ASTM D971-20

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D971 − 12 D971 − 20
Standard Test Method for
Interfacial Tension of Oil Insulating Liquids Against Water
by the Ring Method
This standard is issued under the fixed designation D971; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope
1.1 This test method covers the measurement of the interfacial tension between mineral oil insulating liquid that has a relative
density (specific gravity) less than water and water, under non-equilibrium conditions.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use. See 7.2 for a specific warning statement.
1.3 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D117 Guide for Sampling, Test Methods, and Specifications for Electrical Insulating Liquids
D923 Practices for Sampling Electrical Insulating Liquids
D2285 Test Method for Interfacial Tension of Electrical Insulating Oils of Petroleum Origin Against Water by the Drop-Weight
Method (Withdrawn 2008)
3. Summary of Test Method
3.1 Interfacial Tension is determined by measuring the force necessary to detach a planar ring of platinum wire from the surface
of the liquid of higher surface tension, that is, upward from the water-oil water insulating liquid interface. To calculate the
interfacial tension, the force so measured is corrected by an empirically determined factor which depends upon the force applied,
the densities of both oil the insulating liquid and water, and the dimensions of the ring. Measurements are made under rigidly
standardized nonequilibrium conditions in which the measurement is completed within 60 s after formation of the interface.
4. Significance and Use
4.1 Interfacial tension measurements on electrical insulating oilsliquids provide a sensitive means of detecting small amounts
of soluble polar contaminants and products of oxidation. A high value for new mineral insulating oil indicates the absence of most
undesirable polar contaminants. The test is frequently applied to service-aged mineral oils as an indication of the degree of
deterioration.
NOTE 1—Different liquid matrixes are reviewed in Appendix X1.
5. Apparatus (Figs. 1 and 2)
5.1 Tensiometer —Force measurement on the tensiometer may be made using a torsion wire, load cell, or any other means of
linearly determining tension in the range of 0 to 100 mN/m.
This test method is under the jurisdiction of ASTM Committee D27 on Electrical Insulating Liquids and Gasesand is the direct responsibility of Subcommittee D27.07
on Physical Test.
Current edition approved May 1, 2012April 1, 2020. Published May 2012May 2020. Originally approved in 1948 as D971 – 48 T. Last previous edition approved in
20042012 as D971 – 99aD971 – 12.(2004). DOI: 10.1520/D0971-12.10.1520/D0971-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Tensiometers that use the Du Nouy principle for measuring interfacial and surface tension should be used.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D971 − 20
FIG. 1 Manual Interfacial Tensiometer
FIG. 2 Top View of Manual Interfacial Tensiometer
5.1.1 Horizontal platform to hold the sample container should be capable of movement upward or downward using a mechanical
screw, lever mechanism, electronic drive, or any other means of precisely changing vertical position of the sample.
5.2 Rings made of platinum or platinum-iridium alloy may be used. The ring should be welded into a continuous circle and
attached to at least two parallel stirrups. Circumference of the ring should be minimum 40 mm and diameter of the ring wire should
be about 0.3 mm. Ratio of the ring major radius (R) and the wire radius (r) should be known (R/r) to at least three significant
figures. Stirrups should be at least 25 mm long.
D971 − 20
5.2.1 The ring may be suspended from or rigidly fixed into the tensiometer force measuring system.
5.3 Sample Container—Glass beaker or clear cylindrical container having a minimum diameter of 45 mm.
6. Reagents
6.1 Purity of Reagents—Reagent grade chemicals shall be used in the test.
6.2 Purity of Water—Unless otherwise indicated, references to water shall be understood to mean distilled water or water of
equal purity.
7. Preparation of Apparatus
7.1 Glass containers can be cleaned by rinsing in xylene, hexane or another suitable hydrocarbon solvent, followed by a rinse
in acetone or methyl ethyl ketone. Following these rinses they can be cleaned using detergent in hot water and rinsed thoroughly
with tap and then distilled water. Dry the glass vessel if not for immediate use.
7.2 The ring should be cleaned by immersing it in a small container of xylene, hexane or another suitable hydrocarbon solvent
for at least 5 s while spinning the ring back and forth. Next, repeat this rinse using a container filled with acetone or methyl ethyl
ketone. Flame the ring in a gas flame, again spinning it to flame making sure all areas that will contact liquid obtain rapid, uniform
heating. The ring should barely glow orange and should be heated for no more than 5 s. Clean the ring immediately before each
measurement. (Warning– Solvents are extremely flammable. Take precautions to keep all flammable and combustible materials
away from the flame.)
7.3 The tensiometer should be leveled by placing a bubble level device on the sample platform, if not equipped with one, and
adjusting the tensiometer base until the platform is level.
8. Calibration of Apparatus
8.1 Calibrate the tensiometer against known weights and adjust its zero point according to the procedure of its manufacturer.
Make certain that all portions of the ring are in the same horizontal plane.
9. Procedure
9.1 Preparation of Test Specimens:
9.1.1 Obtain a representative test specimen of liquid, at least 25 mL, liquid to be studied in accordance with Test Method
Practices D923.
9.1.2 Obtain a fresh aliquot of distilled water (50 to 75 mL) at room temperature for interfacial tension measurements.
9.1.3 Test specimens and distilled water should be covered, capped, or sealed immediately after they are obtained. Atmospheric
dust and vapor can easily contaminate water, as can the growth of bacteria. Store test specimen in the darkand distilled water at
room temperature.
9.1.4 Temperature of test specimen and temperature of distilled water should agree within 61°C.
9.2 Determination of Test Specimen Density:
9.2.1 Determine test specimen density at room temperature using a liquid pycnometer or other density measuring device to
60.001 g/cmg/mL. .
9.3 Interfacial Tension Measurement:
9.3.1 Introduce distilled water at room temperature into a clean sample container to a depth of 10 to 15 mm. Place it on the
tensiometer platform so that the ring comes down in the center of the container and is not close to any wa
...