Abstract: The authors made a theoretical analysis regarding slot opening for a bubble cap in operation. Experimental work was carried out on air-water system for a vertical, single, rectangular slot to correlate slot opening with air flow rate. Experiments were carried out on the basis of the work of Cross and Ryder, but with the experimental conditions extended to different slot widths and different immersed depths. Experimental conditions are as follows: air flow rates, 80~16,300 liters/hr; slot widths, 2.06~13.95 mm; immersed depths 0~100mm. The present work confirms the statement of Cross and Ryder that the slot opening formula derived by Rogers and Thiele, V, is only applicable to high air flow rates. The authors further point out that the discharge coefficient u in the above formula is a constant only when the air flow rate is high and its value is 0.63. For low flow rates, the value of u is no longer constant but is a function of Reynolds number. Data obtained also indicate that the slot opening is independent on the immersed depth. In the continuous bubbling region, the relationship between discharge coefficient u and Reynolds number is as follows: 200 3250 : u = 0.63 Since Fraude number = , therefore : 200 3250 : Fr = 0.353 The formula thus obtained will be applicable to any slot widths and any air flow rates ordinarily used providing bubbling is continuous.