Capillary flow porometry by gas-liquid displacement is a well-established technique for measuring pore size distributions in fibrous media within a useful range of typically about 1 to 50 μm. Its operating principle is based on saturating small samples of the media completely with a wetting liquid, and then progressively “blowing out” pores with air by increasing the differential pressure ∆p across the sample. Measurement of the pore size distribution in (identical) glass microfiber filter media were evaluated with 3 commercial porometers as well as an inhouse-designed device and lead to surprisingly divergent results. The causes for these differences as well as the factors that influence the over-all reliability of data obtained by this widely used technique are investigated. Among the key factors studied were the volatility and viscosity of four common wetting liquids, the scan rate (i.e. the holding time interval between increments of differential pressure ∆p or volumetric flowrate (V ̇), and the scan sequence (i.e. dry before wet, or wet before dry scan).
The largest error source by far was the volatility of fluorinated compounds commonly used as wetting liquids. While the vapor pressures of such compounds may be relatively low, their use in combination with a flow of air through the porous matrix can have an enormous effect on the evaporation rate during a scan. Neglecting this effect (which obviously depends on the scan rate) may ultimately result in an error of almost arbitrary magnitude in the pore size distribution. Silicone oil on the other hand has a negligible volatility and provides reliable results for a wide range of operating conditions. The liquid viscosity in the tested range of about 1:20 played a comparatively insignificant role.