ASTM F316: Bubble Point and Pore Size of Membrane Filter Cartridges

ASTM F316 is the test method most often named for the pore size of membrane filters. It covers membranes with a largest pore size from 0.1 to 15 µm, and it is the basis of the bubble point test used to check membrane cartridges before and after use.

What the standard covers

F316 measures two pore size properties of a membrane filter:

• Test Method A, bubble point: the largest pore, expressed as its limiting diameter, which is the diameter of a circle with the same area as the narrowest part of the pore.

• Test Method B, mean flow pore: how common a given pore size is in the membrane, measured from the gas flow through it.

How the bubble point test works

A membrane has pores that run from one face to the other, much like fine capillary tubes. When the membrane is wetted with a liquid, the liquid is held in the pores by surface tension. To push it out of a pore, gas pressure has to overcome that surface tension, and the smaller the pore, the higher the pressure needed. Halve the pore diameter and the pressure doubles.

In the test, the wetted membrane is held in a holder and the gas pressure on one side is raised slowly. The largest pores empty first. The pressure at which a steady stream of bubbles appears on the other side is the bubble point, and from it the largest pore size can be worked out.

The wetting liquid has to be stated with the result, because the bubble point depends on its surface tension. Hydrophilic membranes such as PES and nylon are usually tested wetted with water; hydrophobic PTFE is tested with an alcohol or an alcohol and water mixture.

The mean flow pore test

For Method B, the gas flow through the membrane is measured first with the membrane wet, as the pressure rises, and then dry. Comparing the two flow curves shows at what pressure, and so at what pore size, the flow passes through the membrane, and gives the mean flow pore size and the spread of pore sizes.

What the results tell you

The standard gives three uses for the test: finding the largest pore size of a filter, comparing the largest pore sizes of several filters, and seeing how processes such as filtration, coating or autoclaving change the largest pore size. A low bubble point can also show a damaged membrane, a seal that is not working or a leak in the system. This is why the bubble point is the usual integrity test for membrane cartridges, done without harming the cartridge.

F316 also sets a limit on itself. Its pore size comes from a model of round capillary pores, so it is not a direct measure of the size of particle a membrane will retain, and it should not be the only basis for a retention rating. For bacteria, retention is proved by a challenge test: see ASTM F838.

Vens membrane cartridges and F316

Vens membrane cartridges can meet bubble point requirements measured by ASTM F316, depending on the membrane and grade:

• PES pleated cartridges (SulfoFil)

• Nylon 66 pleated cartridges (NylonFil)

• Hydrophobic PTFE pleated cartridges (FlouroFil)

• Hydrophilic PTFE pleated cartridges (FlouroFil X)

• PFA pleated cartridges (FlouroFil K)

Bubble point testing is one of the integrity tests in the Vens filter test laboratory. When you enquire, tell us the minimum bubble point your specification asks for and the wetting liquid it is measured with.

More: ASTM standards for filters and how to choose a filter cartridge.

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ASTM is a trademark of ASTM International. Vens Hydroluft is not affiliated with ASTM International. The standards are described in our own words from the scopes ASTM publishes; work to the edition named in your specification. Whether a filter meets a requirement depends on the product, its rating and the conditions of use, so we confirm it for each enquiry. See also our product information disclaimer.