Capillary Flow Porometry vs Mercury Intrusion: Which Pore Size Method to Use

At a glance. Use capillary flow porometry when you need the size of the pores that actually carry flow: the bubble point, mean flow pore size and through-pore distribution of a membrane or filter. Use mercury intrusion porosimetry when you need total open pore volume, porosity and density, including dead-end pores, on a material stiff enough to withstand pressures of up to about 400 MPa. For ultrafiltration membranes tighter than roughly 15 nm, liquid–liquid porometry usually serves better than either.

Why do porometry and porosimetry give different pore sizes?

Both techniques rest on the same capillary physics. The pressure needed to push a fluid into, or out of, a cylindrical pore is inversely proportional to its diameter: D = 4γ|cos θ| / P, where γ is the surface tension of the liquid and θ its contact angle on the solid. What differs is the direction of travel and the fluid used.

In capillary flow porometry (CFP), the sample is first filled with a low-surface-tension liquid that wets it completely (θ close to 0°). Gas pressure is raised until liquid is pushed out of the pores and gas starts to flow. A pore only registers when gas can pass all the way through it, so the method sees through-pores only, and each one is reported at its narrowest constriction. ASTM F316 standardises the bubble point and mean flow pore tests on this principle.

Mercury intrusion porosimetry (MIP) works the other way round. Mercury does not wet most solids, with contact angles between 130° and 150°, so it has to be forced in. The instrument records the volume intruded at each pressure step. Every pore connected to the surface fills, whether or not it leads anywhere, so MIP reports total open pore volume and assigns each volume to the pressure at which its entrance was breached. ISO 15901-1 covers the method.

A membrane with a thin selective skin over a sponge-like, macrovoid-rich support will therefore show a small mean flow pore size by CFP, because the skin controls flow, and a broad, much larger distribution by MIP, because the support holds most of the volume. Both numbers are correct. They answer different questions.

What does each method measure?

Property	Capillary flow porometry	Mercury intrusion porosimetry
Pores detected	Through-pores only, at their narrowest point	All open pores, through and dead-end, sized at their entrance
Main results	Bubble point, mean flow pore size, smallest pore, through-pore distribution, gas permeability	Pore volume, porosity, pore-throat distribution, bulk and skeletal density
Working range	About 13–15 nm to above 500 µm	About 3–4 nm to roughly 400 µm
Fluids and pressure	Wetting liquid near 16 mN/m and gas, up to about 35 bar	Mercury, up to about 400 MPa
Sample after test	Usually recoverable once the liquid has evaporated	Contaminated with mercury and not reusable
Best suited to	MF and UF membranes, filter media, nonwovens, hollow fibres	Rigid solids: ceramics, catalyst supports, sintered metals, rock
Reference standard	ASTM F316	ISO 15901-1

The ranges come from Anton Paar's pore size overview, which gives about 13 nm to more than 500 µm for CFP and 3.2 nm to more than 400 µm for MIP, and from ISO 15901-1, which puts the smallest MIP diameter near 4 nm at roughly 400 MPa. The 35 bar ceiling and the 16 mN/m wetting liquid are from Porometer's technical FAQ.

How do I read a capillary flow porometry result?

A CFP run produces two curves of gas flow against pressure: a wet curve on the liquid-filled sample and a dry curve on the same sample afterwards. Three values come from them.

• Bubble point. The pressure at which flow first appears on the wet curve, which gives the largest through-pore. A bubble point calculated from a flow threshold sits slightly below the true first opening, so record which definition your software uses.

• Mean flow pore size. The pressure at which the wet curve crosses the half-dry curve. Half of the gas flow then passes through pores larger than this size.

• Smallest pore. The pressure at which the wet and dry curves merge, because every through-pore is open.

The floor of the method follows directly from the Washburn relation. With a 16 mN/m liquid at 35 bar, D = (4 × 0.016 N/m) ÷ (3.5 × 10⁶ Pa) ≈ 18 nm, and instrument makers put the practical limit at around 15 nm. The conversion also assumes complete wetting, which is worth confirming on new polymers with a contact angle measuring instrument.

When is mercury intrusion the right choice?

MIP is the right tool when the question is about volume rather than flow: total porosity of a ceramic support, pore volume of a catalyst, or bulk and skeletal density of a sintered part. The intrusion and extrusion curves also reveal ink-bottle pores, where a large cavity sits behind a narrow throat, a feature CFP cannot resolve. An automated mercury intrusion porosimeter covers the full range in one automated run.

Its limits matter for membranes. Polymer films, electrospun mats and nonwovens can deform before mercury enters their pores. Lowery, Pai and Rutledge showed that buckling of electrospun fibre mats under mercury pressure significantly changes the pore size distribution that porosimetry reports. Mercury is also toxic, the sample cannot be reused, and both the waste and the instrument need controlled handling.

What about liquid–liquid, liquid extrusion and helium methods?

Liquid–liquid displacement porometry

Two immiscible liquids replace liquid and gas. Their interfacial tension is far lower than a liquid's surface tension, so much smaller pores open at modest pressures. That suits ultrafiltration membranes below the CFP floor and avoids the high gas pressures that can burst or collapse hollow fibres. Results relate well to retention, so pair them with a molecular weight cut-off test.

Liquid extrusion porosimetry

The wetted sample sits on a support membrane whose pores are finer than those of interest. As gas pressure rises, liquid is pushed out of progressively smaller through-pores and the extruded volume is measured. A liquid extrusion porosimeter therefore gives through-pore volume as well as diameter, without mercury.

Helium gas expansion porosimetry

A helium porosimeter applies Boyle's law: helium expands into a chamber holding the sample, and the pressure change gives the grain or skeletal volume. Combined with bulk volume, that gives open porosity, but no pore size. For polymer membranes, the gravimetric wet–dry method in our membrane porosity guide is often simpler. Below about 3 nm, gas adsorption is the method to use.

Which method should I use for my sample?

• MF and UF flat sheets, hollow fibres and filter media where flow matters: capillary flow porometry.

• Tight UF membranes and MWCO correlation: liquid–liquid displacement porometry.

• Ceramics, catalysts and sintered metals: mercury intrusion, with helium porosimetry for skeletal density.

• Electrospun mats and soft nonwovens: capillary flow porometry or liquid extrusion, to avoid compression artefacts.

Step-by-step checklist for choosing a porometer

1. Define the question: the pore size that controls flow, the total pore volume, or both.

2. Estimate the pore range from SEM, rejection or MWCO data, and compare it with each method's floor.

3. Check the sample's mechanics: compressibility, hollow fibre burst or collapse pressure, and maximum test pressure.

4. Choose a wetting liquid that wets fully, does not swell the polymer and has a certified surface tension.

5. Specify pressure and flow ranges: maximum pressure sets the smallest pore, low-pressure resolution sets bubble point accuracy for MF media, and flow range sets the largest pore and the permeability you can report.

6. Specify sample holders for your formats: flat-sheet discs, hollow fibre bundles, tubes or cartridges.

7. Require raw wet and dry curves, ASTM F316 calculations and exportable data, not only a summary report.

8. Validate with a reference membrane of known bubble point and check repeatability between operators.

Frequently asked questions

Is capillary flow porometry the same as a bubble point test?

The bubble point is one result of a capillary flow porometry run: the pressure at which the largest through-pore opens. A full CFP test continues to higher pressures and compares wet and dry curves to give the mean flow pore size, the smallest pore and the distribution in between. ASTM F316 describes both tests.

Why does mercury porosimetry report larger pores than my porometer?

Mercury intrusion counts every open pore, including dead-end pores and the large cavities in a membrane's support layer, and it weights results by volume. Capillary flow porometry reports only through-pores at their narrowest constriction, which in an asymmetric membrane is usually the skin. The two results describe different features of the same structure.

Can capillary flow porometry measure porosity?

No. It measures through-pore sizes and gas permeability, not pore volume. For porosity, use gravimetric wet–dry weighing, mercury intrusion, or helium porosimetry combined with bulk volume.

What is the smallest pore a capillary flow porometer can measure?

With a wetting liquid of about 16 mN/m and a maximum pressure of about 35 bar, the practical limit is around 13–15 nm. Smaller pores need liquid–liquid displacement porometry, which works at lower interfacial tension, or gas adsorption below about 3 nm.

What does a helium porosimeter measure?

It measures the skeletal, or grain, volume of a sample by Boyle's law gas expansion. Combined with the bulk volume, it gives open porosity and skeletal density. It does not give a pore size distribution.

Which wetting liquid should I use?

Use a low-surface-tension liquid, typically a fluorinated liquid near 16 mN/m, that wets the sample completely and does not swell it. Water, at about 72 mN/m, is not recommended because it evaporates during the test and raises the pressures needed.

How Tech Inc. builds porometry equipment

Our AL500 pore size and porosity measuring skid is a capillary flow porometer that reports bubble point, mean flow pore size, pore size distribution and gas permeability for MF and UF membranes, filter media, nonwovens, ceramics and sintered metals. The AL1000 porosity measuring instrument adds finer pressure control and higher-resolution flow sensing for tighter UF membranes. The porometer range sits alongside our mercury intrusion, liquid extrusion and helium instruments, so one supplier can cover through-pore size, pore volume and density.

Each instrument is configured to the client's duty, with pressure range and sample holders chosen for the formats being tested, and is designed, built and factory-tested in our own facilities. Tech Inc. is part of an engineering group founded in 1990, with offices in Canada, India, Estonia and Saudi Arabia, and our characterisation equipment is trusted by universities and R&D laboratories worldwide. For how consistent characterisation data feeds model-driven membrane development, see AI in membrane research.

Send your sample types, expected pore range and test standards to mail@techincresearch.com or call +1 (289) 622-1032, and we will recommend the method and configuration that answers your question.