One of the most important parameters measured by a porometer is the first bubble point or FBP. This point defines the largest pores present inside the material. ASTM F-316-03 defines the FBP as the pressure at which the first continuous gas bubbles are detected. In the calculated FBP operating mode, the FBP is defined at different flows, e.g. at 30, 50, 100 ml/min; the corresponding pressure is recalculated into the pore size. This is however far away from a visual observation of air bubbles and very often, this calculation does not represent the real opening of the largest pores. In the figure below it is clear that increasing the pressure also results in a certain gas flow toward the sample but not actually through the pores of the sample. Erroneous calculations are therefore easily made.

To counter this effect, we developed for the POROLUX^TM 1000 series an actual measured bubble point detection mode. In this mode, the instrument runs in a gas flow control regime, rather than the usual pressure control regime for normal measurements.

With an impregnated, wet sample, the POROLUX™ 1000 sample chamber forms a closed system. Starting from a relative pressure of 0 bar, we add a constant gas flow by the flow regulator. As the volume inside the sample chamber is fixed, this constant flow will result in a linear rise of the pressure above the sample. At the moment the first and largest pore is opened, there will be a change in this linear pressure increase (or its first derivative over time). This chance can be regarded as the true first bubble point of the material. This method to measure the FBP shows an excellent reproducibility and is unique in the market.