a gas needs diffusing into a liquid in the sewage treatment regime other than in the search for efficiency the equality of the bubble size or their absolute minimisation in size may not be imperative rather the capacity to retro fit the arrangement may be more important however in new installations or in other applications where for particular reasons a very small bubble size and very even bubble size distribution is desired the arrangement illustrated in fig4 and 5 may be employed referring first to fig3 two plots are shown of internal pressure against bubble size being formed from two apertures of different size 0 6 and 1 0 mm the pressure increases substantially linearly with increasing volume until the bubble reaches a hemispherical shape thereafter however pressure decreases as the bubble grows further thus at any given pressure a bubble can have two sizes more importantly however if two bubbles are growing from two ports that are supplied by a common source in parallel with one another then as the pressure increases with growing bubble size the growth of the two bubbles in parallel is stable however once the bubble reaches hemispherical the stable growth ends and as one bubble continues to grow its pressure diminishes consequently if there should be any imbalance between the growth of the two bubbles so.