A high quality sparge element with consistent pore sizes is a key feature. There are a few aspects to consider. It could be the inconsistent bubble size, but it could also be a specific sub-set or particular bubble size that is having an impact on the cell health. The range or variations of sparge element pore sizes will obviously dictate the resulting bubbles characteristics and/or foam. It also helps to have the sparge bubbles coming through the shear field of the impeller.