Figure. Bubble dispersion mechanism in the orifice. Display omitted •Pressure drop fluctuation in micro-bubble generation conditions was analyzed.•Micro-bubble flow regime was analyzed with pressure drop fluctuation.•With increasing We/Reg−8 in kerosene, bubble size shows decreasing tendency. The objective of this study was to quantitatively analyze bubble/micro-bubble regime transition point in a bubble column. A test was carried out with a cylindrical stainless-steel column having an inner diameter of 0.097 m and a height of 1.8 m. An air was injected as a gas medium. Kerosene with a density of 800 kg/m3, viscosity of 1.64 × 10−3 Pa•s, and surface tension of 23 × 10−3 N/m at 20 ± 2 °C was used as a liquid. The experiment was conducted on a single nozzle plate with different hole sizes (0.7, 1.5, 2.0, and 3.46 mm). To analyze the standard deviation of the differential pressure signal by air bubbles separated from the jet, 12,000 differential pressure data from 0.05 m to 0.15 m of height were measured at 200 Hz under each condition. As gas velocity at the orifice increased, sizes of bubbles falling apart from the nozzle increased. Standard deviation of differential pressure also increased. Bubbles grew until they fell from the tip of the jet. At higher orifice gas velocity, the standard deviation of the differential pressure was reduced due to the occurrence of micro-bubbles. Since the standard deviation of the differential pressure showed a high peak at We/Reg−8 = 1.2 × 1045 for all cases, this value was selected as a demarcation of bubble/heterogeneous micro-bubble regime transition. The standard deviation of pressure drop fluctuation was the lowest at We/Reg−8 = 6.0 × 1048. This point was determined as the homogeneous micro-bubble regime point.