The POROLUX “true” First Bubble Point detection mode application note

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The POROLUX 1000 is a research grade capillary flow porometer with very pressure and flow control and accuracy. In the POROLUX 1000 a porous sample is inserted, wetted with a so-called wetting liquid. The sample is usually a flat round disk, three different diameters are standard: 13, 25 and 47 mm. Custom made sample holders are optional. The wetting liquid is a liquid which fills up all pores of the sample. Good contact between the liquid and the sample is therefore necessary. Different liquids can be used, the surface tension should however be exactly known and entered in the software prior to the measurement. This value is used for the calculation from pressure to pore size.

A typical capillary flow porometer gradually builds up a nitrogen or other inert gas pressure in between two chosen boundaries. Flow meters follow the flow of gas through the sample. In the graphs, gas flow is shown as a function of pressure. The wetting liquid is pushed out of the large pores at lower pressures, small pores require a higher pressure in order to be emptied. First such a wet curve is measured, after which the same experiment is performed for a complete dry sample.

The “true” first bubble point
Bubble points were for a long time measured in capillary flow porometers by the method described in ASTM F-316 as the pressure at which predefined flow was measured. The POROLUX 1000 can be operated in a calculated of in a measure bubble point mode. In the calculated bubble point mode, the POROLUX 1000 offers several predefined flow to calculate the first bubble point. In the “true” first bubble point bubble point detection mode, the POROLUX 1000 meets the original, physical definition of the bubble point.

The first developed bubble point testers measured a bubble point in the more exact way: a filter was placed in a small tank filled with water or another solvent and connected to a pressure medium allowing a gradual build up of the pressure (see figure 1). The pressure at which the first continuous stream of gas bubbles is observed, is the bubble point pressure. This pressure can be easily calculated into the bubble point pore size.

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