as a European Standard without any modification. In the official version, for Bibliography, the following note has to be added for the standard indicated: IEC 60422 NOTE Harmonized as EN 60422

SIST EN IEC 62961:2019
EN IEC 62961:2018 (E) 3 Annex ZA (normative)

Normative references to international publications with their corresponding European publications 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.

NOTE 1

Where an International Publication has been modified by common modifications, indicated by (mod), the relevant EN/HD applies.

NOTE 2

Up-to-date information on the latest versions of the European Standards listed in this annex is available here: www.cenelec.eu.

Publication Year Title
EN/HD Year ISO 862 -
Surface active agents - Vocabulary
ISO 3675 -

Crude petroleum and liquid petroleum products - Laboratory determination of density - Hydrometer method

EN ISO 3675 -
ISO 12185 -

Crude petroleum and petroleum products - Determination of density - Oscillating U-tube method

EN 14370 -
Surface active agents - Determination of surface tension
EN 14370 -
SIST EN IEC 62961:2019
SIST EN IEC 62961:2019

IEC 62961 Edition 1.0 2018-09 INTERNATIONAL STANDARD NORME INTERNATIONALE 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
® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale ®

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

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé. SIST EN IEC 62961:2019 colourinside

– 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

SIST EN IEC 62961:2019
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

SIST EN IEC 62961:2019

– 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 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. 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. SIST EN IEC 62961:2019

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.

SIST EN IEC 62961:2019

– 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] 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.

a) Typical development of interfacial tension values of a new inhibited mineral insulating liquid b) Typical development of interfacial tension values of a service aged mineral insulating liquid Figure 1 – Typical development of interfacial tension values of new and service aged mineral insulating liquids ____________ 1 Numbers in square brackets refer to the Bibliography. IEC 48,5 49 48 47,5 47 46,5 46 IFT new mineral insulating liquid (mN/m)

New mineral insulating liquid
0 100 200 300 400 500 600 700 Time

(s) IEC 18,5 19 18 17,5 17 16,5 16 IFT aged mineral insulating liquid (mN/m) Aged mineral insulating liquid 0 100 200 300 400 500 600 700 Time

(s) SIST EN IEC 62961:2019
IEC 62961:2018  IEC 2018 – 7 –
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.

IEC 19 17 15 IFT ester liquids
(mN/m) Aged ester 0 200 400 600 Time
(s) New ester 800 1 000 21 23 25 27 SIST EN IEC 62961:2019
– 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: SIST EN IEC 62961:2019

IEC 62961:2018  IEC 2018 – 9 –
fDFπσ2max==(1)=where=1

is the interfacial tension expressed as mN/m; Fmax is the maximum force exerted on the ring when pulled out of the liquid, in mN; 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:

-40,725+4,014×10×+0,012 87fγ∆ρ==(2)=where=f is the correction factor according to Zuidema and Waters [5]; γ

is the interfacial tension without correction in mN/m; ρ∆==is=the=≤i∞∞erence=in=the=≤ensities=between=water=an≤=ins×lating=liq×i≤=at=the=meas×ring=temperat×re=in=g/cm3.=Density=shall=be=meas×re≤=in=accor≤ance=with=IπO=12185=(re∞erence=metho≤),=b×t=IπO=3675=is=accepte≤=as=well.=In=a×tomatic=tensiometers=with=a=b×ilt-in=eval×ation=×nit,=so∞tware=may=per∞orm=the=necessary=corrections=witho×t=a=≤irect=report=o∞=the=meas×re≤=∞orce.=To=obtain=e•act=val×es=∞or=s×r∞ace=or=inter∞acial=tension,=it=is=necessary=to=meas×re=the=ma•im×m=∞orce=on=p×lling=the=ring=o×t=o∞=the=liq×i≤.=Beca×se=o∞=the=great=risk=o∞=≤etachment=in=the=case=o∞=the=man×al=apparat×s,=e•tremely=smooth=manip×lation=is=necessary=since=the=val×e=obtaine≤=imme≤iately=prior=to=≤etachment=o∞=the=∞ilm=is=not=i≤entical=to=the=ma•im×m=val×e.=A×tomatic=tensiometers=can=≤etermine=the=ma•im×m=val×e=electronically=an≤=reverse=the=plat∞orm=movement=promptly=prior=to=≤etachment=o∞=the=∞ilm.=This=makes=it=possible=to=obtain=reliable,=acc×rate=time-consistent=serial=meas×rements=witho×t=tearing=the=∞ilm.=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 ± 10-6 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. SIST EN IEC 62961:2019

– 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).

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. IEC ≤0,4=Pt/lr=10=ø19,5=SIST EN IEC 62961:2019

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 n-heptane, and afterwards with deionized water; – heat in the oxidizing flame for approximately 5 s in an ethanol or natural gas burner to red heat. To prevent mechanical stress on the ring, twist it during this procedure. 6.3 Water used for the test Bi-distilled or deionized water from a glass bottle with a surface tension of > 70 mN/m at maximum 25 °C (permissible range 18 °C to 25 °C) and with a known low conductivity (typically < 0,1 µS/cm). It is crucial that the water used not be from the tap and that it be free from any ions, since they may lead to a significant decrease of the measured interfacial tension. HPLC-grade water may be used, if suitable. NOTE For pure water, the following relationship (Equation 3) between surface tension of water and temperature exists:

0,15T∆=∆σ−==(3)=where=∆1

is the difference between the surface tension values in mN/m measured at two different temperatures; ∆T

is the difference between the two temperatures in °C. 7 Procedure 7.1 General The measurement shall be done in the temperature range between 18 °C and 25 °C. The water and insulating liquid shall be at the same temperature. The density of the insulating liquid shall be determined at the temperature of measurement or can be calculated from a linear extrapolation of density from measurement at a standard temperature (e.g. 20 °C) to the temperature used for the IFT measurement. Round robin test results have shown that variations within this temperature range do not practically influence the test results. The correction according to Zuidema and Waters [5] sh

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