The POROLUX “calculated” 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.
Bubble point detection

The bubble point is defined as the pressure required to blow the first continuous bubbles detectable through a sample. This bubble point test can be performed by pre-wetting a sample, increasing the pressure of gas above the sample at a predetermined rate and watching for gas bubbles downstream to indicate the passage of gas through the maximum diameter pores in the sample (ASTM F316). The first detected gas flow is defined as the bubble point. Home build devices still use this visual detection of the first (stream) of bubbles; the pressure at which this first (stream) of bubbles happens is used to calculate the bubble point. Automated methods use flow meters detecting a certain minimum flow. The first Coulter porometers defined the bubble point as the pressure at which 100 ml/min was measured. In the mean time, mass flow meters have become more sensitive and thus lower flows can be set (e.g. 30 or 50 ml/min).

This implies that there is no uniform definition of bubble point, one should always specify the minimum flow that was used to calculate the bubble point (there are also mathematical methods searching the onset of the flow increase in the flow/pressure graph).

A drawback of this approach is that there has to be a minimum flow in order to detect the bubble point. As a result, the bubble point is calculated at a pressure that is already slightly too high, and therefore this calculated bubble point will be too small. To overcome this drawback, the POROLUX 1000 can also be used in a “true” measured bubble point method (see below).

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