Tensiometer buyer's guide
Optical Drop Shape vs Force Tensiometry: Which You Need
Choose pendant drop if you have microlitres of sample or need interfacial tension between liquids; choose a force tensiometer for CMC series, viscous paints or methods written around ASTM D1331 or ISO 304. For pendant drop on a small budget, our pick is the Dropometer at US$4,990. Runner up for ring work: a du Noüy ring tensiometer from US$7,901.
How do optical and force tensiometers measure surface tension?
They measure the same property in two different ways, and each way has its own inputs and its own sources of error.
- Optical drop shape (pendant drop). A camera photographs a drop hanging from a needle. Software fits the drop outline to the Young Laplace equation, which balances surface tension against gravity. The fit needs the density of the drop and of the surrounding phase, so an unknown density gives an unknown result.
- Force tensiometry (du Noüy ring or Wilhelmy plate). A platinum ring or plate is pulled through the liquid surface while a balance records the force. Ring readings need an empirical correction factor, such as Harkins and Jordan or Zuidema and Waters. The plate method assumes the liquid fully wets the plate, a 0° contact angle.
The surface tension measurement guide covers both methods in more depth, and the pendant drop method entry explains the drop fit.
Optical drop shape vs force tensiometry at a glance
<p>Each row cites a published source; see References. Ranges are typical, and individual instruments vary.</p>
| Question | Optical drop shape (pendant drop) | Force (ring or plate) |
|---|---|---|
| Sample needed | About 10 to 60 µL per drop | Millilitres in a vessel; a test lab lists 20 mL for a Wilhelmy plate run |
| Extra inputs | Density of the drop and the surrounding phase | Ring: correction factor. Plate: a fully wetted plate |
| Interfacial tension between two liquids | Yes, with the drop inside a cuvette of the second liquid | Yes, ring or plate at the interface (ASTM D971, ISO 6889) |
| Fast dynamics | Not below about 1 second of surface age; use bubble pressure for milliseconds | Plate can follow slow changes over time |
| Viscous liquids | Can handle higher viscosity than force methods | Ring unsuitable above about 1,000 mPa·s; plate preferred |
| Opaque liquids | Needs image contrast; a rising drop can help | No optical path needed |
| High temperature and pressure | Up to 400 °C and 690 bar with dedicated chambers | Up to about 300 °C on some force tensiometers |
| CMC series | Possible, one concentration at a time | Usual choice, with automated dosing |
| Standards that name the method | ISO 19403-3 (paints and varnishes) | ASTM D1331, ISO 304, ASTM D971, ISO 6889 |
Published tensiometer prices, checked 25 October 2026
<p>Prices as printed on 25 October 2026, excluding tax and shipping. CSC Scientific does not publish prices itself, so its rows show named resellers. Refurbished KRÜSS units are single listings and change often.</p>
| Instrument | Method | Price as published | Source |
|---|---|---|---|
| Dropometer base kit | Pendant drop, up to 75 mN/m, 0.03 mN/m accuracy; also contact angle | US$4,990 | Droplet Lab product page |
| Gardco DuNouy Precision Tensiometer | Ring | US$7,901.00 | gardco.com |
| CSC Scientific du Noüy ring tensiometer 70535000 | Ring | US$10,222.58 | Thomas Scientific |
| CSC Scientific DuNouy Precision 02-080-015 | Ring, surface and interfacial | US$12,760.00 | Fisher Scientific (US) |
| KRÜSS K20 (refurbished) | Ring and plate | €9,660.00 excl. VAT, sold out | KRÜSS online shop |
| KRÜSS K100 (refurbished) | Ring, plate and more | From €15,181.50 excl. VAT | KRÜSS online shop |
| KRÜSS BP100 (refurbished) | Bubble pressure, dynamic | €22,476.00 excl. VAT | KRÜSS online shop |
| KRÜSS SDT (refurbished) | Spinning drop, very low interfacial tension | €21,604.80 excl. VAT | KRÜSS online shop |
| KRÜSS new force tensiometers | Ring and plate | Quote only | KRÜSS website |
| Biolin Scientific Sigma series | Ring and plate | Quote only | Biolin Scientific website |
| Kibron EZ-Pi Plus | Force | No published price found | None |
When does optical drop shape win?
- You have very little sample. A pendant drop needs microlitres; force methods need millilitres. That matters for biological fluids, new formulations and expensive solvents.
- You need interfacial tension between two liquids and can provide both densities, for example oil and water in emulsion work.
- You work hot or under pressure. KRÜSS documents pendant drop up to 400 °C and 690 bar with dedicated chambers.
- Your liquid is viscous. A drop shape fit is less affected by viscosity than pulling a ring through the surface.
- You also measure contact angle. One optical instrument can cover both, which a force tensiometer cannot.
When does a force tensiometer win?
- Your method names a ring or plate. ASTM D1331, ISO 304, ASTM D971 and ISO 6889 all specify ring, plate or stirrup methods. A pendant drop result will not satisfy a specification written around them.
- You run CMC series. Force tensiometers with automated dosing step through a dilution series without an operator. See critical micelle concentration.
- The liquid is opaque or the density is unknown. A balance does not need an optical path or a density value.
- You need very low interfacial tension or millisecond dynamics. Neither ring nor pendant drop is the right tool: spinning drop covers ultra low interfacial tension, and bubble pressure covers short surface ages.
Do the two methods give the same number?
Close, but not always within the stated uncertainty, and operator choices matter for both.
- Ring vs plate. Moura and colleagues (Acta IMEKO, 2023) compared ring and plate results from three national metrology institutes on six liquids. The plate almost always read higher, by up to 1.38 mN/m, often beyond the stated uncertainty.
- Ring speed. KRÜSS technical note TN308 reports a surfactant solution reading 36.34 mN/m at 10 mm/min and 32.34 mN/m at 0.5 mm/min with a ring, against 30.12 mN/m with a plate at equilibrium.
- Pendant drop fit quality. Berry and colleagues (2015) show that error grows as the drop becomes more spherical, so drop size and needle choice matter.
- The Dropometer. Its pendant drop method was validated against a KRÜSS DSA100E, another drop shape instrument, on water, ethylene glycol and dodecane (Colloids and Surfaces A, 2017). That paper does not compare it with a ring or plate.
If you are switching methods, measure a few of your own liquids both ways and record the offset before you change a specification.
Expert Quote
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.
Which do you need?
- Insulating oil QC to ASTM D971: a ring tensiometer.
- Surfactant and CMC research: a force tensiometer with automated dosing; add pendant drop if samples are scarce.
- Paints and coatings: either. ASTM D1331 recommends the plate for paints and resins, while ISO 19403-3 specifies pendant drop.
- Emulsions and oil water interfaces with small samples: pendant drop in a cuvette.
- A lab that also needs contact angle on a small budget: a pendant drop instrument such as the Dropometer pendant drop tensiometer and goniometer.
- Teaching labs: pendant drop shows the physics directly in the drop shape. See the smartphone surface tension method.
The best tensiometer under US$5,000 guide scores the low cost options side by side.
Who should not buy a Dropometer?
- Your specification names a ring or plate method, such as ASTM D1331, ASTM D971 or ISO 304.
- You run automated CMC series. The Dropometer measures one drop at a time, with no automated dilution.
- You need surface tension above 75 mN/m. That is the top of the published range; force tensiometers publish ranges up to 1,000 mN/m or more.
- You need dynamic surface tension below about 1 second, or ultra low interfacial tension. Bubble pressure and spinning drop instruments are built for those.
- You do not know your liquid's density and cannot measure it. Pendant drop needs it.
Where can you go deeper?
- Surface tension measurement guide and what a tensiometer is
- Pendant drop method, interfacial tension and Young Laplace equation
- Best tensiometer under US$5,000
- ASTM D1331, ASTM D971 and IEC 62961 and ISO 19403-3
- How Droplet Lab measures surface tension with a smartphone
- The two peer reviewed validation papers
About this guide. Droplet Lab makes the Dropometer, which measures surface tension by pendant drop. Prices are as published by the maker or a named seller on 25 October 2026; where none is published we say so. If your method is written around a ring or plate standard, or you run CMC series, a force tensiometer is the better buy. This guide was drafted with Claude Opus 5.5 and reviewed by Droplet Lab.
Not sure which method your liquid needs?
Tell us the liquid, the volume you have and the standard you report to; in a 15 minute call we will say whether pendant drop or a force tensiometer fits, including when a Dropometer is the wrong buy.
Frequently Asked Questions
Is pendant drop as accurate as a du Noüy ring?
Both can be accurate when used correctly, but they fail in different ways. The ring needs a correction factor and is sensitive to pulling speed; pendant drop needs accurate densities and a well shaped drop. Published comparisons show ring and plate results can differ by more than a millinewton per metre.
Do I need the liquid's density for pendant drop?
Yes. The drop shape fit balances surface tension against gravity, so it needs the density of the drop and of the surrounding phase at the measuring temperature.
Can pendant drop measure interfacial tension?
Yes. The drop is formed inside a cuvette of the second liquid, and the fit uses the density difference between the two. The research Dropometer measures interfacial tension this way.
Which method do ASTM D1331 and ISO 304 use?
Both specify force methods: ASTM D1331 covers ring and plate, and ISO 304 covers ring, stirrup or plate. ISO 19403-3 is the standard that specifies pendant drop, for paints and varnishes.
Which method is best for CMC measurement?
A force tensiometer with automated dosing is the usual choice because it steps through a dilution series unattended. Pendant drop works one concentration at a time and suits small or scarce samples.
How much sample does each method need?
Pendant drop uses about 10 to 60 µL per drop. Ring and plate methods need millilitres in a vessel.