56 to discharge directly into first opening 15 of bag 16 aspiration pinch valve 26 closes and calibration pinch valve 28 is opened thus third section 50 fourth section 52 and fifth section 54 of aspiration circuit 48 are isolated from lower section 49 of aspiration circuit 48 the residual vacuum pressure existing in third section 50 fourth section 52 and fifth section 54 pulls air from second opening 17 of bag 16 thus a portion of irrigation aspiration circuit 18 is air vented in another example pump 14 is stopped aspiration pinch valve 26 closes calibration pinch valve 28 opens bridge pinch valve 24 is closed and irrigation pinch valve 22 is open as a result air is drawn from second opening 17 of bag 16 to vent aspiration circuit 48 this results in a slight pressure differential between one end of second section 40 of irrigation circuit 36 and lower section 49 of aspiration circuit 48 a practical result of this slight pressure differential is that material is guided from the irrigation side to stay at the aspiration side of an irrigation aspiration handpiece not shown this characteristic emulates air venting in irrigation aspiration systems which do not utilize a positive displacement pump as is utilized and shown at 14 of ophthalmic irrigation aspiration system 10 as shown in fig2 bag 16 is formed from first wall 58 and second wall 60 joined together at weld 62 disposed within bag 16 is a first air reservoir member 64 made from a cellular material a sponge also a second air reservoir member.