Different scales of liquid mixing exist in bubble columns and it is very important to determine the prevailing mixing scale in each flow regime. Two independent parameters were found to exhibit a monotonous decline in the transition flow regime, which could be attributed to the decrease of the mixing length values L. In this work, a new parameter called "maximum number of visits in a region" (N-v(max)) and the Kolmogorov entropy (KE) were extracted from the gas holdup time-series (60,000 points). The latter were recorded at a high sampling frequency (2,000 Hz) by a wire-mesh sensor. The measurements were performed in a narrow bubble column (0.15 m in i.d., clear liquid height = 2 m) equipped with a perforated plate distributor (14 holes, circle divide 4 x 10(-3) m). Both parameters were capable of identifying concordantly the two main transition velocities at U-trans = 0.022 and 0.112 m/s, which delineate the boundaries of gas maldistribution, transition and churn-turbulent regimes, respectively.