Nippon Sanso K.K developed a compact semi-closed underwater breathing apparatus, the eOBA. It consists of a mouthpiece, manifold with a purge valve, two spring-loaded flexible tubes, a small CO2 absorbent canister (net wt.= 190g), and two compact high pressure bottles (50ccx2 : 190kglcm2 : 80%O2, 20%N2) with a regulator which supplies the gas at the constant flow rate of 1.5 l/min and lasts for 10 min. Thus, a counterlung is not incorporated. However, spring-loaded tubes act as a counterlung since its volume increases to 3.5 1 when fully inflated. Dives to a depth of 5m are also recommended because of no bypass valve. This new eOBA was tested using the mechanical breathing machine and CO2 suppiy system to the circuit. For the various combinations of tidal volumes (0.5-2.51) and respiratory rates (l0-20 breaths/min), the pressure at the mouthpiece, respiratory volume and the CO2 level were continuously monitored. The CO2 absorption rates were then calculated. The thin sloping P-V loops demonstrate that the eOBA is a flow dependent type of apparatus. It was found that the extemal work of breathing (0.1 kg.m/l at 30 g/min) were allowbale. The CO2 absorption rates were sufficient when minute ventilation increased to 30l/min. Thus, results show that the eOBA must be suitable for shallow and short dives.