Figure 6.5 is a schematic of a very simple filter medium holder for determining maximum pore size or bubble point. The medium is clamped in place over a pressurizing chamber. Above the medium is a reservoir, which is filled with a test fluid of known density and surface tension. The pressurizing chamber is connected to a source of pressurized air and to a manometer. The pressure increases when airflow is introduced into the pressurizing chamber. Eventually the air forces its way through the medium and bubbles through the liquid reservoir. The manometer pressure is recorded at the time the first bubble is observed rising through the reservoir. The bubble pointor maximum pore size is calculated as follows: