Abstract

An air pycnometer is constructed based on the concept of energy conservation for the investigation of its sensitivity. The apparatus assumes that the ideal gas law holds and energy transfer between the system and its surroundings is negligible. It comprises two connected chambers—reference and sample. The addition of a ball valve helps isolate them. The volume ratios between them, i.e., 0.52 and 1.00, are explored experimentally for their sensitivity. The predictions for the ratio of 0.20 and 0.33 are also given. Acrylic disks of known volumes are used for testing filling fractions under the volume-ratio conditions. The data are fitted to the model, derived from energy conservation, for the atmospheric pressure and the initial pressure of the reference chamber. The derivative of the model gives rise to sensitivity curves in terms of filling fractions for given volume ratios. Four volume ratios, 0.20, 0.33, 0.52, and 1.00, are explored for their sensitivities and characteristics. The results show that the sensitivity of the pycnometer relates to both the volume ratio between chambers and the filling fraction. When graphed for different volume ratios, the sensitivity monotonically increases as the filling fraction rises for all ratios. The system is more sensitive for a greater ratio only when the filling fraction is more than 75%. For lesser filling fraction, the sensitivity becomes subtle and is unlike that of the nearly-filled chamber. Here, we reveal and discuss this phenomenon. Besides, the results also indicate that sensitivity customization can be achieved. To clarify this statement, we apply the apparatus to determine the void volume, for four filling fractions, of divided solids—full and broken grains of rice and sugar beads—and a fibrous sample—a palm wood disk. The results agree with those of the water saturation approach. Our apparatus gives reliable results, and its sensitivity dependence is elucidated. Locations of the sensitivity degeneracy are identified. The correlation between the filling fraction and volume ratio of the system cavities is described.