Application Guide
How capillary flow porometry, bubble point testing, and complementary methods characterize the through-pores of membranes and filters — and how to report the results consistently.
A filter retains particles because of the narrowest constriction along each continuous path from one face to the other. Dead-end and closed pores contribute to total porosity but not to filtration, which is why membrane QC relies on porometry — methods that only see through-pores — rather than mercury intrusion.
The key quantities are the bubble point (the pressure at which gas first displaces the wetting liquid from the largest through-pore), the mean flow pore size (the pressure at which flow through the wet sample equals half the flow through the dry sample), and the through-pore size distribution between them.
For a wetting liquid in a cylindrical pore, the Young–Laplace relation gives the diameter that empties at differential pressure ΔP:
ASTM F316 instead writes d = Cγ/p with an empirical constant (C = 2860 for p in Pa, or 0.415 for p in psi, d in µm, γ in mN/m), which gives diameters about 0.715 times the ideal-cylinder value. Always state which convention and which wetting liquid were used; the two are not interchangeable.
| Wetting liquid | Surface tension (mN/m, approx.) | Typical use |
|---|---|---|
| Galwick / Porewick (perfluorinated) | 15.9 / 16 | Default for most membranes; wets almost everything |
| Silwick (silicone oil) | 20.1 | Alternative low-surface-tension liquid |
| Isopropanol | ~21.7 | Common for integrity testing of hydrophobic filters |
| Water | 72.8 (20 °C) | Hydrophilic sterilizing-grade filters; higher pressures for the same pore size |
| Membrane type | Typical method | Notes |
|---|---|---|
| Microfiltration (~0.1–10 µm) | Gas–liquid capillary flow porometry | Bubble point, mean flow pore, distribution |
| Ultrafiltration (~2–100 nm) | Liquid–liquid displacement porometry; MWCO by solute rejection (dextrans, PEG) | Gas–liquid CFP needs very high pressures in this range |
| Sterilizing-grade filters | Bubble point, diffusion or pressure-hold integrity tests | Correlated with bacterial retention (ASTM F838) |
| Nonwovens, geotextiles, sintered metals | Capillary flow porometry / bubble test | ASTM D6767 (geotextiles), ISO 4003 (sintered metals) |
Use the calculators to convert test pressures to pore sizes, or email a short description of your sample and what you need to measure.