Contents
Partially compliant

ASTM D971 and IEC 62961: interfacial tension of insulating liquids, such as transformer oil, against water by the ring method

ASTM D971 and IEC 62961 measure the interfacial tension between an insulating oil and water with a Du Noüy ring, to track how the oil ages. The Dropometer measures the same property by pendant drop with the optional cuvette, read at the 60 s or 180 s drop age the standards use. Pendant drop is an alternative method, not the ring method the standards specify: if a contract or customer requires an ASTM D971 or IEC 62961 result, run a correlation study on your own samples first.

What brings you here?

Who this is for
Oil testing labs, utilities and maintenance teams for transformers and other oil filled electrical equipment, and makers of mineral and ester insulating liquids.
Compliance tier
Partially compliant: the Dropometer measures oil and water interfacial tension by pendant drop with the optional cuvette, an alternative method to the ring both standards specify.
Measurement
Pendant drop of oil in water, or water in oil, in the optional cuvette at 60 s or 180 s drop age. No interfacial tension accuracy or range is published.
Setup
About 2 minutes. Battery powered, 1 kg, runs on any bench beside your ring tensiometer.
Last updated
October 20, 2026
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Free one page checklist for ASTM D971 / IEC 62961: what to prepare, how to set up the optional cuvette, which drop age to read, how to screen your test water and what to record. Made for oil testing benches.

Talk to an expert

Book a 15 minute demo on your own oils

Tell us your oils, how dark they get and how you test today. We will run a pendant drop of oil and water in the optional cuvette live, read it at your drop age and show where a ring result still applies.

Book a 15 minute demo
View the official standard
Gurdeep-Saini-Photo
Written by
Gurdeep Singh Saini
Holds a BASc in Mechanical Engineering from Ryerson University (now Toronto Metropolitan University) and an MASc from York University. He develops the automatic edge and baseline detection behind the Dropometer's analysis.
COO at Droplet Lab
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Droplet-Lab logo
Technical Review by
Droplet Lab Team
Droplet Lab builds precision instruments and software for surface science measurement, specialising in contact angle analysis and surface tension characterisation. Used by researchers across materials science, pharmaceuticals, coatings, and advanced manufacturing, Droplet Lab's Dropometer has contributed to studies published in peer-reviewed journals including Advanced Functional Materials (Impact Factor 19). The team combines instrument engineering with deep domain knowledge in wettability science with a focus on practical accuracy.
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Verified against ASTM D971 and IEC 62961
Gurdeep-Saini-Photo
Written By

Gurdeep Singh Saini

COO at Droplet Lab

Holds a BASc in Mechanical Engineering from Ryerson University (now Toronto Metropolitan University) and an MASc from York University. He develops the automatic edge and baseline detection behind the Dropometer's analysis.

Droplet-Lab logo
Reviewed By

Droplet Lab Team

Droplet Lab builds precision instruments and software for surface science measurement, specialising in contact angle analysis and surface tension characterisation. Used by researchers across materials science, pharmaceuticals, coatings, and advanced manufacturing, Droplet Lab's Dropometer has contributed to studies published in peer-reviewed journals including Advanced Functional Materials (Impact Factor 19). The team combines instrument engineering with deep domain knowledge in wettability science with a focus on practical accuracy.

Featured Studies & Publications

Research Paper Contact angle measurement with a smartphone
Research Paper Surface tension measurement with a smartphone using a pendant drop

Evidence box: interfacial tension of insulating liquids

Standards

ASTM D971-20, Standard Test Method for Interfacial Tension of Insulating Liquids Against Water by the Ring Method, from ASTM Subcommittee D27.07. IEC 62961:2018, Edition 1.0, Insulating liquids: test methods for the determination of interfacial tension of insulating liquids, determination with the ring method, from IEC TC 10.

Compliance tier

Partially compliant. With the optional cuvette, the Dropometer measures oil against water interfacial tension by pendant drop, the property both standards report by ring. It is an alternative method and reads at the drop age either standard uses. A reported ring result needs a ring tensiometer or your own correlation study.

Instrument performance

Interfacial tension by pendant drop with the optional cuvette: the datasheet gives no accuracy or range for it. Surface tension against air, used for the test water screen, runs up to 75 mN/m with ±0.03 mN/m accuracy and 0.01 mN/m resolution, static and dynamic (Dropometer product specification). No temperature control: it runs at 10 to 45 °C.

Peer reviewed method

The drop shape method is validated against a KRUSS DSA100E in two peer reviewed papers. See our validation publications.

Customer evidence

We have no published client paper or case study on insulating oil yet. A university shared research facility uses the Dropometer for liquid in liquid work like the optional cuvette setup: pendant drop interfacial tension of lipid solutions in mineral oil against buffer. Read the case study.

What it does not tell you

A pendant drop value from the optional cuvette is not a ring value. It tracks the same property, but the offset to ASTM D971 or IEC 62961 results comes only from a correlation study on your own oils.

Executive summary: insulating liquids against water

What these standards measure, who uses them and where the Dropometer fits

ASTM D971 and IEC 62961 measure the interfacial tension (IFT) between an insulating oil, such as transformer oil, and water by pulling a Du Noüy ring through the interface: a lower value points to more polar ageing products in the oil. Utilities, oil testing labs and makers of mineral and ester insulating liquids use it to trend transformer oil quality in transformers and other electrical equipment. For the terms, see the interfacial tension glossary entry.

ASTM D971-20 reads the value within about 60 s of forming the interface, a non equilibrium reading. IEC 62961:2018 uses the Du Noüy ring method close to equilibrium conditions, at a surface age of approximately 180 s, which IEC says gives a more realistic expression of the real interfacial tension, and adds a drop volume method in an informative annex. Both include a check that the test water's surface tension sits inside a set band before the oil is measured.

The Dropometer pendant drop tensiometer is partially compliant here. With the optional cuvette it measures oil and water interfacial tension by pendant drop, and it can report the value at a 60 s or 180 s drop age to match either standard. Pendant drop is an alternative method, not the ring method the standards specify. If a contract, customer or utility requires an ASTM D971 or IEC 62961 result, run a correlation study against a ring tensiometer on your own samples first. For the optional cuvette, no interfacial tension accuracy or range is published, so set your repeatability from replicate drops.

It also screens the test water: surface tension of water against air up to 75 mN/m with ±0.03 mN/m accuracy (Dropometer product specification). Good bench habits are in sample preparation and contamination control and reproducible surface tension measurement, with water values in our surface tension reference values for common liquids. For trending between lab tests, see screening insulating oil ageing by pendant drop.

For the wider picture, read the electrical and electronics guide, the surface tension measurement guide and why water forms droplets in oil but oil spreads in water. A university shared research facility uses the Dropometer for contact angle and for liquid in liquid work, the setup the optional cuvette serves: pendant drop interfacial tension between oil and aqueous phases.

Further reading: the oil and water classroom experiment, choosing a tensiometer for surfactant and CMC research and a published contact angle study on plasma treated titanium.

How the Dropometer fits transformer oil testing

The ASTM D971 and IEC 62961 measurement, step by step, in our own words

From oil sample to a trended value, with the Dropometer and the optional cuvette

These steps show how the Dropometer measures oil against water by pendant drop, and where a ring result still applies. They do not replace either standard. Buy the current edition and follow its timing, temperature and water rules.

60 s or 180 s

Drop age to match

Read at the ASTM D971 timing or the IEC 62961 surface age, and keep one per trend.

Optional cuvette

Oil against water

For the optional cuvette, no interfacial tension accuracy is published.

About 2 min

Setup

Base plate, phone holder, LED, syringe and cuvette. No PC needed.

1

Take the oil sample and its density

Draw the oil into a clean, dry container using your usual sampling practice for electrical insulating liquids. Keep it sealed and out of light until you test it. Record the transformer or other equipment, the date, the oil type and the oil and water densities: the drop shape fit needs both.

2

Screen the test water

Hang pendant drops of the test water in air and read the surface tension against your accept band. A low reading points to contaminated water or glassware. Clean the syringe, needle and cuvette as described in sample preparation and contamination control.

3

Set up the optional cuvette

For clear, new oil, fill the optional cuvette with oil and hang a water drop from a straight needle, because water is denser. For dark aged oil, light through the oil loses edge contrast, so fill the cuvette with water and release an oil drop upward from an upturned needle, as the sample allows. Use one setup for the whole trend.

4

Read at the standard's drop age

Form the drop, start the timer and record tension against drop age. Report the value at 60 s to match ASTM D971 or at about 180 s to match IEC 62961. Repeat on several fresh drops and reject any with a poor fit or a bubble.

5

Confirm by ring where a result is required

Pendant drop is an alternative method, not the ring method the standards specify. If a customer, utility or contract needs an ASTM D971 or IEC 62961 result, run the ring tensiometer to the standard, or a correlation study on your own oils first.

6

Record and trend

Record the drop orientation, drop age, mean and spread, water screen result, room temperature and sample details. Compare with the unit's earlier results and your maintenance limits. For bench habits, see reproducible surface tension measurement.

7

Watch the full measurement

This video shows a complete Dropometer pendant drop surface tension measurement, from setup to result. It covers the steps above in one run.

Dropometer pendant drop surface tension, start to finish.

Validated measurement approach

Independent benchmarking and publication-based validation references.

Peer reviewed method

The Dropometer drop shape method is validated against a KRUSS DSA100E in two peer reviewed papers, one in Review of Scientific Instruments and one in Colloids and Surfaces A.

Read the validation papers

Customer evidence

We have no published client paper or case study on insulating oil yet. A university shared research facility uses the Dropometer for liquid in liquid work like the optional cuvette setup, including pendant drop interfacial tension between oil and aqueous phases, and for contact angle work across several labs.

Read the case study

Set transformer oil quality limits from your oil plan, not from this page

ASTM D971 · IEC 62961 · oil condition and test water

Neither standard sets a pass or fail value for the oil. Limits come from your oil specification or maintenance guide. IEC 60422 Ed. 5.0 (2024), the IEC guide for mineral oils in service, cites both IEC 62961 and ASTM D971 for this test (source: IEC 60422 Ed. 5.0 preview).

As one lab's published guide, OELCHECK lists at least 40 mN/m for mineral oil as delivered and at least 35 mN/m for oil from a new system. Its floors for usable oil in service are 22 mN/m inhibited, 20 mN/m uninhibited and 18 mN/m for ester based oil (source: OELCHECK interfacial tension guide). These are ring based values. Before you apply them to pendant drop results from the optional cuvette, check the offset in your own correlation study.

The datasheet gives no accuracy or range for the optional cuvette's interfacial tension readings. Set your own repeatability from replicate drops on a reference oil, and recheck it with each new batch of test water.

The test water has its own band. Nanoscience Instruments reports 71 to 73 mN/m for ASTM D971 and 70 to 73 mN/m for IEC 62961 (source: Nanoscience Instruments application note). If you screen water with the Dropometer, set your accept band from clean water measured in your own room, because the Dropometer reads at room temperature and has no temperature control.

Do not mix 60 s and 180 s results in one trend. IEC notes the two readings can differ noticeably, most of all for aged oil (source: IEC 62961 preview).

Example output: new and aged transformer oil

Example data for illustration only. These are not measured results. Your own oils, room conditions, correlation study and limits decide the numbers.

Example: new and aged insulating oils in the optional cuvette, pendant drop interfacial tension against water

Oil sample Drops (n) At 60 s drop age (mN/m) At 180 s drop age (mN/m) Std dev at 180 s (mN/m) Next step
New mineral oil, as delivered545.244.30.3Baseline for the unit's trend
In service oil, unit A533.632.10.4Keep the normal sampling interval
In service oil, unit B524.321.90.5Confirm by ring; plan treatment with the asset owner
Dark aged oil, unit C516.413.50.7Confirm by ring, then act per your guide

How to read this table

Each row is the mean of five oil drops rising in water from an upturned needle in the optional cuvette, at 22.5 °C room temperature, with measured densities. Values are read at the ASTM D971 timing (60 s drop age) and the IEC 62961 timing (180 s drop age). New oil sits in the 40 to 50 mN/m range reported for clean oil by Learn Oil Analysis; the floor for inhibited oil in service is 22 mN/m in the OELCHECK guide. Both sources quote ring results, so confirm any limit in your own correlation study.

Example output chart

The same example data as the table: each oil read at the ASTM D971 drop age (60 s) and the IEC 62961 drop age (180 s). The gap between the two grows as the oil ages.

Grouped bar chart of example pendant drop oil against water tension for four oils, falling from about 45 mN/m for new oil to about 14 mN/m for dark aged oil, with the 180 s reading below the 60 s reading for every oil

Example data, not measured results.

Business impact

What changes when oil is trended by pendant drop between ring tests

Metric Before Dropometer With Dropometer Results
Oil and water tension between lab tests Waits for a ring tensiometer slot Pendant drop with the optional cuvette on any bench No published benchmark
Reading time Ring reading at 60 s or about 180 s Tension against drop age, reported at 60 s or 180 s ASTM D971 and IEC 62961 timings
Reported ASTM D971 or IEC 62961 result Ring tensiometer Still the ring tensiometer, or your own correlation study ASTM D971 and IEC 62961 scope
Test water check Found on the ring tensiometer, in the queue with oil samples Each water batch screened by pendant drop, ±0.03 mN/m surface tension accuracy Dropometer product specification

See it on your own oils

Bring a new oil, an aged one and your test water. We will measure them live with the optional cuvette and show where a ring result still applies.

Book a 15 minute demo Request a quote
ROI estimate

Estimate the cost of the oil tests a bench trend can catch early

Enter your own numbers. The defaults are examples, not Droplet Lab results.

Quality cost snapshot

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Example only. Use what you see in a trial on your own samples.
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Result

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Payback period

Annual exposure combines audit/CAPA cost, customer rejections, and recall probability; avoided exposure applies the risk reduction percentage.

Method settings: timing, clean water and temperature

Lock these before you compare results between runs, labs or standards

Parameter Recommended Setting Technical Rationale
Liquid pair Insulating liquid against water, liquid less dense than water This is the scope of ASTM D971. IEC 62961 covers insulating liquids against water.
Method Standards: Du Noüy ring. Dropometer: pendant drop with the optional cuvette Pendant drop is an alternative method, not the ring method the standards specify. Correlate before you report it as a standard result.
Drop orientation Water drop in clear oil, or oil drop rising in water for dark oil, as the sample allows The camera must see a sharp drop edge. Light through clear water keeps contrast when aged oil is dark.
Reading time 60 s (ASTM D971) or about 180 s (IEC 62961) after the interface forms The value falls as ageing products reach the interface, so the two timings give different numbers.
Densities Oil and water densities at test temperature, measured or from your records The drop shape fit uses the density difference. The ring calculation uses densities too.
Test water band 71 to 73 mN/m (ASTM D971) or 70 to 73 mN/m (IEC 62961) As reported by Nanoscience Instruments. Confirm the band and temperature in your edition.
Temperature As the standard specifies Tension falls as temperature rises. The Dropometer has no temperature control, so condition the liquids and record room temperature.
Fit model and replicates Young-Laplace, the same for every run; several fresh drops per sample One drop cannot show spread. No interfacial tension accuracy is published for the optional cuvette setup.

Is ASTM D971 / IEC 62961 the right test?

Pick the method by the question you need answered

Start: What do you need to know about your oil, water or insulator surface?

Trend oil ageing between ring tests

Signals:

You want oil and water tension on more samples, or more often, than the ring tensiometer queue allows.

Rule-out:

Measure it by pendant drop with the Dropometer and the optional cuvette, at a fixed 60 s or 180 s drop age. See screening insulating oil ageing.

A reported ASTM D971 or IEC 62961 result

Signals:

A contract, customer or utility names the ring standard for the reported value.

Rule-out:

Use a ring tensiometer to the standard. Pendant drop with the optional cuvette is an alternative method; use it in place of the ring only after a correlation study on your own oils.

Is the test water clean enough?

Signals:

Runs fail the water check, or you want to catch bad water before it reaches the oil test.

Rule-out:

Screen the water by pendant drop in air with the Dropometer. See surface tension measurement.

Water repellency of insulator housings

Signals:

You check how well polymer insulators or coatings shed water, new or after service.

Rule-out:

Use the contact angle method in IEC TS 62073. The Dropometer measures contact angle.

Decision tree at a glance

Start from the question you need answered. Blue boxes are measurements the Dropometer runs. A reported ring result to the standard is not one of them.

Decision tree showing that oil ageing trends by pendant drop with the optional cuvette, the test water screen and contact angle to IEC TS 62073 run on the Dropometer, while a reported ASTM D971 or IEC 62961 ring result needs a ring tensiometer or a correlation study

Is ASTM D971 / IEC 62961 the right test?

Interpreting the measurement: interfacial and surface tension

What the oil and water value tells you, and what it does not

Lower value, more polar contaminants: According to the ASTM standard, the ring value is a sensitive sign of soluble polar contaminants and oxidation products. A falling trend points to ageing oil.
60 s and 180 s are not the same number: IEC 62961 reads closer to equilibrium than ASTM D971, and the gap grows as oil ages. Keep one drop age per trend.
Pendant drop and ring values can differ: A drop and a flat interface age differently. Use your correlation study before you compare a cuvette result with a ring limit.
A low water reading voids the run: Water below its band means an impurity in the water or on the glassware, because the measurement depends on water purity. Fix that before you test oil, or the oil value reads low.
Oil surface tension is not the oil and water value: Ageing products adsorb at the oil and water boundary. The oil's surface tension against air is a different quantity.
Expert perspective

Why drop age and clean water matter for oil and water measurements

Not all contact-angle measurements are created equal: the right method depends on the application. A sessile drop and a tilted plate can give very different, and equally correct, numbers.

Dr. Alidad Amirfazli

Department of Mechanical Engineering, York University, Co-founder and Scientific Advisor, Droplet Lab

Common pitfalls with oil against water by the ring method

Honest limits of the method and of the Dropometer

Pendant drop with the optional cuvette is an alternative method, not the ring method ASTM D971 and IEC 62961 specify. Correlate before you report it as a standard result.
For the optional cuvette, no interfacial tension accuracy or range is published. Do not borrow the surface tension accuracy figure for it.
Dark aged oil lowers edge contrast when you look through it. Use an oil drop rising in water where the sample allows, or send unclear samples to the ring lab.
Wrong densities shift the result, most of all for ester liquids, whose density is close to water. Measure both densities at test temperature.
The Dropometer has no temperature control. Condition the liquids, record room temperature and keep the room steady across a trend.
Mixing 60 s and 180 s readings, or water in oil and oil in water setups, in one trend hides real change. Note both on every result.
FAQ

Frequently asked questions

What does ASTM D971 measure?

It measures the interfacial tension between an insulating liquid that is less dense than water and water, using a Du Noüy ring, under non equilibrium conditions. ASTM describes it as a sensitive way to detect soluble polar contaminants and oxidation products, so it is often used in transformer oil testing to track ageing of mineral oil in service.

How is IEC 62961 different from ASTM D971?

Both use a Du Noüy ring. The older ASTM D971 reads within about 60 s of forming the interface. The new IEC 62961 standard, first published in 2018, reads at a surface age of about 180 s, closer to equilibrium, which IEC says separates differently aged ester liquids better. IEC 62961 also gives a drop volume method in an informative annex (source: IEC 62961 preview).

Can the Dropometer measure ASTM D971 or IEC 62961 interfacial tension with the optional cuvette?

Yes. The interfacial tension is measured by pendant drop of one liquid in the other, so its tier is partially compliant. It reports the value at a 60 s or 180 s drop age to match either standard. For the optional cuvette, the datasheet gives no interfacial tension accuracy or range, so set your repeatability from replicate drops. It also screens your test water's surface tension against air, up to 75 mN/m with ±0.03 mN/m accuracy.

Is pendant drop an equivalent method to the ring in ASTM D971 or IEC 62961?

No. Pendant drop is an alternative method, not the ring method these standards specify. IEC 62961 lists a drop volume method in an informative annex, but not pendant drop. If a contract, customer or utility requires an ASTM D971 or IEC 62961 result, run a correlation study against a ring tensiometer on your own samples first. Never treat the two as interchangeable without it.

Should I hang a water drop in the oil or float an oil drop in the water?

Water is denser, so a water drop hangs from a straight needle in an oil filled cuvette. That works while the oil is clear enough to light through. Aged oil can be dark, so fill the optional cuvette with water and release an oil drop upward from an upturned needle, which keeps the light path in clear water. Choose as the sample allows and keep one setup per trend.

What value means the transformer oil needs attention?

Neither standard sets one. Limits come from your oil specification or maintenance guide, such as IEC 60422 for mineral oil. As one lab's guide, OELCHECK lists at least 40 mN/m for new mineral oil as delivered and a floor of 22 mN/m for inhibited oil in service (source: OELCHECK). These are ring values, so map them to pendant drop results through your own correlation study.

Can I measure the oil's surface tension instead?

The Dropometer can measure the surface tension of an oil against air by pendant drop, without the cuvette. Neither standard uses that value, and it is not a stand in for the oil and water value, because ageing products gather at the oil and water boundary.

Legal note

This page summarises ASTM D971 and IEC 62961 in our own words and does not reproduce either standard. Buy the current editions from ASTM or IEC before you write a procedure. The Dropometer is partially compliant: it measures oil and water interfacial tension by pendant drop with the optional cuvette, an alternative method to the ring. Droplet Lab does not certify compliance. Your own method validation does.

View the official standard Book a 15 minute demo

How this page was created

Sources, review and tools

Transparency 4 checklist items
01

Sources

ASTM D971-20 listing; IEC 62961:2018 listing (IEC webstore); IEC 62961:2018 preview: scope, introduction and contents (VDE Verlag); Nanoscience Instruments: ASTM D971 and IEC 62961 water bands and timing, the Dropometer product specification and Droplet Lab validation papers.

02

Drafting assistance

Drafted with Claude (Anthropic) from the sources listed, then reviewed by Droplet Lab.

03

What we did not do

We did not copy text from the standard. Buy the current edition for the full procedure.

04

Last checked

October 2026

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References

1. ASTM D971-20 / IEC 62961:2018. store.astm.org/d0971-20.html
2. IEC 62961:2018 listing (IEC webstore). webstore.iec.ch/publication/27653
3. IEC 62961:2018 preview: scope, introduction and contents (VDE Verlag). vde-verlag.de/iec-normen/preview-pdf/info_iec62961{ed1.0}b.pdf
4. Nanoscience Instruments: ASTM D971 and IEC 62961 water bands and timing. nanoscience.com/
5. Droplet Lab. Peer reviewed validation papers in Review of Scientific Instruments and Colloids and Surfaces A. dropletlab.com/validation/publications/
6. Droplet Lab. Dropometer product specification, 2026. dropletlab.com/products/dropometer/