The methods of harvesting cells are provided, wherein the methods comprise introducing a processing material and a source material into a processing loop. The processing loop comprises a processing chamber and a filtering device. The processing material and the source material are circulating through the processing chamber and the filtering device, wherein the processing chamber has a mass; balancing an influx of the processing material into the processing chamber with a permeate flux of the filtering device to maintain the mass of the processing chamber at a constant value; and collecting the cells in a collection chamber. Cell harvesting devices are also provided for processing and harvesting cells using a control law to balance the mass of the processing chamber through the entire process.