One of the most important parameters measured by a porometer is the first bubble point or FBP. This point corresponds to the largest pore(s) present inside the material.
The ASTM F-316 standard defines the FBP at the pressure at which the first continuous bubbles are detected. This is based on the traditional approach of placing the sample in a housing with liquid placed on the top side. Then a pressurised gas is applied under the sample and the pressure is gradually increased over time and when a constant flow of rising bubbles is observed on the top side of the sample, it is assumed that the gas pressure has reached bubble point. However, this is a visual and, therefore, subjective approach. When do we consider that the first continuous bubbles are detected? It differs significantly depending on the person.
For that reason the FBP can be defined at different flow rates, e.g. at 30, 50, 100 ml/min. So for a certain target flow the pressure required to achieve it is used to calculate the pore size, using the Young-Laplace equation as previously explained. Because with this approach there is already flow at the FBP, by definition, this calculated FBP is always smaller than the real bubble point and thus the calculated FBP never represents the real opening of the largest pores.
There are multiple criteria to select the pressure to calculate the FBP. A POROLUX™ user has the choice to select among different calculation methods.
There is another, more accurate, approach for detecting the largest pore. It is the so-called measured bubble point.
The fully wetted sample inside the sample chamber forms a closed system. If we increase the pressure on the sample using a small, constant flow of gas towards the sample chamber, as the volume 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 the linear pressure increase. This change can be regarded as the true first bubble point of the material and this pressure is used to obtain the pore size. This method to measure the FBP shows an excellent accuracy and reproducibility.